To gain insight into the duration and quality of the scientific peer review process, we analyzed data from 3500 review experiences submitted by authors to the SciRev.sc website. Aspects studied are duration of the first review round, total review duration, immediate rejection time, the number, quality, and difficulty of referee reports, the time it takes authors to revise and resubmit their manuscript, and overall quality of the experience. We find clear differences in these aspects between scientific fields, with Medicine, Public health, and Natural sciences showing the shortest durations and Mathematics and Computer sciences, Social sciences, Economics and Business, and Humanities the longest. One-third of journals take more than 2 weeks for an immediate (desk) rejection and one sixth even more than 4 weeks. This suggests that besides the time reviewers take, inefficient editorial processes also play an important role. As might be expected, shorter peer review processes and those of accepted papers are rated more positively by authors. More surprising is that peer review processes in the fields linked to long processes are rated highest and those in the fields linked to short processes lowest. Hence authors’ satisfaction is apparently influenced by their expectations regarding what is common in their field. Qualitative information provided by the authors indicates that editors can enhance author satisfaction by taking an independent position vis-à-vis reviewers and by communicating well with authors. The scientific peer review process is one of the weakest links in the process of scientific knowledge production. While it is possible to review a paper in less than a day (Ware and Mabe www.scirev.sc). On this website, researchers can share their experiences with the peer review process regarding manuscripts they have submitted to scientific journals. This information can subsequently be used by their colleagues when selecting a journal to submit their work. Information is available on several important aspects of the peer review process, including the duration of the first review round, total review duration, the time editors take to inform authors about an immediate (desk) rejection of a manuscript, the number and quality of referee reports, the time authors take to revise and resubmit their manuscript, and the overall quality of the process as experienced by the authors.To gain insight into the duration and other key aspects of the peer review process, we analyze data from 3500 review experiences submitted by authors to the SciRev.sc website  seriously? Is a reasonable motivation for a (desk) rejection given? Does the editor take an independent position vis-à-vis reviewers when making important decisions? Does the editor advise authors on the importance of specific reviewer comments? Together these aspects affect the author’s experience with the journal and to a certain extent may turn a rejection into a good experience or an acceptance into a bad one. We therefore also analyze the authors’ overall evaluation scores given to the journals for their peer review performance as well as the motivations given by authors for their scores. Because an author’s review experience is influenced by many factors , we study the overall scores in a multivariate way and also analyze the authors’ scoring motivations.There are around 28,000 scientific journals worldwide, which publish 2.5 million scientific articles annually, produced by a research community of 6–9 million scientists  reject it. If desk rejected, the corresponding author receives a message from the editor that the manuscript is considered not fit for publication in the journal, with or without a brief motivation given for the rejection. A manuscript that has passed this first stage will then be send out for peer review, whereby experts in the field (peers of the authors) evaluate the manuscript and write a referee report. On the basis of these reports, the editor decides either to reject the manuscript or gives the author(s) an opportunity to revise and resubmit it, or—in exceptional cases,—directly accepts it. In case of a revise-and-resubmit, several additional review rounds may follow before a final decision regarding acceptance or rejection is made. If the process takes exceptionally long, the author may decide to withdraw the manuscript and submit it to another journal.Given the fact that reviewers are often overloaded with academic work, that they are generally not paid for their review work, and that reviews are mostly anonymous, there are few incentives to give high priority to this work Azar . Hence, It is therefore not surprising that one of the most important criticisms of the peer review system is that it is much too slow Lotriet . There aFrom the author’s perspective, first response time is particularly important, i.e., the time that elapses between submission and first response from the editor, be it rejection, acceptance, or a revise-and-resubmit. First response time is important because it often delays the publication of an article more than once, as many manuscripts are rejected once or several times before acceptance Azar . There aDurations vary substantially between scientific fields and even within the same broader discipline. Kareiva et al. , for insWith respect to the number of times the average manuscript is rejected before it reaches the journal that will publish it, Azar  arrives Editors are also worried about these developments. ‘Amongst journal editors there are growing concerns that the quality—and duration—of the review process is being negatively affected as “referees are stretched thin by other professional commitments”. This often leads to “challenges in finding sufficient numbers of reviewers in a timely manner” Lotriet , p. 27.’www.publons.com), a website where reviewers can upload information on anonymous review work they performed. This information is then verified with the journals and can subsequently be used as ‘proof’ of the peer review work done by the reviewer. This initiative provides a solution to the recognition problem. However, it does not help solve the problems of duration and quality as neither the time reviewers spent writing the reports nor the quality of their reports are registered.Several suggestions have been done to make it more attractive for scientists to act as reviewers. Free subscription to journal content, annual acknowledgement on the journal’s website, more feedback about the outcome of the submission and quality of the review, appointment of reviewers to the journal’s editorial board and financial incentives , provided it has been subjected to peer review.Authors who submitted a review to SciRev.sc were asked about their affiliation, which was checked by asking them for their institutional email address and sending a confirmation link to that address. Authors who registered with a noninstitutional email address, because for various reasons they could not provide an institutional one , were asked for additional information to check their identity. Reviews were only accepted if the author’s identity was confirmed. Reviews of accepted papers were additionally checked at the journals’ websites; these reviews were only included if the author had indeed published a paper in the journal during the period mentioned.Although the data are not based on a representative sample of author experiences, they are interesting because they paint a broad picture of the range of author experiences from different fields of study. Each submitted review represents the experience of an author and is important as such. If other authors report similar experiences, this would point toward a specific pattern. And if the resultant patterns differ among scientific fields, this would indicate that the prevalence of specific experiences differs among those fields.There is little reason to expect authors from different fields to be fundamentally different in the way they experience the different aspects of the peer review process. However, there might be different expectations between fields about review duration and hence about what is considered a long process. Besides by field differences, experiences may also be colored by the process outcome and the journal’s impact factor. We therefore split the figures presented in this paper according to scientific field and process outcome (accepted/rejected) and also study relationships with the journal’s impact factor. Information on the impact factor was derived from the journal’s website and other Internet sources. This information could be found for 3126 reviews. In our analysis, we use the natural logarithm of the impact factor, as more journals are concentrated in the lower ranges of the impact factor.Of the 3500 review experiences, 572 (16.3%) referred to manuscripts that were rejected without being sent to reviewers, 693 (19.8%) that were rejected after the first review round, 2128 (60.8%) that were accepted after one or more review rounds, 43 (1.2%) that were immediately accepted without peer review process, and 64 (1.8%) that were withdrawn by the author. Given the relatively small number of reported cases of manuscripts that were withdrawn or immediately accepted, these were not included in our analysis. We also removed some