In a variable capacitor configuration, a bias of 0.15 Flexible dielectrics that harvest mechanical energy via electrostatic effects are excellent candidates as power sources for wearable electronics or autonomous sensors. The integration of a soft dielectric composite (polydimethylsiloxane PDMS-carbon black CB) into two mechanical energy harvesters is here presented. Both are based on a similar cantilever beam but work on different harvesting principles: variable capacitor and triboelectricity. We show that without an external bias the triboelectric beam harvests a net density power of 0.3  Among these recognized technologies, triboelectric transducers (TENG) enjoy a growing interest, in particular thanks to Pr. ZL Wang's pioneering work2. Their performances are notable; for example, Wang et al. succeeded in harvesting 150 3. From the shape of the current intensity, one may assume that the average power is at least five or ten times lower than the peak power, leading to an estimated power from 15 to 75 g4. Based on electrostatic effects, TENG can be compared to capacitive transducers. Indeed, in both systems charge density is a key factor. But charges are injected via different mechanisms. For triboelectric systems, electrical charges come from a physical contact between the two dissimilar dielectrics facing with each other5, whereas in capacitive harvesting systems they come from a DC-voltage bias6. This is one of the main drawbacks of triboelectricity as charges densities are very sensitive to moisture and can also disappear due to air breakdown9. On the contrary, electrical charges in capacitive harvesting systems are less sensitive to external conditions. But their accumulation is limited by the leak resistance of the capacitor. Another distinction is the theoretical level of understanding of the two systems. Indeed, capacitive transducers are based on a well-established capacitor principle, whereas the microscopic mechanisms of triboelectricity are still debated. For example, after centuries of research, it is still not clear whether electrons, ions or even bulk material transfer is responsible for the observed charging10. The research for developing new TENG’s is still in progress, especially to improve the harvested power and to establish a clearer theory on the subject.A large amount of untapped energy sources surrounds us. They derive from human activities or come from ambient energies such as chemical, thermal, radiant and mechanical energies. Contrary to batteries, they are not time-dependent and can power low-power electronics  for long periods. After the radiant sources , mechanical energy  is the second main environmental source of energy. It can be found almost anywhere: from ocean waves to industrial facilities. Considering its ubiquity, vibrational energy is of particular interest to be converted into electricity. Different transducers  are studied for decades, their operating principle and limitations are well known11, in particular thanks to recent progresses on dielectric elastomers which have a higher permittivity than air15. Typically, dielectric elastomers are inserted between compliant electrodes to form a multilayered electrical capacitor with deformation dependent capacitance. Several structures of capacitive transducers have been studied and applied to different sources of mechanical energy from wave energy16 to human motion17. Largely employed for piezoelectric generators, the design and realization of cantilever beams have been less investigated in the context of electrostatic transducers (capacitive and triboelectric transducers). However, the structure of cantilever beam is generally simple and well suited to produce large strain in particular at the mechanical resonant frequency. An efficient way to design a capacitive transducer as a beam is to insert a flexible dielectric material between two electrodes, and to clamp the so created capacitance to a shaker. When the shaker is subjected to vibrations, the cantilever beam oscillates thereby varying the capacitance formed by the soft dielectric and the two electrodes . At resonant frequency, the beam oscillates with a maximal amplitude and generates large variations of capacitances. This induces oscillating displacements of electrons when the dielectric is polarized, and therefore the production of electrical energy along a cycle.In addition to this new field of research, capacitive transducers have been the subject of renewed interest18). It offers a charge-pump to drive electrons through an external resistance load, in which current flows back and forth between the electrodes as alternating current. Contrary to a capacitive transducer, a triboelectric transducer does not require any external polarization.Based on the same cantilever architecture, a triboelectric energy harvester can be easily implemented by inserting two dielectrics facing each other. Indeed, triboelectricity occurs when two dissimilar materials get charged by contacting and separating them  the recovery powers are discussed. Considering losses of the systems, we show that the capacitive transducer beam provides a positive energy balance. We measured a net power density of 0.15 g and a frequency of 40 Hz under a bias voltage of 50 mV/Here, we study a new soft dielectricThe manuscript is organized as follows. First, we remind the general working principle of a capacitive transducer to highlight key optimization parameters. Second, we describe the transducer architectures and the properties of the emulsion based PDMS/carbon black composite. Third, we discuss the results. We then conclude with comparison between the two systems and future ways of improvement.In contrast to triboelectricity, electrical charges from a capacitive transducer are provided by a DC bias voltage. The change in charge density is directly influenced by the change in capacitance of the transducer due to a mechanical energy input.Capacitance from a capacitive transducer can change in time thanks to a geometrical change of the electrode arrangement (change in electrode distance or surface) but also from a change in the dielectric permittivity (electrostriction), see Table If we suppose that during a time interval Since Using the relation Applying the Green’s theorem to (2) it leads toFinally, The whole point of capacitive transducers is to maximize this area to enhance the mechanical–electrical conversion.6. In this circuit all voltages and currents of the circuit are continuous functions of time.The simplest conditioning circuit demonstrating a generation of electrical power out of variation of a capacitance is the “continuous conditioning circuit”Its design is presented in Fig. The circuit is composed of a time variable capacitor The equation describing the electrical behavior of the continuous conditioning circuit is given by Kirchhoff's voltage law:The current is written as the derivative of the charge with respect to the time:When combining (4) and (5) with (6), the differential equation governing the circuit is given by:Finally, we obtain the first order following differential equation:In order to study the operation of the continuous conditioning circuit, we numerically solve, by an Euler explicit scheme, the differential Eq.  governinWe approximate the time derivative function For the study, the capacitive transducer is assumed to move according to the sinusoidal law:We run the simulation for several values of the load resistance and the DC bias voltage. The numerical values of the parameters used in the simulation are given in Table Figure entclass1pt{minimaentclass1pt{minimaIn the entclass1pt{minima21 (free charge carriers and dielectric losses). This is a drawback for energy harvesting techniques based on capacitive transducer. In this work, we insert an insulating pure PDMS layer onto the CB/PDMS composite. The insulating layer acts as a blocking layer and prevents discharge of the capacitor. This concept and its theoretical development were recently discussed elsewhere by our research group20. So as to identify the optimal carbon concentration to increase the dielectric constant, we investigate the dielectric properties of the CB/PDMS composites at rest with and without PDMS insulating layer /PDMS composite made by using a water-in-oil (W/O) emulsion template . This process provides a fine control of the location of the conductive inclusions which remain confined in the materials pores. The direct carbon dispersion in the PDMS matrix gives a very inhomogeneous dispersion with large aggregates. In the field of dielectric composites, working near and above the percolation threshold allows a huge increase of the materials permittivity. However in theses regimes, the composites suffer from a weakness due to the significant increase of electrical lossesyer Fig. . The CB/The dielectric composites without insulating layer have a percolation threshold around 5 wt% Fig. A. This c22. The capacitive harvester involves the CB/PDMS composite (soft dielectric) and a copper electrode (bottom electrode) as two surfaces facing each other while the triboelectric harvester involves a CB/PDMS composite and a Kapton/copper layer.For the next steps, the 10 wt% CB/PDMS materials with the PDMS insulating layer are chosen to be integrated into the cantilever beams as they showed the highest dielectric constant. The architectures of the cantilever beams under investigation are illustrated in Fig. For the two systems, the two bases of the triangular surfaces are glued together while the tips are simply fixed together by a magnet. As shown in the Video When the cantilever is up, the top layer is detaching from the bottom layer, the capacitance is at this moment minimal as the dielectric is changing from CB/PDMS to air, the distance is increasing and the contact surface is decreasing Fig. D. The caFor the measurements, the capacitive harvester is connected to a resistance As a result, to get The cantilever is subjected to vibrations using a shaker (4810 Bruel & kjaer) which drives it into different mechanical frequencies from 1 to 50 Hz. An accelerometer  is fixed on the cantilever to measure the shaker acceleration.It is worth mentioning that mentclasspt{minimasing Eq.  one exam23, the Kapton has a tendency to be negatively charged. We will consider that the composite CB/PDMS with be positively charged. Once the two polymers separate  are brought into contact with each other Fig. A. Video ate Fig. B,D, a poOnce the cantilever is down Fig. C, reductTo make the measurements, the triboelectric harvester is connected to a resistance We note that in the following text, the term power refers to average power and not to instantaneous power or peak power as may be done in some works dealing with triboelectric studies. The peak power will be reported under the notation The total harvested power g  of the capacitive transducer, it leads to a maximum raw power density of 0.24 g and a bias voltage of 96 V. For lower acceleration amplitudes, the harvested power is decreasing following a quadratic power law (exponent close to 2)g and a = 7.3g), the power laws are respectively Taking into account the surface 7.5 cm of the cIt should be noted that in the absence of polarization, the power recovered in this system is zero Fig. D. This mtial Eq.  governinentclass1pt{minimaNow we expose the calculations of the loss power for the capacitive harvester. When the harvester is subjected to a bias voltage Thus, the power loss is calculated using EQ. Table Table For comparison, the