extreme cases regarding immediate rejection time , duration of first review round and total review duration , and duration of revision after first review round . The extreme cases were not concentrated in specific fields.n = 172), (2) Natural sciences (n = 1408), (3) Engineering , (4) Mathematics and Computer sciences (n = 375), (5) Medicine (n = 640), (6) Public health , (7) Psychology , (8) Economics and Business , (9) Social sciences (n = 553), and (10) Humanities (n = 178). Given that a substantial number of journals have a broad scope and therefore include more than one scientific field, the sum of the reviews in the different fields is higher than the total number of reviews.Information on the various aspects of the peer review process is presented for all review experiences, separately for accepted and rejected papers and for ten major scientific fields: (1) General journals  category. For 289 cases the impact factor was missing. These missings were addressed using the dummy variable adjustment procedure [imputing the mean and including a dummy indicating the missings  to 5 (excellent). The outcome of the peer review process is a dummy indicating whether the paper was accepted (1) or rejected (0). The duration of the first review round is measured in days. To indicate language background, we included a dummy indicating whether (1) or not (0) the organization where the author works is located in a country where English is the main language used in daily life . Of the 3500 reviews, 2516 were submitted by authors from non-English-speaking countries. Regarding the distribution of reviews over continents, 557 were obtained from Canada and the USA, 96 from Latin America and the Carribean, 2099 from Europe, 470 from Asia and the Pacific, 190 from the Middle East, 83 from Africa, and 5 of which the continent is not known. For the dummies for scientific field, deviation from mean (effects) coding is used. The dummies therefore indicate to what extent the overall rating within the field is higher or lower than the mean of the fields Hardy .After rating the overall review experience, authors are given the opportunity to motivate their rating in a few words or sentences. These motivations are published online with the reviews, if permission is given by the author. They paint a sometimes revealing picture of what researchers experience in their attempts to get their work published. To supplement the figures presented in this paper with qualitative information, we analyzed the 1879 motivations available in the 3500 reviews studied.For authors, the duration of the first review round, or first response time, is probably the factor they are mostly interested in, as this takes up a substantial part of the total manuscript evaluation time and to a large extent determines how much time is lost if the outcome is negative. First response time includes the time taken by the journal for a first evaluation of the manuscript, finding reviewers, the time the latter require to do their work, and the time the editor then requires to evaluate the manuscript in light of the referee reports and to inform authors about the decision.As can be seen in Table While writing a peer review may take between 4 and 8 h, in only 19% of all reported cases authors were informed about the outcome in less than a month. In about one third of the cases (32%) authors had to wait 3 months or more and in 10% of the cases even more than 6 months before being informed. Duration differs widely between scientific fields. In Social sciences and Humanities, only 7–8% of the authors were informed within 1 month versus 25% in Natural sciences and 27–28% in Medicine and Public health. In Economics and Business and Mathematics and Computer sciences over one sixth (18%) of authors had to wait 6 months or longer.It is yet unclear to what extent the long duration of the first review round is the result of the peer review process as such and to what extent it is due to (in)efficient manuscript handling at editorial offices. Given that immediate rejection times are often long (see Table P = −0.29) as well as for all scientific fields separately, with General journals (P = −0.51), Mathematics and Computer sciences (P = −0.27), and Natural sciences (P = −0.26) having the highest correlations. The only exception was Humanities, where no significant correlation between first response time and impact factor was found. This might be because this field traditionally values publishing books more than publishing in journals , General journals (2.18), Economics and Business (2.16), and Social sciences (2.15) showing a higher average number of review rounds.Total review duration correlates significantly and negatively (−0.27) with a journal’s impact factor, thus indicating that total review duration is shorter for higher impact factor journals.Immediate rejection time is the time an editor takes to inform authors that he or she is not interested in the manuscript (and will therefore not send it to reviewers). Our figures clearly show that immediate rejection time is a major source of unnecessary time loss in the peer review process Table . On averOn the positive side, in half (50%) of the reported immediate rejection cases, the editor informed the author(s) within 1 week. However, the data also show that in 17% of cases authors had to wait more than 4 weeks to be informed of the rejection. Several authors even had to wait for more than 3 months, or withdrew their manuscripts after hearing nothing for an even longer period. These are clearly unacceptable practices.The situation is best in Medicine, where 62% of authors are informed about an immediate rejection within 7 days, followed by Natural sciences and Public health where this figure is 54%. Immediate rejection time is longest for authors in the Social sciences and Mathematics and Computer sciences, where in about 30% of reported cases it took the editor 4 weeks or more to inform author(s) that he or she was not interested in the manuscript and would not to send it to reviewers. There is a significant negative correlation (−0.18) between immediate rejection time and the journal’s impact factor, which indicates that journals with a higher impact factor have editors who work faster and editorial offices that are more professionally organized.Reviewers are generally blamed for long processing times, but our findings indicate that manuscript handling at editorial offices plays an important role too. If editors take a month for an immediate rejection decision, they are probably also slow in finding reviewers and processing referee reports.The average number of referee reports is about 2.2 in all scientific fields  between these factors. When the revision was experienced as more difficult, authors were also more satisfied with the improvement. Regarding the difficulty experienced, revision processes were perceived as easiest in Mathematics and Computer sciences and in Public health (2.6), and as most difficult in Economics and Business (3.3). Regarding the experienced improvement of the manuscript as a result of the revision, authors from Social sciences, Economics and Business, and Humanities reported somewhat higher figures (3.8 and 3.9) compared to the other scientific fields (3.7).There is a small positive correlation (0.07) between the difficulty experienced regarding the referee reports and the impact factor of the journal. Thus, reviewers of more highly ranked journals tend to make somewhat greater demands on the authors. The degree of improvement experienced regarding the manuscript is not significantly related to impact factor.The time from the first submission date to the final decision date is not only influenced by the time the manuscript is at the editorial office or being reviewed, but also by the time authors take to revise their manuscript. It is therefore important to look also at the duration of the revision time. Table Table Regarding the relationship between the journal’s impact factor and the time authors take to revise their manuscript, we expected authors who received a revise-and-resubmit from a high-level journal to be more motivated to complete the revision of their manuscript quickly. However, no significant correlation was found between revision time and the journal’s impact factor.The SciRev questionnaire gives authors the opportunity to provide an overall rating of the review experience on a scale from 0 (very bad) to 5 (excellent); see Table To determine how the various factors might affect the satisfaction of authors with the peer review process, we turn to the results of multivariate analyses , the sign and significance of the main effects stay the same. The interaction analysis shows that the negative effect of a longer duration of the first review round and the negative effect of more review rounds are less profound for accepted papers. Hence it seems that authors are willing to accept extensive revision work if this is rewarded with the acceptance of their paper. At the same time, they seem especially disappointed if the manuscript is still rejected after a long review process.The negative interaction between a paper being accepted and the number of referee reports indicates that authors of rejected papers may consider a higher number of reports as a sign that their paper was taken seriously and might be content with extensive feedback. For obvious reasons, authors of accepted papers are more positive when the journal has a higher impact factor. Authors from English-speaking countries are less negative about the peer review process when their paper is accepted and when they receive more referee reports but find a long process more problematic. This might reflect that they have higher expectations that their paper will be accepted and that the peer review process will be short and efficient compared to authors from non-English-speaking countries.When the duration of the first review round is longer, or when the impact factor of the journal is higher, authors are more concerned about a higher number of review rounds. In those cases, they might expect a smooth continuation of the process and be more disappointed when this proves not to be the case. A longer duration of the first review round is considered less negative by authors who receive more referee reports.The motivations authors give for their rating of the peer review process on SciRev.sc contain important qualitative information on author experiences. We analyzed these motivations and registered the author’s major concern(s). A first important observation is that about half (918) of the 1879 comments is positive. Many authors, in particular of accepted papers, are satisfied with the process and express their gratitude in their motivations. Of the 961 comments with a negative connotation, 371 (39%) express concerns about the duration of the review process. This aspect of long review duration is included in the quantitative outcomes and has been discussed in the preceding sections.A more informative source of discontent, mentioned 437 times (45%), concerns the role of editors and editorial offices. Poor communication of editors/offices—in particular not reacting to information requests—are a major source of frustration mentioned by authors. We received reports of authors who waited over 6 months without hearing anything of the journal or receiving reactions to information requests. Also editors who ‘hide’ behind reviewers and do not take an independent position vis-à-vis them are perceived as problematic. In particular when referee reports are contradictory—as often happens—it is important that editors provide guidance and indicate the comments on which authors should focus in their revision.Poor quality of referee reports is mentioned in 141 (15%) of the critical comments. Referee reports are often perceived to be superficial, contradictory, unreadable, ask unreasonable modifications, or convey the impression that the reviewer did not read or understand the paper. Some other issues mentioned are the addition of completely new comments in the second review round, the theft of ideas, or asking for unnecessary references.In this paper we study various aspects of the peer review process on the basis of 3500 review experiences reported in the last 3 years on the SciRev.sc website. Aspects discussed include the first response time (duration of the first review round), total review duration , the immediate rejection time, the time authors take for their first revision (revision time), the number, quality, and difficulty of referee reports received, and the overall rating of the process.We find considerable variation between the ten scientific fields distinguished. Whereas the reported first response time is 8–9 weeks for Medicine and Public health, it is 11–14 weeks in Natural sciences, Engineering, Psychology, and General journals and 16–18 weeks in Economics and Business, Social sciences, Mathematics and Computer sciences, and Humanities Table .The long duration of the peer review process is often blamed on reviewers taking much time to complete their reports. However, our figures indicate that inefficient editorial processes are also important. The reported immediate rejection time Table , which iThe total time between submission of a manuscript and the final decision of the editor is not only influenced by the time reviewers take to submit their reports and the time editorial offices take to handle the manuscript, but also by the time authors take to revise and resubmit their manuscript Table . In thisP = −0.29), total review duration (P = −0.27), and immediate rejection time (P = −0.18), all indicating that review processes of more highly ranked journals are more efficient. We also found a small but significant positive correlation (P = 0.08) between experienced difficulty of the referee reports and impact factor, indicating that reviewers of more highly ranked journals are somewhat more demanding.Most characteristics of the peer review process studied are related to the journal’s impact factor. More highly ranked journals have a shorter duration of the first review round  also less bothered by a longer duration of the first review round and by more than one review round. We also find that authors rate the process more positive if they receive more referee reports, in particular after a long first review round and when the manuscript is rejected. This indicates that authors appreciate the work of reviewers and the feedback given on their manuscripts. Compared to authors from non-English-speaking countries, those from English-speaking countries are generally less satisfied with the process, particularly when their manuscript is rejected or in case of more than one review round. This suggests that authors from English-speaking countries have higher expectations of the peer review process.


The material characteristics such as size effect are one of the most important factors that could not be neglected in cutting the material at nanoscale. The effects of anisotropic nature of single crystal materials in nano-cutting are investigated employing the molecular dynamics simulation. Results show that the size effect of the plastic deformation is based on different plastic carriers, such as the twin, stacking faults, and dislocations. The minimum uncut chip thickness is dependent on cutting direction, where even a negative value is obtained when the cutting direction is {110}<001>. It also determines the material deformation and removal mechanism  with a decrease in uncut chip thickness. When material is deformed by shearing, the primary shearing zone expands from the stagnation point or the tip of stagnation zone. When a material is deformed by extruding and rubbing, the primary deformation zone almost parallels to the cutting direction and expands from the bottom of the cutting edge merging with the tertiary deformation zone. The generated surface quality relates to the crystallographic orientation and the minimum uncut chip thickness. The cutting directions of {110}<001>, {110}<1-10>, and {111}<1-10>, whose minimum uncut chip thickness is relatively small, have better surface qualities compared to the other cutting direction. Ultra-precision cutting is one of the most efficient and low-cost methods in realizing the nanometric surface roughness and sub-micrometric form accuracy. However, the machined surface quality is affected by many factors, such as material properties –3, machiBesides that, when the UCT is comparable to the cutting tool edge radius, the tool could no longer be simplified as a sharp edge. The interactions between the cutting tool edge and the materials make the material deform in different ways. For instance, the shearing plane which used to be a plane –13 extenIn this study, the plastic deformation and surface generation of single crystal aluminum in nano-cutting are investigated employing MD simulations. The effects of the crystallographic orientation and the tool edge radius are considered in terms of dislocation evolution, stacking fault evolution, shearing plane evolution, atom displacement, cutting force, surface morphology, and material removal mechanism. This study contributes to a better understanding of the surface generation for single crystal materials and even polycrystalline materials in nano-cutting.z direction in the model to reduce the size effect of the nano-cutting process.MD simulation is employed to investigate the plastic deformation of aluminum during nano-cutting. As shown in Fig. x direction. The cutting distance of the model is about 40 nm. Initial temperature of the cutting model is equal to the constant temperature in thermostat layer.Seven cutting directions, including {100}<001>, {100}<011>, {100}<012>, {110}<1-10>, {110}<001>, {111}<1-10>, and {111}<11-2>, are employed to investigate the effect of crystallographic orientation on plastic deformation mechanism in nano-cutting. UCT is at the range from 0.1 to 5 nm and to reduce the simulation time, the cutting speed is set to 100 m/s at the negative E is given as the following function:Fi(ρi) is the embedding energy to embed atom i into the electron density ρi, and ϕi,j(rij) is the pair potential energy between atoms i and j. The electron density ρi can be calculated by the following form:fj(rij) is the electron density casing by atom j which has a distance of rij to the location of atom i.The embedded atom method (EAM) potential  is emploE is the pair potential energy, D0 is the cohesion energy, α is a constant determined by material properties, r0 is the distance