performances of the triboelectric harvester using the same composite CB/PDMS, contact area, tip magnets and excitation frequency are investigated. The main difference between the previously described harvester is the absence of bias voltage see Fig. B.g is measured . At the beginning of the experiment the bell jar is not closed and the air inside is identical to the surrounding air (50% of relative humidity (RH)), see Fig. see Fig. A.Figure g). Compared to the capacitive harvester, humidity is unfavourable to the triboelectric one. As shown in Fig. 9 or that changes in friction coefficient of the materials occur making the contact between them less favourable24. It should be noted, however, that it seems possible to encapsulate the triboelectric device in PDMS to make it non-sensitive to moisture.Figure g is measured in triboelectric configuration  and where the size of the harvester (presence of a polarization battery) is not important. Nevertheless, even if humidity is not a problem for the performance of the capacitive harvester, the presence of liquid water, especially salty water, may be the cause of a short circuit between the electrodes or corrosion of the cables. To prevent these phenomena, an encapsulation of the structure will be necessary.We presented a new cantilever resonator, specifically developed for real vibrational energy harvesting. Our composite CB/PDMS was associated with a thin insulating layer of pure PDMS to limit electrical losses. We succeeded in evaluating the energy scavenging abilities of this cantilever in two transducers modes: capacitive and triboelectric transducers. We recall that in this work the term power refers to averaged power and not instantaneous power or peak power as it can be done in some work dealing with triboelectric studies. At the resonant frequency of 40 Hz a recovery power of 0.3 29, in particular to match very low frequencies . In addition, the resonant variable capacitor cantilever could find applications for low-cost frequency or accelerator sensors in vibrating structures.Further improvements of the presented systems lie in the design of the capacitive cantilever. We believe that optimization of the layers thickness could increase the power density by increasing the strain of the electroactive material. Indeed, the change of capacitance, a key parameter for increasing the harvested energy, is dominated by the applied strain. Moreover as mechanical vibrations are broadly dispersed, a larger resonant frequency bandwidth can be simply done by a parallel arrangement of single beam structures using different seismic masses20. We first prepare an emulsion and disperse a solution of carbon black particles in water in a mixture of PDMS (RTV615 Momentive), and curing agent . To ensure the stability of the emulsion we use lauryl PEG-8 dimethicone  as a surfactant. The exact composition of the water phase is 5g of Arabic gum (Sigma Aldrich) for 95g of deionized water. To homogeneously disperse the desired amount of carbon black powder (Alfa Aesar) we use a sonicator (for 1 h at 400 W). We progressively add the carbon black dispersion to the oil phase under manual stirring up to a water:oil mass ratio of 80%wt to emulsify the system. The water-in-oil emulsion loaded with carbon black particles is spread with a stencil into a circular plastic mold. The diameter of the mold is 24 mm and its depth 125 microns. We set a second plastic surface on the emulsion to confine this latter between two flat surfaces. The PDMS polymer is cured without evaporation in a warm water bowl (90 °C) for 4 h. The relative humidity in these conditions is 100%. Then, the solid material layer is removed from the two plastic surfaces and dried in an oven for 1 h at 150 °C. As PDMS is permeable to water vapor, droplets containing carbon black particles dry. At the end of the process we obtain a foam structure with spherical-shaped pores covered by carbon black particles.This process has already been described by our research group30. All the experiments are performed at room temperature.We measure the electrical conductivity and dielectric permittivity of the samples between two metallic disc electrodes at a frequency of 100 Hz under an applied voltage of 1 V using an impedance analyzer . A calibration procedure removing the contribution of the polarization of the electrodes is used to determine the dielectric permittivity and the conductivity of the sample as a function of frequency2 triangular piece of 125 m2 × 1.8 mm) is cut. A silver conductive ink is coated on the composite to get the top electrode. The coating is protected by a thin layer of PDMS. Finally, the two triangular parts (PET and composite) are maintained contiguous: the triangular base of the two parts are glued together while the two tips are stick by two magnets (0.35g each). Figure A 7.5 cmThe structure of the triboelectric harvester is very similar to the capacitive harvester. Surface and thickness are same. The main difference is the presence of a Kapton sheet (100 The recovery device as well as the vibrating pot are placed in a hermetic bell (Kartell) where a reservoir of water had been placed. This bell is equipped with a sensor of humidity and temperature  and allowed the passage of the various cables of the electric circuit. At the beginning of the experiment the bell is not closed and the air inside is identical to the surrounding air (50% of relative humidity (RH)). Then the bell is hermetically sealed, and the water in the tank begins to evaporate. While the water is evaporating, the humidity and harvesting performances are both recorded. The oscilloscope continuously acquires the voltage across the load resistance. A FFT (Fast Fourier transform) is performed directly by the oscilloscope on a 2-s signal. The maximum peak value of the FFT (mentclasspt{minimaSupplementary Information.Supplementary Video 1.Supplementary Video 2.


One of the most common tasks is the identification and representation of clusters. However, this is non-trivial in heterogeneous datasets since the data needs to be analyzed from different perspectives. Indeed, highly variable patterns may mask underlying trends in the dataset. Dendrograms are graphical representations resulting from agglomerative hierarchical clustering and provide a framework for viewing the clustering at different levels of detail. However, dendrograms become cluttered when the dataset gets large, and the single cut of the dendrogram to demarcate different clusters can be insufficient in heterogeneous datasets. In this work, we propose a visual analytics methodology called MCLEAN that offers a general approach for guiding the user through the exploration and detection of clusters. Powered by a graph-based transformation of the relational data, it supports a scalable environment for representation of heterogeneous datasets by changing the spatialization. We thereby combine multilevel representations of the clustered dataset with community finding algorithms. Our approach entails displaying the results of the heuristics to users, providing a setting from which to start the exploration and data analysis. To evaluate our proposed approach, we conduct a qualitative user study, where participants are asked to explore a heterogeneous dataset, comparing the results obtained by MCLEAN with the dendrogram. These qualitative results reveal that MCLEAN is an effective way of aiding users in the detection of clusters in heterogeneous datasets. The proposed methodology is implemented in an R package available at  Determining the number of clusters in a dataset is a frequent problem in data clustering, and is a distinct matter from the algorithm of actually solving the clustering problem. The correct choice of the number of groups is often ambiguous depending on the shape and scale of the points in a dataset and the desired clustering resolution by the user. The optimal choice of clusters depends on the intended use, but in general, it strikes a balance between the maximum compression using a single cluster and the highest resolution of the data by assigning each data point to its own cluster.Several clustering algorithms have been proposed for partitioning datasets . Most ofAs Hierarchical clustering is a widely used and effective algorithm to answer these questions, as it provides a framework for viewing the clustering at different levels of detail by imposing a hierarchy on it using a tree . During Clustering methods often are a fixed process: loading a dataset, setting parameters, running the algorithm, and plotting the results. In other words: clustering is used generally to analyze the data, not to explore it . The inthttps://bitbucket.org/vda-lab/mclean.We suggest a novel and generic clustering and exploration approach called MCLEAN (Multilevel Clustering Exploration As Network) for grouping and visualizing multiple granularities of the data that enables: (1) exploration of the dataset using a overview-plus-detail representation, (2) simplification of the dataset using aggregation based on the similarity of data elements, (3) detection of substructures by means of community detection algorithms, and (4) inclusion of the human in the process of selection the number of clusters. Our methodology follows a synergistic approach that combines the strengths of connectivity-based algorithms, community detections techniques and the ability of humans to visually detect patterns, to explore moderately large datasets. It is a visual exploratory and clustering method that permits the user to interact with the algorithm results. The method combines hierarchical clustering algorithms with interactive tools to find optimal clusters and visualize them in a simplified network representation. Network visualizations are an effective means to understand the patterns of interaction between entities, to discover entities with interesting roles, and to identify inherent groups or clusters of entities . The MCLThe remaining part of this paper is organized as follows. In the section ‘Background’ we give an overview of related work in multilevel clustering and graph visualization techniques as an exploratory tool. The section ‘Methods’ describes the proposed visualization technique for clustering exploration in detail, followed by the section ‘Evaluation’, in which we present an evaluation of our approach. Finally, the section ‘Conclusions and Future Work’ presents conclusions and possible directions for future work.The proposed framework allows the user to employ tacit knowledge in the clustering process in order to detect substructures. This process provides a multilevel environment through overview-plus-detail offering both a general outlook of the data grouping and the precise union of a subset of elements using graphs. To set our work in context, we present a set of examples of visual multilevel clustering and the network transformation of data to identify patterns.There are several methods to perform clustering analysis, but only a few of them support visual analysis. Even fewer provide interactive exploration capabilities of the clusters in different levels of detail. However, the importance of visual interaction for performing clustering analysis is increasingly recognized , as the Hierarchical clustering has been long used in many different fields including biology, social sciences, and computer vision due to the ease of interpreting the output by the user. The selection of the clusters is based on a single similarity threshold, where the tree