at equilibrium, and r is the distance between two atoms.The interaction between the carbon atoms is ignored due to the diamond is much harder than aluminum and the diamond tool is thought as rigid. The interaction between the rigid diamond tool and aluminum atoms is depicted by the Morse potential:The MD simulation is based on the Large-scale Atomic/Molecular Massively Parallel Simulator, and the microstructural evolution of the workpiece under the cutting process is analyzed based on common neighbor analysis, an algorithm to characterize the local structural environment for pairs of atoms and dislocation analysis using dislocation extraction algorithm  with sofThe snapshots of the MD simulation with UCT of 5 nm are shown in Figs. At the cutting direction of {100}<001>, as shown in Fig. z direction with cutting direction of {100}<001>. The displacement vectors of workpiece atoms have abrupt changes at the stacking fault boundary on {111} plane. This plane is seen as the shearing boundary or the shearing plane. The included angle between it and the cutting direction is shearing angle. In this condition, the shearing angle is supposed to be 45°. However, due to the workpiece in front of the cutting edge has a 10° pile-up, the shearing angle in this figure is actually about 35°. In front of the cutting edge, there is a zone in which the displacement vectors almost equal to zero. It means that the atoms are entrapment by the cutting edge. It takes more time and cutting distance for the atoms in the zone to determine whether to be a part of chip or machined surface. The zone is also called as stagnation region. The shearing boundary starts at the tip of the stagnation region. The shearing boundary on the {111} planes is not perpendicular to the cutting plane, while the included angle between them is 55°. Therefore, in different slice distance in z direction, the starting point of the shearing boundary as well as the position of the stagnation region are changed, as shown in Fig. z direction. The stagnation region is split into two small regions influencing the movement of the atoms around the tool edge.Figure When the cutting direction is {100}<011>, the minimum included angle between {111} planes and the cutting direction is 54.7° which is too large to initiate the dislocations sliding continuously along {111} planes. Therefore, the shearing boundary or the shearing plane is not on the {111} planes, but a dislocation slide plane which is perpendicular to the cutting plane generates in front of the tool edge, including perfect dislocations, Shockley partial, Frank partial, stair-rod dislocations, and other dislocations on it as shown in Fig. As shown in Fig. With the cutting direction of {110}<001>, as shown in Fig. Similar to simulation results obtained with cutting direction of {100}<011>, dislocations moving along {111} planes are not initiated at the cutting direction of {110}<1-10> as shown in Fig. The {111}<1-10> cutting direction which is a crystallographic slip direction of single crystal aluminum makes a large number of Shockley partial dislocations initiate in front of the cutting edge accompanied with ISF, as shown in Fig. At the cutting direction of {111}<11-2>, stagnation region is also formed in front of the cutting edge and is larger than cutting on the {100} and {110} crystallographic plane, as shown in Fig. Statistical results of the shearing angles with UCT of 5 nm at different cutting directions are displayed in Fig. The processing forces normalized by the cutting width and the ratios of processing force at cutting direction Figure entclass1pt{minimaAt different cutting direction, the separation height and recovery height are displayed in Fig. With large UCT, shearing planes are formed in front of the cutting tool edge making the material removed in shearing mechanism. However, when the UCT is smaller than or similar to the minimum UCT, the plastic deformation and material removal mechanism are different from the former shearing mechanism. In this study, MD simulations have been employed in investigating the plastic deformation of material with UCT around the separation height obtained above. The displacement vector plots of different cutting directions and UCTs in Fig. When the cutting direction is {100}<001>, no shearing plane is formed in front of the tool edge. For the UCT of 0.45 and 0.7 nm which is less than or equal to separation height, workpiece surface is firstly pressed down by the tool edge in elastic deformation. Then, the upper layer, usually the first layer, of the workpiece material is removed by the cutting tool in an extrusion way. The rest part of the materials flow to the flank face of the cutting tool experience the elastic-plastic deformation and form the machined surface. For the UCT of 0.95 nm which is larger than separation height, more materials are removed to form the chip. However, no shearing plane forms in front of the tool edge and the materials are still removed by extruding. Similar results obtain at cutting direction of {100}<011>{110}<1-10> and {111} <11-2>. Minor difference in the cutting processes is that the cutting tool edge rubs on the workpiece material surface and almost no materials are removed, when the UCT is less than the separation height at these three cutting directions. In rubbing mechanism, the workpiece material experiences elastic-plastic deformation due to the interaction with the tool edge and flank face, which, to some extent, affects the generated surface. For the cutting direction of {110}<001> whose separation height is negative, the upper layer materials could be removed even the UCT is 0.1 nm. This is because the materials in front of the tool edge firstly pile up by shearing mechanism and then are extruded or secondly sheared by the cutting tool edge. The material could also be removed when the UCT is 0.25 nm for the cutting direction of {111}<1-10>, as shown in Fig. Therefore, when the UCT is smaller than separation height, the material undergoes the rubbing and extruding mechanism. In this condition, no material or just the first layer of the material is removed. When the UCT is larger than the separation height, more material would be removed in extruding mechanism. Thus, the separation height with the UCT of 5 nm which is equal to the tool edge radius could be simply seen as the minimum UCT. It determines the material deformation and removal mechanism with a decrease of UCT.Figure Figure From another direction, the surface generation quality, to some extent, relates to the contact length which reflects the interaction of the workpiece and the flank face of cutting edge, as shown in Fig. During the nano-cutting, the size effects of materials make the generation of shearing plane based on different plastic carriers, such as the twin, stacking faults, and dislocations on different crystal planes. The shearing angle has little relationship to the ratio The separation height with the UCT of 5 nm which is equal to the tool edge radius could be simply seen as the minimum UCT. Its value changes in different cutting directions, and even negative value is obtained in the cutting direction of {110}<001>.The minimum UCT determines the material deformation and removal mechanism with a decrease of UCT. When the UCT is considerably larger than the minimum UCT, the material is removed by shearing mechanism. When the UCT is smaller than or similar as the minimum UCT, the material is removed by extruding. For further decreasing the UCT, rubbing happens and no material is removed.At the shearing mechanism, the PDZ, SDZ, and TDZ exist at nano-cutting process. The PDZ expands from the stagnation point or the tip of stagnation zone. However, in rubbing and extruding mechanism, the PDZ is almost parallel to the cutting direction and expands from the bottom of the cutting edge or just merges with the TDZ. The differences of the shear strain between the extruding and rubbing mechanisms are that the strain happens at surface or subsurface of the workpiece material.The generated surface relates to the crystallographic orientation and the UCT. The cutting directions of {110}<001>, {110}<1-10>, and {111}<1-10>, whose minimum UCT is relatively small, have better surface qualities compared to the other cutting directions. The surface quality gets worse when the UCT is similar as the minimum UCT for cutting directions of {100}<001> and {100}<011>. The surface generation quality also relates to the contact length which reflects the interaction of the workpiece and the flank face of cutting edge.The effects of crystallographic orientation on plastic deformation and surface generation of single crystal aluminum in nano-cutting are investigated employing MD simulations. The conclusions can be drawn as follows:


The traditional British pork pie consists of roughly chopped pork cooked in a hot water pastry crust. Due to shrinkage of the meat during cooking, the gap formed around the meat is usually sealed using a gelatin based jelly to exclude air and thus help to preserve the pie. The properties of the jelly are such that it will ingress into the pastry crust causing undesirable softening. The jelly is traditionally produced by simmering pig trotters with seasoning for several hours. In this work we demonstrate the potential of magnetic resonance imaging (MRI) as a tool for investigating the conditions required for producing jellies with different properties and present two examples of this use. Firstly we demonstrate that MRI can determine the ability of water to diffuse through the jelly which is critical in minimizing the amount of moisture moving from the jelly to the crust. Secondly, the impact of jelly temperature on the penetration length into the crust is investigated. These examples highlight the power of MRI as a tool for food assessment.  Examples of food studies using MRI include the nature of water distribution in bread during storage [Magnetic Resonance Imaging (MRI), most well-known for medical applications, has been increasing in popularity as a technique for use in the assessment of food and drink products ,13,14,15 storage ,10,11,12 storage ,17. Work storage . Due to  storage  but did The jelly was produced by boiling 2 kg of coarsely chopped pig trotters in 6.5 L of water for 2 h. After this time the pig’s trotters were removed and the solution was allowed to boil for a further 2 h during which the jelly was reduced as a result of the water boiling off. We refer to this period as the concentration time, during which samples were taken every 15 min. There were 5 samples of 10 mL taken at each of 9 concentration times which were sieved to filter particulate matter before being stored in 10 mL glass screw top vials. Once the samples were collected they were stored in a refrigerator at 6 ± 2 °C in order to prevent the gelatin structure from breaking down. The viscosities of the jellies were determined using a CSL2 rheometer . The pastry samples were made from a traditional hot water paste: 500 g of plain flour, 2.5 g of salt, 300 g of lard and 300 mL of water. To combine the ingredients the lard and the water were heated together until all the lard was melted before slowly adding it to the flour and salt. The mixture was then mixed until malleable and cool enough to handle. Dough samples were baked in a 5 cm diameter and 2 cm deep cupcake mould to give them a similar shape to a traditional pork pie without the meat filling. A circular cutter was used to get an even sample size of 6.5 cm in diameter. Once samples were cut out and placed in the mould, a small weight was placed inside the pastry to mimic the effect of the meat, ensuring that the sample kept its shape and to prevent the pastry rising. The pastry was placed in an oven at 220 °C for 30 min, after this time the anti-rising weight was removed and the samples were left to bake for a further 10 min. This replicates the conditions experienced by the area of the pastry which is most susceptible to moisture changes, the side walls of the pastry.Once the samples were cooked to the point where the pastry appeared golden brown they were removed from the oven, and the mould, and left to cool. The jelly was removed from the heat and filtered before cooling after which 5 mL of one of 9 different concentrations was added to the pastry with samples produced in triplicate. The temperature was monitored  and a thermocouple probe ) as it cooled to allow jelly samples to be taken from 70 to 30 °C in steps of 10 °C. The jelly was immediately added to the pastry samples and left to cool before scanning. This method was carried out for each concentration.2 or T2eff contrast respectively. SPI is a slow sequence (11 min per image for this study) in which each point in k-space requires an excitation pulse in contrast to RARE which collects a number of k-space lines for a single excitation pulse making it very rapid (16 s in this study). SPI is used because it allows for collection of very short T2 values (on the order of milliseconds) to which RARE is not sensitive   and Rapisequence  allowingThe viscosity of the jelly as a function of concentration time is shown in Two example MRI images of pie crusts which have had samples of jelly at 70 °C introduced to their surface are shown in At this temperature, the jelly sample that has been concentrated for 30 min almost entirely enters into the crust. For increased concentration times some of the jelly remains on the surface. In this work we have shown that MRI may be used to determine the extent to which water diffuses within traditionally produced jelly. This is an important factor in minimizing the amount of moisture moving from the jelly to the crust of a pork pie, resulting in an undesirable pie crust softening. In our focus on demonstrating the suitability of MRI measurements to this application we have not included the taste or mouth feel which would be required to provide all of the information for product development. Investigating the effect of the temperature of the jelly added to the crust has shown that the penetration length of the jelly into the pie crust can be extracted from the MRI images and that lowering the temperature that the jelly is added at, could provide a possible means of reducing penetration. Although images have not been included here, we have also shown the ability of MRI to determine if the jelly has completely sealed in the meat or if there is still air contact. This could be a useful tool for online process monitoring of pie production. We have demonstrated in this work that MRI is indeed a powerful tool for product development in the food processing industry and has the potential to provide useful information that could lead to product improvements.


A water droplet pinning on inclined hydrophobic surface is considered and the droplet heat transfer characteristics are examined. Solution crystallization of polycarbonate is carried out to create hydrophobic characteristics on the surface. The pinning state of the water droplet on the extreme inclined hydrophobic surface  is assessed. Heat transfer from inclined hydrophobic surface to droplet is simulated for various droplet volumes and inclination angles in line with the experimental conditions. The findings revealed that the hydrophobic surface give rise to large amount of air being trapped within texture, which generates Magdeburg like forces between the droplet meniscus and the textured surface while contributing to droplet pinning at extreme inclination angles. Two counter rotating cells are developed for inclination angle in the range of 0° < δ < 20° and 135° < δ < 180°; however, a single circulation cell is formed inside the droplet for inclination angle of 25° ≤ δ ≤ 135°. The Nusselt number remains high for the range of inclination angle of 45° ≤ δ ≤ 135°. Convection and conduction heat transfer enhances when a single and large circulation cell is formed inside the droplet. The droplet contact angle and the contact angle hysteresis depend on the texture parameters and the free energy of the solid surface2. In this case, two possible states can be generated at the surface, which include Wenzel and Cassie and Baxter states4. On the other hand, the adhesion force resulted between the droplet and the solid surface is mainly governed by the droplet contact angle hysteresis, the droplet size, and the surface tension of the droplet fluid. Increasing the contact angle hysteresis enhances the adhesion force, which is more pronounced for high surface tension fluids and large droplet diameters. Increasing adhesion force at the interface of the droplet and the solid surface modifies the droplet dynamics significantly. In the case of large adhesion forces, the droplet pins on the hydrophobic surface despite the hydrophobic surface undergoes large inclinations. Although surface inclination alters the line of action and the magnitude of the static force in a sessile droplet, the droplet remains on the hydrophobic surface and the initial shape of the droplet deforms according to the force balance. The geometric deformation of the droplet alters the advancing and receding contact angles while changing the adhesion force generated at the droplet and the solid interface. Since local heating generates fluid motion inside the droplet, it contributes to the static behavior and geometric feature of the sessile droplet. Consequently, investigation of the droplet pinning on the inclined hydrophobic surface under local heating conditions becomes essential.Water droplet on inclined hydrophobic surfaces gives rise to different dynamic behavior when subjected to a gravitational force at different angle of inclination. Depending on the magnitude of adhesion, friction, and shear forces, rolling/sliding or pinning of droplet takes place on the surface. The main forces governing the droplet dynamics in terms of rolling and sliding are the droplet weight and adhesion force generated on the hydrophobic surface due to large droplet contact angle hysteresis. Localized heating of the droplet generates the Marangoni and the buoyancy currents in the droplet fluid while contributing to the droplet dynamics on the hydrophobic surfaceet al.5. They indicated that the existing adhesion force model could not predict the sliding angle correctly because of consideration of the three phase contact line; however, they introduced a new model incorporating the droplet area in contact with the surface. The shape evolution of a sessile water droplet during freezing incorporating the supercooling effect was examined by Zheng et al.6. They suggested