is cut at a uniform height. Unfortunately, large and heterogeneous datasets usually require a more flexible approach allowing the user to explore different clustering scenarios. Some methods have been proposed to cut the tree at different levels. Partition-based clustering techniques such as k-means and CLARANS  attempt The dendrogram visual representation is not scalable to larger datasets. A technique presented by Given a matrix whose entries represent the similarity between data items, many methods can be used to find a graph representation. In fact, modeling data items as a graph is a common conceptualization used in hierarchical clustering algorithms. In a more general approach, Ploceus  offers aThe WhatsOnWeb system  takes adSystems presented for clustering and exploration in The network exploration of MCLEAN can be considered close to the solutions proposed for the navigation of the clustering results for large-scale graph visualization systems, such as The MCLEAN method takes a similarity matrix of all data records as input, and produces a simplified graph representation showing a higher abstraction of the clustering process. MCLEAN combines two visual representations. First, an overview plot (barcode-tree), related to a dendrogram and topological barcode plot, shows how the general cluster structure changes for different values of a parameter ε, indicating how close points need to be in the multi-dimensional space to be considered belonging to the same cluster. Second, a node-link plot represents the clustering results at a given ε. For this ε, clustering information in the node-link diagram is dual-layered. First, graph connected components correspond to data clusters at this threshold ε. Second, different colours within a connected component indicate that this subnetwork would be split when using a more stringent ε; in other words, it indicates substructures in this cluster.A connected component is a subgraph in which all the vertices are directly or indirectly connected. We use connected components to define the clusters in the dataset. In addition, MCLEAN employs community detection algorithm to find subclusters inside connected components. As a result, user knowledge (tacit or other) can inform on whether a cluster is distinct or is a part of a larger cluster. This ambiguity is common in heterogeneous data sets.ϵ) that defines the distance of union between two data elements. We find similarities between the MCLEAN approach and topological data analysis (TDA). Analyzing the multidimensional spaces from a topological structure perspective, interpreting the persistent homology by calculating the number of connected components (b0 from betti numbers) and using the persistence concept to define the optimal threshold of network representation prove that although the aims are distinct they share a same philosophy of analysis  projects the data elements in reduced dimension ordination space. Two or three dimensions are often used, which is based on ease of visualization rather than on the dimensionality of substructures in the data. Unfortunately, in some cases, these projections blur patterns due to the heterogeneity of the distances and the limitations of the space visualized. Therefore, a change to the spatialization  can help to overcome the limitations of complex datasets. An example of these weaknesses can be seen in the MDS applied to the Synthetic dataset in Although the distance matrix does not contain explicit network semantics, MCLEAN uses this approach to transform the encoding of distances by the use of links in the network. Moreover, the algorithm employed in the final drawing of the network  is optimized to avoid overlapping between the nodes.ϵ which defines the radius that designates points to be lying in each other’s neighbourhood. In DBSCAN, a second parameter numPts is used to define the minimal number of points that can constitute a cluster. In MCLEAN, however, all datapoints are considered network nodes, and datapoints that are within a distance ϵ from each other are linked. The result of this step produces a graph where there exists a path between two nodes if and only if they belong to the same connected component. At this stage of the methodology clusters are represented as connected components in topological space. ϵ applied to the same dataset. As ϵ increases, the number of links grows between the nodes.The graph transformation step of MCLEAN is similar to the DBSCAN method , in thatIn case of large datasets, the node-link representation can become visually overwhelming for the user without a proper level of aggregation. The challenge is to extract understandable information buried in the structure of multiple nodes and links. In addition, a layout of the entire graph is costly to compute. MCLEAN simplifies and highlights the structure of the raw network. This process of simplification is founded on the use of aggregating nodes (meta-nodes) that represent a subgraph at a higher level of abstraction.Node aggregation is based on degree centrality, where the degree of a node is defined as the number of connections that the node has within a network. This value is computed for all nodes, and the highest one is the first candidate to be the center of an aggregated node (meta-node). All nodes connected directly with the candidate are converted into an aggregated node. All connections with other data elements are inherited in the meta-node keeping the structure of the connected component. The result of node aggregation for the graphs created in Our simplification graph approach was designed to preserve the structure of the input graph. According to  1.m1 and m2 if and only if there exists an edge between two leaves in the input graph l1 and l2 such that l1 is a descendant of m1 and l2 is a descendant of m2.Edge Conservation: an edge exists between two meta-nodes  2.Connectivity Conservation: any subgraph contained inside a meta-node must be connected.By respecting these two properties, we ensure that the resulting graph preserves the topological features of the initial graph: edge conservation guarantees that any edge in the simplified graph is present in the initial graph, while connectivity conservation ensures that any path can continue through any meta-node .In MCLEAN, meta-nodes are created through the densest nodes (highest degree) in a connected component. The node with the highest degree is the best candidate to be the center of the meta-node. The simplified network representation  preserveϵ, as can be seen in the four subplots in As indicated above, the generated connected components depend on the value of parameter ϵ and clustering results, MCLEAN generates these graphs across a range of ϵ values. These are subsequently combined in the tree representation called barcode-tree, which is inspired by both a clustering dendrogram and barcode representation . The individual components are arranged along the vertical axis of the plot. At any given threshold, the number of connected components is the number of lines that intersect the vertical line through the threshold. Meta-nodes are formed in the join points that are aggregations of individual data elements or existing meta-nodes at a smaller threshold (see ‘Node aggregation’ section). This tree overcomes the limitations of binary structure of a dendrogram, allowing for a more clear representation of branches. Moreover, the barcode-tree implements a leaf ordering method motivated by the MOLO algorithm presented by The barcode-tree  is a visϵ1 ≤ ϵ2 ≤ ϵ3 ≤ ⋯ ≤ ϵN−1 ≤ ϵN then M1⊆M2⊆M3⊆⋯⊆MN−1⊆MN with Mi being the meta-nodes in network i. If the user is interested in understanding the structure of the input data, then topological hierarchies are useful tools to explain the origin of all edges viewed in a cut. Both the objective for the barcode-tree view in MCLEAN and the barcode in TDA is to find the persistent topological structures across a range of thresholds. Those structures which persist over an extensive range are considered signals of the underlying topology. As the threshold changes, the topological structures of network change accordingly.Meta-nodes for a small threshold are aggregated into new ones created by the larger threshold: if ϵ from 0 to 300. For ϵ = 100, we see 600 connected components because there are no connections amongst the individual elements in the dataset. For ϵ = 220, we see a big connected component and a significant subset of individual elements, reflecting the fact that some vertices have joined into a larger connected component. For ϵ = 290, we see a single connected component that indicates the joining of all data elements.In The resolution of the plot depends on the number of evaluations, showing a general overview with only a few thresholds  or allowAlthough the exploration of the connected components path around a threshold of interest can give an intuition of the resulting network, the analysis of the connected components using the network representation  is a necTo understand the implications of the proposed methodology and the interaction between the visuals, we performed a qualitative evaluation regarding learnability and usability of MCLEAN. We recruited six participants, including four doctoral and two post-doctoral researchers in the area of data science with knowledge of clustering techniques  and dimensionality reduction techniques . None had seen or used MCLEAN before the evaluation test. The goal of the evaluation was to identify qualitative insights about how well MCLEAN supports the identification of patterns according to the simplification of the dataset.We gave a brief introduction to MCLEAN, explaining the fundamentals of the methodology and demonstrating the main functionalities of the interface developed to interact with the network and barcode-tree including the bidirectional selection of elements between the two visuals see . A trainThe evaluation exercise was split into three parts: detection of patterns using the barcode-tree, selection of thresholds comparing the dendrogram and barcode-tree, and detection of patterns combining the network representation and barcode-tree. After the exercise, the questionnaire was provided to obtain user satisfaction and additional feedback.ϵ 0 to 300 by steps of 5 , and that these approaches used in conjunction are crucial to allowing a user-centric approach to information discovery, to exploit heterogeneous data sources better.Although the presented network and barcode-tree representations help the user in gaining insight in their data, there are some clear points for future work. For example, the current approach relies on single linkage clustering whereas average and/or complete linkage clustering might be more useful for particular datasets . In addition, the current visual encoding of the barcode-tree shows visual artifacts  depending on the granularity level used. Finally, it will be useful to investigate further methods for directly comparing how data elements are integrated across thresholds.In conclusion, incorporating the domain user in the clustering process itself allows for retaining the richness of multilevel patterns in cluster results. MCLEAN facilitates integrating tacit or other user knowledge in clustering result interpretation and exploration, while simplifying the representation of groups especially in the presence of noise or outliers. We argue that the MCLEAN approach provides new opportunities beyond existing techniques for cluster visualization and exploration.