that the final droplet profile was less dependent on the degree of supercooling because it was less affected by the freezing rate. As the cold plate temperature was reduced, the freezing rate increased and the freezing time decreased. The influence of interfacial heat transfer, surface tension, and contact angle on the formation of plasma-sprayed droplets was studied by Zhang et al.7. They indicated that the interfacial heat transfer was the key parameter influencing the droplet cooling process and the final splat morphology. Increasing the substrate preheating temperatures delayed solidification and low surface tension promoted liquid projection; however, the contact angle was of less importance in changing the surface morphologies. Heat transfer from the heated superhydrophobic surface to droplet for evaporation was investigated by Hays et al.8. They demonstrated that for supercritical substrate temperatures, as the cavity fraction increased, nucleate boiling was delayed to higher substrate temperatures and the Leidenfrost point occurred at a lower temperature. Marangoni convection instability in a sessile droplet with low volatility on heated surface was examined by Shi et al.9. The findings revealed that with increasing contact angle, the critical Marangoni number for the onset of convection instabilities first decreased and then increased. The critical cell number of the steady Benard-Marangoni convection kept on only one for contact angle less than 15° whereas it increased sharply after angle lager than 15°. The study for the wettability and condensation heat transfer of sine-shaped micro-grooved surfaces was carried out by Qi et al.10. They showed that the net force and the sliding velocity increased as droplets grow, and larger geometrical size was favorable to droplets falling. The velocities of sliding down on horizontal grooved surface were less than that corresponding to the smooth surface; however, the velocities of sliding down on vertical grooved surfaces could remain higher than that on smooth surface. A model study for impact, spreading and freezing of a water droplet on horizontal and inclined superhydrophobic cooled surfaces was introduced by Yao et al.11. They presented a regime map which depicted the different responses of droplets as a function of normal Weber number and Ohnesorge number. The simulation of droplet detachment from solid walls under the influence of gravity was carried out by Tilehboni et al.12. They showed that the droplet could detach from wettable wall in low Eotvos numbers (Eo ≤ 6); however, in hydrophobic walls, the droplet detached from the wall for Eotvos number in in the range of 3 ≤ Eo ≤ 48. The influence of gravity on the shape of liquid droplet was studied by Ren et al.13. They demonstrated that the negligible gravity effect enabled to improve the optical performances of the liquid lens by choosing suitable liquids without importance of the density mismatch.Considerable research studies were carried out to examine the mechanics and heat transfer characteristics of the droplet on the hydrophobic surfaces. A study on the adhesion energy of the liquid droplets located on a hydrophobic flat surface was carried out by Kim 15. Heat transfer in a sessile droplet on a hydrophobic surface was investigated previously18; however, the main focus was the internal fluidity of the droplet on the horizontal surfaces. In these studies, influence of the gravitational force on the flow field and heat transfer was not incorporated. The gravitational force becomes important for reshaping the sessile droplet on the inclined hydrophobic surfaces. This in turn alters the heat transfer rates and modifies the Marangoni and buoyant forces inside the droplet while altering the Nusselt number for different the inclination angles. Consequently, investigation of the heat transfer characteristics of a droplet, which pins on the inclined hydrophobic surface, becomes necessary. The current study presents the new considerations and covers original approaches for droplet pinning and heat transfer on the inclined hydrophobic surface. In the present study, a water droplet attachment (pinning) on an inclined hydrophobic surface is considered and heat transfer from the inclined hydrophobic surface to the pinned droplet is examined. The droplet shape and the heat transfer rates are evaluated for various inclination angles and the droplet volumes. In the analysis, the droplet volume is varied from 10 μL to 25 μL and the inclination angle is changed from 0° to 180°. Experiment is carried out to monitor the droplet shape on the inclined hydrophobic surface. Temperature and flow field inside the droplet are simulated incorporating the experimental conditions. The flow field inside the sessile droplet on the inclined hydrophobic surface is validated through the high speed camera measurements.The water droplet attachment and dynamics on the inclined hydrophobic surface is important in terms of heat transfer during condensation or evaporationρ is the water density and V is the liquid velocity.Internal fluidity of a sessile droplet is simulated in line with the experimental conditions of the inclined hydrophobic surface. The coupled flow and thermal fields are considered simultaneously in the simulations. The continuity equation for transient flow is:β is the thermal expansion of the water. The momentum equation can be written as:p is the pressure, μ is the dynamic viscosity of the liquid, g is the gravity and po is the hydrostatic pressure corresponding to density ρo and temperature To.For natural convection, the density variation is mainly caused by the thermal expansion of the fluid and can be expressed from Boussinesq approximation as:Cp is the specific heat capacity and k is the thermal conductivity.The flow field should satisfy the energy balance according to:Initially water at stagnant condition is considered inside the droplet; in which case, the flow velocity is set to zero, the pressure is set to the Laplace pressure, and temperature is set to be uniform, which is same as the ambient temperature (300 K). Hydrophobic surface is considered to be at 308 K in line with the experimental conditions.h = 10 W/m2K) and the radiation boundary condition is considered at the interface between the droplet free surface and its air ambient, which has temperature of 300 K. Moreover, the natural convection and radiation boundary condition is considered for the free surface of the hydrophobic sample, which is not occupied by the water droplet, i.e., the natural convection and radiation boundary condition is adapted at the hydrophobic surface exposing to the ambient air. Since the hydrophobic surface is maintained at constant temperature, constant temperature boundary condition is adopted at the interface between the droplet bottom and the hydrophobic surface. In addition, no-slip boundary is adopted at droplet and surface interface.Figure 19 is incorporated to simulate the flow field and temperature distribution inside the droplet. Simulations are carried out incorporating 10 μL, 15 μL, 20 μL, and 25 μL water droplet volumes and the inclination angle of the hydrophobic surface are varied within the range of 0° ≤ δ ≤ 180°. Laminar two-phase flow moving mesh model coupled with heat transfer in fluid model is used during the simulations. The models used, solve numerically continuity, momentum and energy equations simultaneously to obtain the flow field and temperature distribution in the solution domain. 3-dimensional simulation of the flow field and temperature distribution is very expensive because of the excessive mesh requirements for the accurate solutions. The comparison of the flow field obtained from 2-dimensional and 3-dimensional simulations are provided in the previous study18; in which, it was reported 2-dimensional simulations results in very close flow field and temperature distribution inside droplet relative to that obtained from 3-dimensional simulations. Consequently, 2-dimensional simulation of the flow field is adopted in the analysis. In the numerical approach, finer meshes are located in the region where the fluxes are high. Mesh independence tests are conducted for each droplet contact angle considered in the simulations. Figure 20. The selection of time step is critical to ensure the accuracy of the scheme; in which case, it is in the order of 10−4 s. The residuals of flow parameters are set as Since the time taken for the experiment and the simulation of experimental conditions is short, which is in the order of 30 s, the evaporation from the droplet surface is neglected in the simulations. This situation is verified during the experiment and droplet images are taken after 30 s and 100 s periods and it is compared with the image corresponding to the droplet at the onset of the experiment. It is noticed that droplet in both images have identical diameters and heights. Therefore, the omission of evaporation from the droplet surface is justified. COMSOL Multiphysics software21. The hierarchical texture was obtained after solution crystallization of the polycarbonate surface. The crystallized surface composed of micro-/nano-spherules and fibrils. The sessile drop tests were conducted on the crystallized sample surfaces using a goniometer (Kyowa model - DM 501) through which the water droplet contact angle was measured incorporating the technique proposed in the previous studies23. High-precision drop shape analysis (HPDSA) was incorporated in the measurements23. The water droplet contact angle is found to be varying from 134° to 136°. A test rig was designed and realized to investigate the droplet pinning on the hydrophobic surface22; in which case, the data produced were enabled to analyze the dynamic behavior of the droplets on the inclined hydrophobic surface. To monitor and record the droplet geometric feature on the inclined hydrophobic surface with different angles, Dantec Dynamics (SpeedSense 9040) high-speed camera was utilized. The sample rate was kept at 100 frames per second, which provided image resolution of 960 × 720 pixels.The solution-crystallized polycarbonate surface had hydrophobic characteristics and it was used in the study. The polycarbonate wafers of 3 mm thickness were cleaned prior to immersing into acetone in which the samples were kept for 4 minutes in line with the methods indicated in an earlier study24. In the simulations, the initial and the boundary conditions for momentum and energy equations were set while mimicking the experimental conditions. The trajectory of the hollow glass particles provided tracing of the flow velocities inside the droplet. Table To validate the flow field inside the droplet, desalinated water was mixed with 3% (by volume) of the hollow glasses with the nominal size of 10 μm. The droplet of this mixture was used to form a droplet located on the 25° inclined solution crystallized surface, which had a constant temperature (308 K). Prior to the measurements, a laser beam plane-illumination was adapted while the beam was passing through the cross-section of the droplet. In the experiments, the droplet volume was selected as 25 μL. The Particle Image Velocimetry (PIV) was used to record the motion of the hollow glass particles across the droplet cross-section. The PIV lens was focused on the droplet cross-section where the laser beam was illuminating and the PIV data were recorded instantly after focusing the the PIV lens. Due to the lens aberration correction of the PIV system, the width of the illuminated plane was slightly larger than the width of the focused plane at the droplet cross-section. However, this difference is considerably small. In addition, simulations were carried out to predict the flow field developed inside the droplet while resembling the water-hollow glass mixture. In this case, the the discrete phase model was used to solve the governing equations of flow while the hollow glass particles were present in the solution domain. Since the hollow glass particles concentration was low in the droplet fluid (3% by volume), a slurry-single fluid model was assumed in the solution of governing equations. Consequently, the effective thermal properties were used in the solution of the equations. The details of the formulation and numerical solution are found in the previous studyPinning of water droplet on an inclined hydrophobic surface is considered and the influence of gravitational force on the droplet shape is examined. Heat transfer from inclined hydrophobic surface to water droplet is simulated in line with the experimental conditions. In the simulations, various inclination angles of the hydrophobic surface and four droplet volumes are incorporated.25. During crystallization, a fetus is formed as the polymer chains align in a parallel way and as the process progresses gradually, the chains are added to the fetus. As the fetus size becomes large enough, the growth of nucleus initiates spontaneously25. The nucleation resulted in the bundle-like or lamellar crystals formations. The types of crystallization depend on the length of primary nucleus and free energy of the surface normal to the chain direction per unit area26. In the case of solution crystallization, the mixture of bundle-like and lamellar nucleus can be formed through the build up by a series of additions of the repeating units during the solution crystallization process. Figure 27. This arrangement results the crystallinity (fc) values in the order of 13.2%, which is similar that reported in the previous study (13.6%)27.Figure 28. A liquid droplet has liquid-solid and liquid-vapor interfaces and the apparent contact angle equation incorporates the contributions of these interfaces. Hence, the equation for the apparent contact angle becomes28:θc is the apparent contact angle, f1 is the surface fraction of liquid-solid interface, f2 is the surface fraction of liquid-vapor interface, θ1 is the contact angle for liquid-solid interface, and θ2 is the contact angle for liquid-vapor interface. For the air-liquid interface, f1 can be represented as f, which is the solid fraction, and air fraction (f2) becomes (1 − f). The parameter f ranges from 0 to 1; in which case, f = 0 is the case where the liquid droplet is not in contact with the surface and f = 1 is case where the surface is completely wetted. However, the contact mode changes from Cassie-Baxter state to Wenzel state29 when the surface texture becomes sparse or when the droplets impact the surface with high velocity30. The solid fraction of the wetted surface area (f) can be associated with the surface roughness ratio (r). Therefore, the solid fraction of the surface can be formulated from the projected area of spherules over the total projected area of the textured surface. This corresponds to the fraction of the area covered by the spherules at the crystallized surface. The fraction of the area wetted by the liquid can be written as,On the other hand, the solvent crystalized surface possesses hierarchical texture with micro/nano size spherules and fibrils and the wetting of the surface is governed by the Cassie and Baxter state. In this case, the resulting surface texture gives rise to the air pockets, which are trapped in the texture while resulting in Cassie-Baxter state on the crystallized polycarbonate surface. The apparent contact angle including surface roughness can provide more realistic formulation of the liquid droplet contact anglef ≤ 0.6. It should be noted that the solid fraction is determined from 3-D Optical Profilometer data  and it corresponds to the projected of area of all spherules over the projected area of the whole surface. The solid fraction obtained from 3-D profilometer is also compared with the data obtained from AFM 3-D image of the surface. Several repeats of 3-D optical imaging are carried out to cover the large area of the surface, which is in the order of 100 mm2. The findings reveal that the solid fraction varies with in 0.4 ≤ f ≤ 0.6; however, statistically, over 90% of the data remains within the range of 0.6 ≥ f ≥ 0.57 and the average value of the solid fraction is in the order of f = 0.58. Moreover, several tests are carried out for the water droplet contact angle measurements on the crystallized surface in line with the previous studies23 to ensure the correct measurement of the droplet contact angle. Table θhysteresis = θA − θR, where θA is the advancing angle and θR is the receding angle. The attainment of large values of the contact angle hysteresis indicates the presence of the large adhesion force, which acts along the three phase contact line at the interface of the droplet on the crystallized surface.The data related to the areas of the textured and the air liquid interfaces are determined from AFM image. Figure ∀ < 0.11 μL31. Therefore, the droplet volumes, which are considered in the analysis (10 μL ≤ ∀ ≤ 25 μL) are larger than those reported in the previous study31. In addition, the droplet rolling is also not observed during the experiments. On the other hand, the force required to detach a droplet from the surface is same as the force needed to overcome the work of pinning32. The adhesion composes of the lateral force (in the plane of the hydrophobic surface) and the normal force . The normal force becomes important when the hydrophobic surface is extremely inclined (δ ≥ 90°). The pressure of air captured within the surface texture, below the droplet bottom meniscus, remains slightly higher than the atmospheric pressure because of the curvy nature of the droplet meniscus. It should be noted that droplet meniscus forms a curvy arc on the top of the texture. Once the droplet is inclined on the surface, the curvature of the meniscus arc changes while altering the pressure of the trapped air in the texture. Depending on the angle of inclination of the hydrophobic surface, the air volume in the texture can either be increased or decreased slightly. Once the inclination angle increases more than 90°, the air volume in the texture increases while reducing the trapped air pressure and generating Magdeburg like forces. These forces are related to f is the solid fraction, ΔP is the trapped air pressure change within a single texture gap (cavity), and Atotal is the total area of textured surface. The formulation of Magdeburg like forces and trapped air pressure change in the texture gap is given in Supplementary Material S1. These forces contribute to the adhesion of the droplet on the inclined textured surface. However, at some texture locations, pressure may remain as the atmospheric