During the last decades, the rate of caesarean section is increasing and this can increase the mortality and morbidity. Up to one third of the caesarean sections are attributed to the elective repeat caesarean section (ERCS). This study aims to evaluate attitudes and factors affecting the choice of pregnant women with one previous caesarean section regarding their mode of delivery in their second pregnancy. By assessing these attitudes, this study can help the efforts in developing strategies to increase the rates of vaginal delivery.A cross-sectional design was conducted by a structured questionnaire on 166 pregnant women who had delivered once by caesarean section for their first pregnancy and were in the third trimester of their second pregnancy. Any women with an absolute indication for caesarean section was excluded. The study comprises women who attend the clinic at our center in Northern of Jordan. Proper statistical tests were performed to assess the association between the choice of delivery and selected demographic and clinical factors.About 55.4% responded that they would choose ERCS (n = 92) and the remaining participants chose trial of labour after caesarean section (TOLAC) (n = 74). Fear of pain was the most common reason for choosing caesarean section, accounting for 55.4%. Interestingly, our study did not show a significant association between the mode of delivery and demographic factors, such as age, educational level and occupation. The single independent significant factor influencing patients’ choice that our study revealed was “being informed about the complications of TOLAC”. The choice of TOLAC was almost four times higher for those participants who had been informed about the complications, compared to those who had not been informed.Proper counselling is a main factor that affected the patients’ choice toward the mode of delivery. Proper pain management may encourage patients to choose TOLAC because fear of pain was a main reason that patients requested ERCS instead of TOLAC. •Pregnants with a previous caesarean section has the right to choose TOLAC or ERCS.•Pregnants should be aware about the risks and benefits for each mode of delivery.•Proper counselling pain management are the main factors affect their choice.•Knowledgeable pregnant women tend to choose TOLAC over ERCS. In Jord22.1This study was conducted at a tertiary care center. After obtaining approval from the Institutional Review Board approval, we invited and interviewed women who underwent one previous caesarean section to assess the factors and attitudes that would influence their decision regarding the mode of the delivery in their second pregnancy. The study was conducted between June and December 2018 in an observational analytical cross-sectional design. The confidentiality of the participants was protected by providing a code number for each participant during the data collection and analysis phases of the study. All women were interviewed in the outpatient clinics. Also, all women who had delivered only once (parity 1) by a caesarean section and were in the third trimester of their second pregnancy were included in the study. In addition, any women with an absolute indication for caesarean section was excluded from the study . All women underwent their previous caesarean section in our unit with good care and monitoring.Our center is located in Northern of Jordan and provides the care for a large population from different social backgrounds, and for both urban and rural populations. In addition, the center provides the care for the population of all financial levels because most of the patients have the health insurance.2.2The structured interviews were done by two senior residents and the data was collected using a structured questionnaire. The structured questionnaire includes demographic variables (age and monthly income), as well as clinical variables such as indications of the first caesarean section, whether the operation was an elective or emergency one, the type of anaesthesia used, and any complications during or after the caesarean section. In addition, the choice of delivery whether TOLAC or ERCS, the reasons for their choices, and the source of their information  were investigated. Moreover, they were assessed if they had been informed about the complications of a trial of labour after a caesarean section and about the complications of repeated caesarean sections. The utilized tool in this study was derived from different literature sources. Face validity was conducted by four experts in the field and the results clearly indicated that the measuring tool used in our study was valid. Content validity was also assessed by three other experts and the Content Validity Index was 85%. The questionnaire was also clear and understandable for all participants.2.32 test was used to investigate the differences in categorical variables between groups. Student's t-test was performed to examine the significance level of continuous normally distributed variables. Binary logistic regression was applied to study the contributory effect of different independent variables on the choice.Statistical analyses were performed using IBM SPSS Statistics Software (v. 21). Data were presented as frequency distributions for categorical variables and mean ± standard error of the mean for continuous variables and recorded in a spreadsheet; the significance level was 0.05. The Pearson χBefore that, a pilot study was conducted using fifteen patients who possessed the same inclusion criteria, and these fifteen were not included in the final sample size. The results showed there had been no problems during collection, coding and analysis.The sampling method includes the random selection of women who attend the obstetric clinics at our center which provides the care for a large population from different social backgrounds. The size was calculated according to the power of analysis formula with the following assumptions: power of 0.9, alpha of 0.05, beta of 0.1, the anticipated incidence for the ERCS group of 65% and the anticipated incidence for group who will choose TOLAC is 35%. According to that, the total required sample size is 112.33.1One-hundred and sixty-six women participated in this study. The mean age of the women was 28.2 years with a range of 19-to-41 years old. Around 52% of the participants were unemployed (n = 86), 84% held a university certificate (n = 140) while the remaining 16% had graduated from a secondary school or less. (n = 26).Furthermore, the results showed that almost 85% of the sample (n = 140) have no history of fertility treatment. The previous mode of delivery for all participants was caesarean section. Also, more than two thirds of the women had delivered through emergency caesarean section (n = 114), while the remaining by elective caesarean section. The general anaesthesia was conducted in more than two thirds of the women (n = 107). The mean birth weight for the new-born babies in the first delivery was 3.05 kgs. and ranged from 0.9 to 5 kgs.Moreover, 83% of the participants did not experience any antenatal complications during their second (current) pregnancy. Also, only 13% of the participants were admitted to the hospital during their second (current) pregnancy (n = 22).In addition, the findings showed that the treating physicians were the main source of information about the labour and delivery and accounted for 48% (n = 81), while 33% of the participants received their information from family and friends (n = 55) and only 18% received the information from the media. In addition, the results showed that almost half of the participants were informed about the complications of ERCS (n = 82). On the other hand, only 30% of the participants were informed about the complications of TOLAC (n = 49). The majority of the sample had plans for a future pregnancy (n = 143). Sample characteristics, namely, demographics and the clinical factors for participants, are presented in 3.2Participants were asked about their choice of delivery for their second pregnancy. More than half of them (55.4%) responded that they will choose ERCS (n = 92) and the remaining participants said they will opt for TOLAC (n = 74). The attitude of participants towards the choice of delivery was assessed according to the different reasons given by them. Regarding those participants who decided ERCS, the main reason was the fear from labour pain, accounting for 55.4% (N = 51 out 92). Also, 15.2% (n = 14 out 92) indicated that there was a specific medical reason for their choice . Another 15.2% (n = 14 out 92) claimed that ERCS was safer than TOLAC. About 9.8% (n = 9 out of 92) did not want to repeat their previous experience (of failed-to-progress in labour) and 4.3% believed that TOLAC involved more complications than ERCS The attitude of participants towards their choosing TOLAC was also assessed using the various reasons they had given for their choice. Some participants had chosen more than one option at the same time. The most important reason given by the participants for choosing TOLAC was its natural process; this accounted for 62.1% (n = 46 out of 74). The second most important reason was that the participants believed that they might be able to have more children in the future if they achieved vaginal delivery this time; this accounted 45.9% of the participants (n = 34 out of 74). Also, 25.7% of the participants believed that TOLAC resulted in a shorter hospital stay (n = 19 out of 74), and 21.6% of the sample claimed that TOLAC was safer than ERCS (n = 16 out of 74). The economic reason was the least important reason and only 2.7% of the sample said that TOLAC costing less than ERCS (n = 2 out of 74). The choices for delivery and the attitudes toward the choice of delivery are presented in 3.3There was no significant association between the choice of delivery and the following variables: age, employment status, educational level, history of fertility treatment, level of urgency of the previous caesarean section (elective versus emergency), antenatal complications during the second (current) pregnancy, the source of information, being informed about possible complications with ERCS and birthweight of the previous pregnancy.P = 0.03). The causes of these admissions did not include any absolute maternal or fetal indication for CS.On the other hand, the results revealed that there was a significant association between the choice of delivery and "the previous admission during the second (current pregnancy)". Women with previous admissions were more likely to choose ERCS  than to choose TOLAC  (P = 0.001). The informed complication was a 0.5–1% rate of uterine rupture.In addition, the results revealed that there was a significant association between the choice of delivery and "being informed about the complications of TOLAC", women who were informed about the complications of TOLAC were more likely to choose TOLAC  than to choose ERCS  (P = 0.02). Moreover, a significant association between the choice of delivery and planning for a future pregnancy was found. Women planning for future pregnancy were more likely to choose TOLAC  than to choose ERCS  (P < 0.001). After controlling for all factors, "informing patients about the complications of TOLAC" was the single independent variable that would affect the choice of delivery (P = 0.001). The odds ratio for informing patients about the complications of TOLAC was 3.7, indicating that the choice of TOLAC was almost four times higher for participants who had been informed about the complications compared to those who had not been informed.To examine the predictors that may contribute to the choice of delivery, a binary logistic regression was applied . The fol4Pregnant women with one previous caesarean section should be properly counselled about the mode of delivery during their antenatal visits, and this counselling has to be documented clearly in their medical records. They should be informed that the rate of success for vaginal birth after a caesarean section is about 70%, according to RCOG and ACOG.Shared decision-making regarding the mode of delivery following a previous caesarean section is a relatively new concept in Jordan; it is still unclear whether our patients chose the mode of delivery based on a clear understanding of the benefits and risks of TOLAC and ERCS. Our study found that nearly half of our participants received their information about labour and delivery from the physicians treating them. In other words, about half of the participants gained their information from unprofessional sources, which probably provided them with misleading facts. This means that our current practice is deficient and is against the recommendations, which clearly state that every single patient should be counselled thoroughly by her physician about the benefits and risks of TOLAC and ERCS. Our study also showed that not all the participants who received their information from their physicians were sufficiently aware of the risks and benefits to be in a position to actively share in the decision-making process, as only 30% of the participants had been informed about the complications of TOLAC. We believe that this is a serious situation and can even lead to dangerous outcomes, if left unnoticed and not corrected. Therefore, adopting the idea of introducing a clear written consent form for patients with a previous caesarean section could be an effective way of ensuring that every patient has received the required information, clearly and comprehensibly delivered, before making a safe decision. This is reflected by the homogeneity of our sample, for example 85% held a university certificate, and 85% didn't receive fertility treatment which in turn reflects the level of education in the community regardless the social and finical status. This homogeneity reflects that these factors; as the level of education; did not influence or explain the increasing rate of ERCS over TOLAC. On the other hand, this emphasises the importance and necessity of patients' education and counselling about TOLAC and ERCS to raise the rate of TOLAC regardless if the pregnant woman has a university certificate or not.Our study showed that 55% of the participants showed a positive attitude towards ERCS, which was comparable to the results from a similar study conducted in Kenya on patients with a previous caesarean section, which was 67% . More thOn the other hand, about 45% of the participants showed a positive attitude towards TOLAC, which is close to what was found in the Kenyan study as nearly a third of them opted for TOLAC . The mosIt is also important to highlight that there is another crucial factor that can affect the success rate of TOLAC which is the lack of clinical guidelines and the relative variance in TOLAC indication criteria. This is obvious by the high success rate of one study that reached 91% after considering the appropriate criteria .The main strengthen point is that this study is the first conducted study to assess maternal attitudes toward the mode of delivery. In addition, the utilization of systematic conductance of well-prepared questionnaire is another strengthen point. Also, it sheds the light on important issue regarding the proper counselling of the patients. On the other hand, the relatively small sample size is a limitation. Moreover, conditions that may occur just before the delivery and can change the attitude of the women were not assessed.5There is no doubt that each patient with a previous caesarean section has the right to choose TOLAC or ERCS. However, at the same time, she has the right to be fully informed and made aware of the risks and benefits for each mode of delivery by a well-trained obstetrician. This is because that the proper counselling and education, including checklists and informed consent are main factors that affected the patients’ choice toward the mode of delivery. Also, proper pain management is one intervention that might encourage patients to opt TOLAC because fear of pain was the main reason that patients requested ERCS instead of TOLAC.This study was conducted according to the Strengthening the reporting of cohort studies in surgery (STROCSS) 2019 Guideline .The datasets generated and analyzed during the current study are available from the corresponding author.Not commissioned, externally peer reviewed.Written informed consent was obtained from the patients for publication. Institutional approval was obtained from the Institutional Review Board at Jordan University of Science and Technology (19/117/2018). This study was conducted in accordance with the Declaration of Helsinki.Institutional approval was obtained from the Institutional Review Board at Jordan University of Science and Technology 19/117/2018.This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.All authors contributed significantly and in agreement with the content of the article. All authors were involved in project design, data collection, analysis, statistical analysis, data interpretation and writing the manuscript. All authors presented substantial contributions to the article and participated of correction and final approval of the version to be submitted.researchregistry5369https://www.researchregistry.com/browse-the-registry#home/?view_2_search=researchregistry5369&view_2_page=1.Dr. Amer Sindiani.The Authors declare that there is no conflict of interest.


Wettability control of carbonates is a central concept for enhanced petroleum recovery, but a mechanistic understanding of the associated molecular-scale chemical processes remains unclear. We directly probe the interface of calcium carbonate  with natural petroleum oil, synthetic petroleum analogues, and aqueous brines to understand the molecular scale behavior at this interface. The calcite–petroleum interface structure is similar whether or not calcite was previously exposed to an aqueous brine, and is characterized by an adsorbed interfacial layer, significant structural changes within the calcite surface, and increased surface roughness. No evidence for an often-assumed thin-brine wetting layer at the calcite–petroleum interface is observed. These features differ from those observed at the calcite–brine interface, and for parallel measurements using model synthetic petroleum mixtures . Changes to the interface after petroleum displacement by aqueous brines are also discussed. It is normally assumed that pore fluid wetting is controlled by the presence of a thin brine film, located between the rock surface and the crude oil, whose behavior depends on the interactions at the brine–oil and rock–brine interfaces4. The extent of the proposed aqueous film thickness ranges from a few molecular layers to thicknesses of ~ 10 nm6. Increasing attention has been given to the chemical interactions at the rock–fluid interface and its control of wettability alteration in carbonate rocks during low-salinity water-flooding for enhanced oil recovery9. For example, measurements of surface forces and adhesion at mica and glass surfaces reveal that thin brine films are stabilized at salt concentrations below, and pH values above, critical values12. These ideas have been explained on the basis of the well-kjnown Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory10.Wettability is a central concept for understanding oil displacement from geological reservoirsdouble layer expansion (DLE) model14 posits the presence of an aqueous wetting layer between the carbonate and petroleum phases, whose thickness is controlled by changes in the diffuse layer screening in low ionic strength brines. This concept implicitly assumes that the carbonate surface has a pH-dependent surface charge , which is screened by counter ions, either adsorbed on the surface or distributed in a diffuse ion layer (with a screening length that is controlled by the ionic strength of a solution)21. This picture is well-established for oxide–water interfaces, such as the rutile (TiO2) (110) surface24. In contrast, molecular scale measurements of the well-defined calcite (CaCO3) (104) cleavage surface have found that its speciation does not follow this prediction25, and separate measurements of ion adsorption suggest that the net charge of a calcite surface is quite small26. This suggests that one of the conceptual foundations for the DLE model is not supported by direct observations of the intrinsic chemical reactivity of well-defined model systems.The mechanisms to explain this behavior generally fall within two distinct concepts: the carbonate surface reactivity model posits structural and compositional changes at the carbonate–petroleum interface within the internal pores of the rock due to a combination of carbonate dissolution and surface adsorption30. For example, it has been suggested that ionic aggregates in thin brine films act as anchors to hold oil components in the vicinity of the substrate, and that these anchors are disrupted in low salinity water in favor of the water-wet state31. These two mechanisms are clearly distinct, but have in common the implicit assumption of a reactive carbonate surface that provides sites for adsorption from the brine or the petroleum phase.In contrast, the In spite of the wealth of information concerning the enhancement of oil displacement by saline waters, there are very few studies that have explicitly characterized, through in situ observations, the interfaces between rock surfaces with petroleum oil and brines at a level that provides a direct test of these ideas. Consequently, the appropriate conceptual model of the associated interfacial interactions has not been tested directly. The goal of this study, therefore, is to provide direct observations of the structure and reactivity of model carbonate interfaces in contact with aqueous brines, petroleum oil, and synthetic oil mixtures to provide new insights into the intrinsic molecular-scale structures and chemical interactions at these interfaces.32, synthetic aqueous brines of various ionic strengths and compositions, and synthetic oil mixtures in order to test different conceptual models of carbonate–petroleum interactions.  single crystals formed during the diagenesis. We probe the structure of freshly cleaved calcite (104) single-crystal surfaces in contact with a suite of fluids, including natural petroleum oil0 μm sing37. The specular reflectivity signal , the fraction of the incident X-ray beam that is reflected by the surface) is measured as a function of incident angle, θ, with respect to the surface plane. The scattering condition for each data point is characterized by the “momentum transfer”, Q = (4π/λ)sin(θ), where λ is the wavelength of X-rays, as shown schematically in Fig. We probe calcite–fluid interfaces using the technique of X-ray Reflectivity (XR)37 in which a calcite sample is held in contact with a thin fluid layer . Next we show the evolution of the interfacial structure for samples that are equilibrated with aqueous brines before exposure to petroleum, and then subsequently exposed to a brine to simulate the displacement of fluids in a production environment. The comparison of these results for calcite samples with, and without, previous exposure to a brine allows us to constrain the role of aqueous wetting layers separating the calcite surface from the petroleum. Finally, we present results from a calcite sample reacted with synthetic oil mixtures to explore the possible role of the different petroleum components in controlling the observed behavior. and the presence of an interfacial hydration layer40 . In order to make comparisons of these results to the calcite–petroleum interface, we report characteristic values of the interfacial structure: the average vertical atomic shift magnitudes of Ca and CO3 within the top three calcite layers , the average carbonate tilt angle magnitude in the first three layers with respect to the bulk value (<|∆θCO3|> = 4.7°), and the number of water molecules in the primary surface hydration layer per unit cell area of calcite (104) (ΣOi = 2.3). These data are also sensitive to the calcite surface roughness, e.g., due to the presence of topographic steps, which is quantified in terms of a root mean square (rms) variation in its height (σint)41. The calcite–HSW interface is found to have only minimal interfacial roughness, σint = 1.5 Å . This value is similar in magnitude to what was observed previously in CSS38.The calcite–brine interface in high salinity water ace Fig.  are visi−1 and 2.8 Å−1. These differences are understood using the same structure model used for the calcite–brine interface to quantitatively reproduce the XR signals. This best-fit model reveals three significant changes in the molecular-scale interfacial structure with respect to that seen in HSW 42. We find a combined occupation factor of 10.9 WEq per unit cell area for the first two adsorbed fluid layers, a ~ 5-fold increase with respect to that observed in brine. The observed electron density is due to the combination of the atomic number of each species, Z, and its number density. This suggests that this interfacial layer does not consist primarily of water, because the observed electron density is significantly higher than that for any known phase of water or ice near ambient temperatures and pressures. The data also reveal a large interfacial roughness, σint = 7.3 Å, a ~ 4-fold increase with respect to that observed in HSW, suggesting some dissolution or growth of the calcite substrate when placed in contact with petroleum. In summary, these results reveal that the calcite interfacial structure experiences significant differences in the brine vs. petroleum environments and that XR measurements are highly sensitive to these differences.The XR data for a freshly cleaved calcite surface in contact with natural petroleum oil Fig.  provide 38 in spite of their very large differences in ionic strength . In contrast, we find that the occupation factor for two adsorbed layers (5.2 WEq) and the surface roughness (σint = 5.9 Å) are very similar to those observed in petroleum. That is, the calcite surface in LSW brine after displacement of petroleum has mixed characteristics: structural changes within the calcite are similar to those seen in water while the electron density of the adsorbed layers and the calcite surface roughness are similar to those seen in petroleum. A similar mixture of characteristics also was observed after displacing the petroleum with HSW  and CSS (from previous work)40, suggesting that adsorption of ions at this surface (if any) is weak and that the net charge of the calcite surface is small.From these results, we can therefore reach the following tentative conclusions.adsorption of a petroleum component to the calcite surface or by reaction of the calcite surface with the petroleum component.One of the challenges raised by the above results is to interpret the structural properties of the calcite–petroleum interface. While the XR measurements provide strong constraints on the electron density and thickness of the adsorbed interfacial layer, they do not define its chemical composition. For example, we can conclude that this interfacial layer does not consist primarily of aliphatic hydrocarbon molecules, since the observed electron density is ~ 2- to 4-fold higher than that of pure alkanes. The data, above, suggest that these adsorbed layers derive from petroleum, either by 43  may be responsible for the high electron density layers on the calcite surface.Given the high compositional and chemical complexity of petroleum oil, additional constraints into the source of these interfacial behaviors can be obtained by comparing these results with that of calcite in synthetic mixtures of pure compounds that contain the major components of natural petroleum. Guiding this work was an initial working hypothesis that the high