pressure because of the fully connection of the texture gaps to the free atmosphere. In this case, the forces generated within texture gap, due to pressure drop during volume increase of the trapped air within the texture, become zero. On the other hand, the lateral adhesion force is formulated heuristically by approximating the three-phase contact line with a single ellipse and using the experimentally obtained polynomial function for the dependence of the contact angle on the position along the three-phase contact line33. Therefore, the resulting equation for adhesion force becomes34:γLV is the surface tension of the liquid on the solid surface, D is the droplet diameter prior to deformation, θR is the receding angle, and θA is the advancing angle. The polycarbonate surface is crystallized and it possesses a hierarchical texture; therefore, the roughness parameter can be introduced in Equation 35. Consequently, Equation f is the solid surface fraction (solid/liquid contact fraction) as obtained from AFM data, which varies within the range of 0.4 to 0.6. The force diagram resembling the droplet attachment on the surface is shown in Fig. Fτ is the shear force generated by the flow field inside the droplet , which is the same order as the shear force acting on the wetted surface due to flow circulation in the droplet, FmL is the lateral component of the Magdeburg like force, m is the droplet mass, g is the gravity, and δ is the inclination angle of the surface , which is generated due to rate of fluid strain in the region of the droplet bottom, is significantly smaller than that of the lateral adhesion force (4 × 10−5 N). Therefore, to sustain the droplet pinning at 90°, the force generated becomes higher than the lateral adhesion force. The contribution of Magdeburg like force in the lateral direction is considerable, which is in the order of 10−4 N  corresponds to the ratio of the maximum droplet height (H) over the wetted length (L) of the droplet on the hydrophobic surface while lateral deformation parameter (W/L) is the ratio of the maximum width (W) of the droplet over the wetted length (L) of the droplet on the hydrophobic surface. It should be noted that the wetted length of the droplet on the hydrophobic surface changes with the droplet volume; however, it remains constant for a unique droplet volume. Figure Two deformation parameters are introduced to assess the geometric changes of the droplet due to bulging under the various inclinations of the hydrophobic surface. These include the vertical and lateral deformation parameters. The vertical deformation parameter  over the thermal diffusivity of the droplet fluid (αT) should remain greater than unity , local (∂V/∂t), and radial  effects remains low. The scale analysis for the acceleration depicts that the maximum radial accelerations is in the order of 10−5 m2/s, the convective acceleration is close to 0.005 m2/s and the local acceleration is about in the order of 10−4 m2/s. In this case, convection acceleration takes over the flow acceleration inside droplet during inclination of the hydrophobic surface. The shear stress and shear force due to the rate of fluid strain in the bottom region of the droplet are in the order of 3.3 × 10−6 N/m2 and 8 × 10−12 N, respectively, which is considerably smaller than that of the adhesion force  mainly influences the flow stability in the buoyant convectionmentclasspt{minimaFigure The pinning characteristics of a water droplet on an inclined hydrophobic surface are investigated and heat transfer from hydrophobic surface to the droplet is examined. Solution crystallization of a polycarbonate surface is carried out to texture the surface towards achieving the hydrophobic features. The resulting surface is characterized incorporating the analytical tools including 3-dimensional optical, scanning electron and atomic force microscopies, and X-ray diffraction. Water droplet contact angle and contact angle hysteresis are evaluated during the inclination of hydrophobic surface. The geometric features of the droplet for each inclination angle are recorded and the change of the droplet geometry due to bulging on the inclined hydrophobic surface is evaluated. The lateral and normal adhesion forces required for the pinning of the droplet on the inclined hydrophobic surface are formulated. Heat transfer from the inclined hydrophobic surface to the droplet is considered and the flow and temperature fields in the droplet are simulated in line with the experimental conditions. The predictions of flow field are validated incorporating the PIV data for flow velocity inside a droplet, which is pinned on the horizontal hydrophobic surface. It is found that the lateral adhesion force of the droplet overcomes the gravitational force in the direction of surface inclination, which is true for all the inclination angles and droplet volumes considered. The shear stress developed at the droplet bottom due to the rate of fluid strain is considerably smaller than that of the lateral adhesion force; hence, its contribution to the droplet pinning is negligibly small. The normal force component generated on the hydrophobic surface due to droplet surface tension is larger than that of the gravitational force for the surface inclination angles within the range of 70° ≤ δ ≤ 115°. As the inclination angle of the hydrophobic surface increases further, the normal component of the adhesion force remains less than the gravitational force resembling the component of the weight in the normal direction. Since the droplet pins on the inclined surface beyond 115°, the contribution of the Magdeburg like forces to the droplet pinning becomes critical. These forces are associated with the change of pressure in the air trapped within the texture gaps when the meniscus geometry of the droplet bottom changes during the inclination. Although some of the textures are fully connected and the air trap in these textures is exposed to atmospheric conditions, some of the textures are not connected and they appear as closed packed nature while causing the air pressure being different than the atmospheric pressure. The air trapped in these textures is responsible for the generation of the Magdeburg like forces at the interface of the droplet meniscus and the surface texture. Heating of the droplet causes formation of two contour rotating circulation cells inside the droplet for the surface inclination angles within the range of 0° < δ < 25° and 135° < δ < 180°. However, increasing the inclination angle further, a single circulation cell extends inside the droplet and occupies a large droplet volume, which is more pronounced for the inclination angles of 45° ≤ δ ≤ 135°. This behavior is attributed to the buoyancy and the Marangoni currents, which influence the flow field inside the droplet. The Nusselt number remains high for the inclination angles 45° ≤ δ ≤ 135°, which is attributed to the convection heat transfer inside the droplet; in which case, a single circulation cell is formed inside the droplet, and the enhancement of heat diffusion in the droplet fluid due to temperature gradient. The present study provides insight into the droplet adhesion on the inclined hydrophobic surface, and the forces acting on the lateral and the normal directions on the hydrophobic surface. A new concept of Magdeburg like forces contributing to the droplet adhesion on the extreme inclination angles of the surface is introduced. The details of the droplet heat transfer on the inclined hydrophobic surfaces are presented, which will be useful for fundamental underrating of droplet heating for practical applications.Supplementary Material S1


The stroke-cam flapping mechanism presented in this paper closely mimics the wing motion of a hovering Rufous hummingbird. It is the only lightweight hummingbird-sized flapping mechanism which generates a harmonic wing stroke with both a high flapping frequency and a large stroke amplitude. Experiments on a lightweight prototype of this stroke-cam mechanism on a 50 mm-long wing demonstrate that a harmonic stroke motion is generated with a peak-to-peak stroke amplitude of 175° at a flapping frequency of 40 Hz. It generated a mass lifting capability of 5.1 g, which is largely sufficient to lift the prototype's mass of 3.39 g and larger than the mass-lifting capability of a Rufous hummingbird. The motor mass of a hummingbird-like robot which drives the stroke-cam mechanism is considerably larger (about five times) than the muscle mass of a hummingbird with comparable load-lifting capability. This paper presents a flapping wing nano aerial vehicle which is designed to possess the same lift- and thrust-generating principles of the Rufous hummingbird. The application is indoor flight. We give an overview of the wing kinematics and some specifications which should be met to develop an artificial wing, and also describe the applications of these in the mechanism which has been developed in this work. Summary: The stroke-cam flapping mechanism closely mimics the wing motion of a hummingbird. It is the only lightweight hummingbird-sized flapping mechanism which generates a harmonic wing stroke with both a high flapping frequency and a large stroke amplitude. Recent innovations in microelectronics, material science and