density layer might be due to the adsorption of organic functional groups that have an intrinsically higher electron density than aliphatic carbon. We chose two synthetic oils to test this idea. The first was n-dodecane as a representative of the aliphatic component of petroleum. We also used a synthetic oil mixture that included dodecane, toluene and asphaltene, as representative of the aromatic and high molecular-weight compounds in petroleum. This mixture is used to see if the presence of structurally denser aromatic rings in the asphaltenezCa|> = 0.08 Å, <|∆zCO3|> = 0.13 Å, and <|∆θCO3|> = 6.1°), and the adsorbed interfacial layer is consistent with a dense “lying down” two-dimensional layer of dodecane , small surface structural changes  from the synthetic oil mixture to the calcite interface is observed, but it appears to be fully reversible under these conditions and inconsistent to that observed in petroleum.The same surface was then exposed to a synthetic oil mixture that incorporates toluene and asphaltene components in natural petroleum  Fig. a. The XRum Figs.  and 3. Tum Figs. b is gene−1 and 2.9 Å−1. The data and optimized model , the surface atomic shift magnitudes are large , along with a > 2-fold increase in the density of the adsorbed layer. Finally, the same sample was then exposed to the synthetic oil mixture, and the structural characteristics associated with calcite–petroleum interfaces were mostly retained. This result indicates that natural petroleum is able to effectively displace the synthetic oil mixture and that the specific chemical interactions and structures that are responsible for the calcite-petroleum interfacial structure are effectively irreversible and due specifically to the interaction of calcite with the petroleum.When the same sample was then exposed to natural petroleum, the XR data showed significant changes including the notable decrease in XR signal near Q = 1 Ådel Fig. b have chdel Fig. , Table 3These results reveal a number of insights concerning the interfacial structure of petroleum oil and brines interacting with the calcite (104) surface.The calcite–petroleum interface exhibits multiple distinct characteristics, including: (1) the formation of a high-density adsorbed layer; (2) the incorporation of significant structural distortions within the top few layers; and (3) an increase in the surface roughness (implying dissolution or growth). These behaviors were observed whenever the calcite surface was in contact with petroleum oil, independent of whether it was a freshly cleaved surface or had previously been exposed to any of the aqueous brines described here , or even the synthetic oil mixture. Also the adsorbed layer observed in petroleum oil was not dissolved into the synthetic oil mixture once it formed. The large magnitude of the calcium carbonate interfacial distortions and the effectively irreversible adsorption of the adsorbed layer can be thought of as a “fingerprint” of the intrinsically strong calcite–petroleum interactions. These observations suggest that the properties of this interface are controlled by a component of the petroleum that is strongly attached to the calcite surface . We saw no evidence for a thin water wetting film separating the petroleum from the calcite surface  implying that the pristine calcite surface is intrinsically “oil-wet”.smaller than water. Our measurements with the synthetic oil mixture was designed to test whether this could be explained by the adsorption of components of petroleum that contain aromatic groups . The inclusion of this species in the synthetic oil mixture led to behavior that was opposite to that seen in petroleum oil  and its interaction with calcite was reversible . These results, therefore, suggest that the observed adsorbed layer at the calcite surface seen in petroleum is not due to the components included in the synthetic oil mixture. However, the behavior could be sensitive to the specific composition of the synthetic oil mixture, especially with respect to saturated, aromatic, resin and asphaltene (SARA) fractions, and further work will be needed for a definitive conclusion.A primary characteristic of the observed interfacial layer is that its electron density is too large to be attributed to that of a residual thin film of water. The adsorbed layer is present for samples without any prior exposure to brine. This immediately implies that this layer derives primarily from the interaction of calcite with petroleum. The composition or identity of the adsorbed surface film is not uniquely determined by the XR results. However, the data analyses provide some constraints. The high electron density of the adsorbed layers (~ 3–5 times higher than that of water) cannot be explained by adsorption of simple aliphatic molecules  from petroleum oil, as they have electron densities that are typically similar to or Another possibility to explain the high electron density of the interfacial layer is that it may be the result of a reaction of calcite with a petroleum component resulting in an interfacial complex. In this scheme, the incorporation of elements  having a higher atomic number (Z) would increase the electron density of this interfacial layer. The systematic increase in the observed roughness of the calcite surface after contact with petroleum demonstrates that the morphology of the surface is altered when the calcite is exposed to the petroleum. This increase in roughness is due to changes in calcite surface topography in the form of a laterally variable surface height  although the XR do not distinguish between dissolution and growth. A possible mechanism for the surface roughening is the dissolution of ions from the calcite surface through strong chelation with one or more components in the petroleum. This is fully consistent with strong calcite–petroleum interactions between the adsorbed layer and the calcite surface. It is also consistent with the high electron density of the interfacial layer, as the inclusion of ions in this layer from the dissolution of calcite would increase its electron density with respect to the molecular components of petroleum.3 ions each lose one Ca–O bond in its coordination shell. The structural distortions within the calcite surface can be used as a measure of the structural and chemical perturbations at the interface in response to this change in the surface coordination environment. In the case of the calcite–water interface, the adsorption of water molecules to the surface Ca and CO3 ions completes the coordination shell of these groups and the interfacial structural displacements are generally small38. In the absence of water, computational studies show significantly larger structural distortions44. The structural changes seen for calcite in contact with petroleum oil are even larger than those predicted for a bare calcite surface, suggesting the presence of strong interactions between the calcite surface and a surface-active component that we infer derives from petroleum.The differences in atomic structural distortions within the calcite surface can be interpreted from a crystal chemistry perspective. Creation of the calcite surface requires that the top layer Ca and CO30, through a combination of surface adsorption of a petroleum component and its complexation with ions from the calcite surface due to carbonate dissolution.Together, these results, including the structural distortions of the calcite surface, the presence of an adsorbed layer, and surface roughening, suggest that the adsorbed layer modifies the intrinsic calcite surface. It therefore appears likely that this adsorbed layer and its interaction with the petroleum oil will also strongly influence the petroleum wettability of the surface. This interpretation is generally consistent with the conceptual picture of carbonate surface reactivity as the primary control over carbonate wettability in petroleum38 in terms of the characteristics of the surface hydration layer, minimal surface roughness, and small structural distortions of calcite. These similarities suggest that there is little or no adsorption of the primary solute ions in these brines  are essentially similar to that observed previously in calcite-saturated solutiones Table  to the c20. This conclusion derives from two observations. First, the calcite–petroleum interface is essentially similar whether or not the calcite surface was previously exposed to a brine before exposure to petroleum. Given the high sensitivity of XR to interfacial structure, this implies that little or no water is present at the calcite–petroleum interface. The second line of evidence derives from what was not observed. Thin film structures are well-studied using XR and lead to a periodic oscillation in the reflectivity signal as a function of Q47 due to the interference between X-rays that are reflected by the top and bottom of the film. For the case of a film of thickness, L, having a uniform density, ρ, the XR signal will exhibit an oscillation of period, ∆Q = 2π/L, where the magnitude of the intensity oscillation is controlled by factors such as the film density (relative to that of the substrate and fluid) and the interfacial roughnesses. Such wetting layers are inferred to have thicknesses in the range of ~ 10–100 Å4 which would correspond to intensity oscillations with a period of ∆Q ~ 0.6 to 0.06 Å−1 in the XR data. This is not observed in any of the XR data .In other words, the XR data show no evidence for a distinct water wetting layer at the calcite–petroleum interface after the brine is displaced by petroleum . These results are in conflict with two underlying concepts of double-layer expansion (DLE) as the primary mechanism for mediating carbonate–petroleum interactions  leads to a calcite–fluid interface whose characteristics are intermediate between those observed in water and natural petroleum. While the presence of high electron density layers and high surface roughness is typical of that seen at the calcite–petroleum interface, this interface has smaller structural distortions that are more similar to that seen at the calcite–brine interface. This immediately implies that the brine does not effectively displace the interfacial components seen in the petroleum phase.The large calcite surface structural distortions and high surface roughness that are observed in petroleum are reduced after the petroleum is displaced by the brine, but are still larger than those observed in the brine without prior exposure to petroleum. At the same time, the observed electron density of the adsorbed layer does not change significantly after the displacement of oil by either LSW or HSW brines. From the context of interfacial crystal chemistry discussed above, the reduction in the interfacial structural distortions that were observed after the petroleum was displaced by LSW suggests that water may be able to penetrate into this layer and thereby reduce the chemical stresses imposed by the adsorbed layer through its interaction with calcite. At the same time, the adsorbed interfacial layer was not displaced by the brine, as the enhanced electron density of this layer was similar to that found for measurements of petroleum.Synthetic mixtures of the primary components of petroleum oil  were used as a proxy for natural petroleum to guide and constrain our interpretation of the observed behaviors. Measurements of the calcite surface in the mixture did not reproduce any of the primary characteristics observed for calcite in natural petroleum. The small but reversible changes observed for calcite in dodecane vs. dodecane/toluene/asphaltene mixtures imply that toluene and/or asphaltene may be surface active species, but they interact weakly with the calcite surface and are readily displaced by dodecane. These results suggest that the observed interfacial behavior of calcite in petroleum is not associated with the primary petroleum components included in the synthetic oil mixture.intrinsic carbonate-petroleum reactions are best described by a conceptual picture that is based on carbonate surface reactivity30 rather than double-layer expansion (DLE)20. That is, our results directly reveal the presence of an interfacial layer that is seen only after interaction of calcite with petroleum, which is formed whether or not the calcite was previously in contact with an aqueous brine, and which is not displaced either by brines or the synthetic oil mixtures. These results suggest that this interfacial layer may play an important role in the wettability of the carbonate mineral surface by petroleum, and presumably the displacement of oil from the surface.The present results suggest that the More work needs to be done to connect the observations that are obtained using model systems to the phenomenon of enhanced oil extraction. For example, the present results do not incorporate the high structural and morphological complexity of the natural carbonate rock. Differences in behavior could derive from a number of factors including geometrical factors (confinement and curvature of rock pores), mineralogy , or interfacial crystal chemistry . Consequently, additional measurements will be needed to test the applicability of the insights from these model systems to the production environment. Additional work will also need to identify the composition and reactivity of the observed adsorbed interface layers, and how these characteristics depend on the brine composition. Finally, additional studies will need to discern the role of injection brines in controlling rock wettability and oil displacement in natural carbonate rock samples to understand whether those behaviors are comparable to that seen here on single-crystal calcite surfaces.The present results provide new molecular-scale insights into the intrinsic interactions and structures that occur at the interface between calcium carbonate  and the fluids associated with oil production, including natural petroleum oil, aqueous brines , and synthetic oil mixtures. These results show that the intrinsic interactions between calcite and petroleum are better described as an “oil-wet” instead of “water-wet” interface. The properties of this interface appear to be controlled by the presence of an interfacial layer . This interfacial species appears to be strongly bound since it is not displaced by either the aqueous brines or the synthetic oil mixture. However, its composition is, as of yet, undefined. Additional features of this interface, especially when compared to the calcite–brine interfaces, include significant structural distortions within the top few calcium carbonate layers, and a significant surface roughness. That is, the results reveal that the molecular-scale structure and interactions of carbonate surfaces with natural petroleum and brines are distinct and that these differences are likely to be key to establishing the fundamental chemical controls over its wettability.40. Briefly, rods of calcite were cut with diamond saw from a large (~ 1–2 in.) sized calcite crystal, with the rods having a cross section of ~ 5 mm × ~ 10 mm, with the axis of the rod oriented orthogonal to the calcite cleavage plane. The calcite surfaces were then created by cleaving the calcite crystal with a razor blade, by tapping with a hammer, exposing a clean calcite (104) surface.The single crystal calcite surfaces were prepared as described previously32. The compositions of these solutions are shown in Table 2SO4, NaHCO3, NaCl, CaCl2, CaCl2·2H2O, MgCl2·6H2O). The natural petroleum oil was obtained from a carbonate reservoir, and its composition was previously characterized32. The synthetic oil mixture (with a composition indicated in Table 48.The aqueous brine solutions are similar to those used previously49) to penetrate fluid layers when the fluid thickness exceeds even a few nm’s. The availability of experimental probes such as synchrotron X-ray scattering has enabled a revolution in our understanding of solid–water interfaces50. Such understanding obtained by the use of single crystal surfaces, provides an avenue to better comprehend the intrinsic fluid–solid interactions at the molecular-scale. In particular, substantial work has been done to understand the structure and reactivity of the calcite(104)–water interface which is flat, free from contaminants, and has a unique termination25.The ability to probe solid–liquid interfaces through direct in-situ observations can extremely challenging due to the inability of most interface-sensitive probes  to understand the intrinsic structure of carbonate interfaces in contact with natural and synthetic petroleum oils and brines. The penetration of X-rays through petroleum is similar to that of water and so the XR capabilities previously developed to understand mineral–water interfaces can be applied directly to the structure and composition of carbonate–petroleum interfaces.Probing the interaction of carbonates with petroleum presents additional challenges, including the high compositional complexity of petroleum oil37 probes interfacial structure through the variation of the specular  reflected intensity, IR, as a function of the angle of incidence . It is convenient to recast this information in terms of the interfacial reflectivity, R(Q) = IR(Q)/I0, as a function of the “momentum transfer”, Q = (4π/λ)sin(θ), where I0 is the incident beam intensity. In particular, the measured XR signal, R(Q), is related directly to the laterally averaged electron density profile at the solid–liquid interface, ρ(z), as a function of the height, z, above and below the interface through the relation:e = 2.818 × 10–5 Å, and AUC = 20.2 Å2 is the surface unit cell area of the calcite(104) surface. That is, the reflectivity signal is related to the Fourier transform of the electron density across the interface. This simple and direct relationship between the unknown structure and the measured signal allows models of the interface to be quantitatively tested and optimized to reveal the interfacial structure. Specifically, the interfacial structure is obtained through least-squares fitting of the data using molecular-scale models that include the mineral surface structure, the presence of any adsorbed species from the fluid, and the average surface roughness. The models ultimately specify the height, zi, occupation, Oi, and root-mean square distribution, σi, for each atom, i, at the interface. In many cases, we make use of well-established chemical constraints, such as the description of the carbonate group as a rigid object defined by its height and its rotation. This is described by a tilt angle, θ,  and a twist angle, ϕ, . Within this picture, the reflectivity signal can be written as:Briefly, the XR techniquesub(Q), the interfacial region, Fint(Q), and the fluid above the surface, Ffluid(Q). In this representation, the contributions from the substrate and fluid are known a priori . The only unknown in the reflectivity signal derives from the structure at the interface, including the top few calcite surface layers and the near-surface adsorbed layers. Previous work has shown that the interfacial structure differs from the bulk structure by depth-dependent atomic layer shifts that are measurable for the top four calcite layers. Similarly, it has been found that the calcite–CSS interface is characterized by two layers of adsorbed water molecules  followed by a bulk-like fluid density. The structure of the interfacial fluid layer depends on the chemical interactions at the solid–fluid interface, and can extend as far as ~ 1 nm from the solid surface.These three terms represent the structure factors, F(Q), of the crystalline substrate, FIn the case of the petroleum–carbonate interface, it is important to recognize that some caution is needed in interpreting the optimized structural models. The model implicitly assumes that the interfacial structure is laterally uniform  and that it can be described by a unique substrate structural distortion, a few adsorbed species at the calcite surface, along with a bulk-like fluid above the surface. The composition of the petroleum oil, unlike simple brine solutions, is highly complex, with hundreds of individual molecular components, each of which may behave differently. Furthermore, the inferred interfacial structural distortion is described by large structural changes that are likely to be controlled by the specific interaction with adsorbed species that may include large molecules and oligomers. Therefore, the actual interfacial structure is likely to be laterally heterogeneous at the molecular-scale. As such, the derived structural models should not be taken literally , but instead are best interpreted as representing the effective  interfacial structure. Consequently, our interpretation emphasizes the large-scale trends in the structural models  to reveal the key characteristics  of these calcite–fluid interactions.


R = −1, R = −0.7, R = −0.5 and R = −0.3. The experimental results have been used in the process of a fatigue life estimation performed for a random generated narrowband stress signal with a zero and a non-zero global mean stress value. The calculations have been performed within the time domain with the use of the rainflow cycle counting method and the Palmgren−Miner damage hypothesis. The mean stress compensation has been performed with the S-N curve mean stress model proposed by Niesłony and Böhm. The model has been modified in terms of torsional loading conditions. In order to obtain an appropriate R = 0 ratio S-N curve fatigue strength amplitude, the Smith−Watson−Topper model was used and compared with literature fatigue strength amplitudes. The presented solution extends the use of the correction model in terms of the torsional loading condition in order to obtain new S-N curves for other R values on the basis of the R = −1 results. The work includes the computational results for new fatigue curves with and without the mean stress effect correction. The results of the computations show that the mean stress effect plays a major role in the fatigue life assessment of the tested aluminum alloys and that the method can be used to assess the fatigue life under random conditions.The paper presents the experimental fatigue test results for cyclic constant amplitude loading conditions for the case of the torsion of the PA4 (AW-6082-T6), PA6 (AW-2017A-T4) and PA7 (AW-2024-T3) aluminum alloy for a drilled diabolo type test specimen. The tests have been performed for the stress asymmetry ratios  The central motivation of this paper is the fatigue life assessment of the PA4, PA6 and PA7 aluminum alloys under random loading conditions under torsion stress with a global non-zero mean stress value. Aluminum alloys have many applications these days due to their weight, and they can be used in the automotive, maritime, as well as in the aviation industry. These applications are very often connected with the bearing of torsional forces, as the materials are being subjected to variable loading conditions. These conditions cause the premature failure of the material, which can be explained as the effect of material fatigue. This can be visualized as we analyze the material for the initiation of micro-cracks, which propagate into larger cracks which later on cause the failure of the material. As correctly stated by Wang et al. : “failurIt is well known that aluminum alloys do not have a visible fatigue limit, as is usually the case for steel materials. While analyzing these alloys, we can take an arbitrary value of two million cycles as the fatigue limit as presented, e.g., by Boller and Seegers  in theirR, which can be defined via the use of the maximum maxτ and minimum minτ value of the stress during the cyclic tests:The planned torsional stress state which should be obtained during the experimental tests was connected with the expected stress asymmetry ratio mτ and stress amplitude aτ: The ratio can also be connected with the value of the mean stress As for the mean stress and stress amplitude, they can be defined as:tM is the torsional moment, and oW is the section modulus for the torsion calculated with the use of the outer diameter D and inner diameter d:That means that in order to obtain a certain ratio, we need to often add an extra static stress value to the material. The torsional stress value is obtained during experimental tests under a controlled torsional moment, and therefore it can be calculated with the use of Equation (5):R = −1 (afτ), as well as any other fatigue strength amplitude. The model works on the basis of the interpolation of the values between two S-N curves. In the case where we compensate for the R values between −1 and 0, we will obtain an interpolated value for every compensated amplitude. In the case of this study, it will be for R = −1 (afτ) and R = 0 (afRτ = 0). It can be presented in the iterated form of Equation (7):In order to take the mean stress effect into account, we need to transform the stress amplitude. The literature presents a number of transformation models which are widely used. These models use mechanical properties of materials such as the yield strength or ultimate strength, as in, for instance, the Goodman , Gerber R = 0, the model is simplified into the form:The values in the brackets are constant and do not change with every loading cycle. Generally, we can obtain the fatigue strength values from different sources, e.g., Boller and Seeger  materialR = 0, we have taken two approaches, the first being with the Smith−Watson−Topper model (SWT). This model is mainly suggested for axial loading cases and is rarely discussed or used for the pure torsional state, but we can find some recent papers, such as Gates and Fatemi [In order to obtain the missing value of the fatigue strength amplitude for d Fatemi  or Wu etd Fatemi , which dd Fatemi . Thus, id Fatemi :(9)τaT=τR = 0, aτ = mτ = afRτ = 0, the model takes the form of Equation (10):Due to the fact that in the case of bM) or torsional moment (tM). The maximum allowable value of the bending moment is 80 Nm, and the value of the static moment is 60 Nm. The maximum frequency that can be obtained during the bending or torsional test for the stand is 29 Hz. Taking the limitations of the stand into account, a simple static finite element method (FEM) calculation has been performed in order to assess what maximum stress values can be obtained during the experimental tests for the dimensions and mechanical properties of the specimen. The shear stress distribution along the specimen is presented in For the torsional fatigue tests, we have used a modified diabolo specimen. The shape and dimension of the new type of experimental diabolo specimen is presented in R = −1 and for individual results for R = −0.7, R = −0.5, R = −0.4 and R = −0.3. These results were used in order to obtain an S-N curve for R = −1, with its values of the slope m, fatigue strength stress amplitude, as well as the fatigue strength cycle value. This information has been used in order to calculate the fatigue life with the use of the rainflow cycle counting method and the Palmgren−Miner damage accumulation hypothesis. The rainflow cycle counting method was first introduced by Endo and Matsuishi [m, the fatigue strength stress value afτ, the fatigue strength fN, as well as the transformed stress amplitude resulting from the mean stress effect aiTτ:The tests were performed under cyclic loading conditions for a stress asymmetry ratio value of atsuishi . Since tatsuishi . The Palatsuishi  is one oobsN is the observation time taken from the rainflow algorithm.The final formula for the fatigue life assessment takes the following form:To perform the main task of the paper, which is to assess the fatigue life for a random signal, a narrow band stress signal was generated. The dominating frequency of the signal was 5 Hz. A section of the generated signal is presented in R = −1 and also singular points of experiments performed under different asymmetry ratios. At least two samples have been tested for each asymmetry ratio level and serve as illustrative values beside the S-N curve obtained for R = −1. Some outliers, e.g., the two points at R = 0.3 in The experimental results of the fatigue tests are presented in m and fatigue strength stress amplitude afτ corresponding to the value of the fatigue strength Nf have been calculated and are presented for all the tested materials in mR or yield strength eR. On the basis of the experimental results, S-N curve coefficients such as the slope R = −1, and in the second case it had a global mean value equal to the global value of the maximum amplitude. In the second case, this resulted in the fact that the random generated loading signal had a value of R = 0. The calculation results for PA4, PA6 and PA7 for the generated narrowband signal in the cases of R = −1 and R = 0 are presented in R = 0 using the Niesłony−Böhm model have been presented together with the two approaches for obtaining the missing fatigue strength (SWT and literature data). The area beneath the curves represents the safe area of the generated narrowband stress signal in relation to the value of the maximum amplitude that can be applied for the given loading case.The obtained coefficient values have been used in the calculation process for the generated random narrowband loading signal. This is one of the most important steps in the process of the fatigue life assessment, as in this case we want to calculate two new S-N curves for the random signal without taking the mean stress effect into account as well as taking it into account. The generated signal had a global zero mean stress value for the case of R = 0. The calculation procedure presented in this paper focuses on the application of two popular models, which are the Smith−Watson−Topper model for obtaining the fatigue strength value for R = 0 and the Niesłony−Böhm model, which serves as the transformation model for taking into account the effect of mean stress and creating reference S-N curves for the analyzed signal for R = −1 and R = 0. The second approach was to use literature data to obtain the missing fatigue strength for R = 0. As for the obtained results, we can note that the narrowband random results obtained for R = −1 are above the cyclic stress curve due to the fact that the whole stress history did not have a constant value but was a multiple level amplitudes composition. A scatter band has been generated for the constant amplitude cyclic tests with the value of 20%. As can be seen in R = −1 are almost within this band, and in the other two cases they are way above the scatter band. Meanwhile, for the calculation results for the narrowband stress signal for R = 0 for the Niesłony−Böhm model + SWT, most of the results are within the 20% scatter band for PA4, PA6 and PA7. In the case where the mean stress compensation is performed with the use of the Niesłony−Böhm model + data, all results are within the 20% scatter band. Therefore, the presented method can be applied with success in order to generate S-N curves for generated random narrowband loading signals, taking into account the effect of non-zero mean stress. An important topic of discussion is the use of the SWT model in order to obtain the missing fatigue strength for the Niesłony−Böhm model. We can note that, in comparison to literature data, it underestimates the overall fatigue life, which means that it is rather a safe method. What seems to be important to highlight about this research is the fact that we have tested a brand new specimen shape, whose purpose was to start the initiation process mostly through shear stress. Given the shear stress distribution performed with the finite element method, we were expecting the place of the initiation of the fatigue crack to be on the surface of the specimen in the lower parts, as it was mounted to the experimental stand. The individual specimens with the non-zero global mean stress value had their crack initiation spot on the upper side of the surface due to the pre-tension of the specimen. The planned task, which was to answer the question of how this new type of diabolo sample will react to the experimental procedure seems to be clear, as it reacted exactly as we assumed. Another important point of discussion seems to be the lack of experimental results for torsional tests for these types of materials in the broader literature. Therefore, the authors hope that these results may be used by other researchers, as well as for industry applications, in order for them to perform their own tests as well as modify their calculation procedures by taking into account the mean stress effect in situations where we are missing the experimental curves for R = −1 and R = 0 obtained with the use of the two used approaches has been presented. We can note once again that the SWT model underestimates the ratio between the S-N curves.In The new design of the test samples has served its purpose, as it allowed us to obtain a pure shear distribution inside the sample.The S-N curve Niesłony−Böhm (N-B) mean stress compensation model can also be applied to torsional loading conditions.R = 0 used with the Niesłony−Böhm model have improved the results substantially in comparison to fatigue strengths obtained with the SWT model.The literature results for the missing fatigue strength amplitude for R = −1, and two mean stress effect compensations for R = 0 with different approaches to obtain the fatigue strength for the N−B model.The calculation results obtained for the generated narrowband loading signal have allowed us to perform calculations for three new S-N curves in the case of no mean stress correction for the ratio R = 0 were within the scatter band of the cyclic results for R = −1, within or below the 20% area, whereas the calculation results for the narrowband curves for R = −1 were only in the scatter band of the PA6 alloy.It can be noted that the narrowband curves for The following conclusions and observations can be drawn from this paper’s results and discussion: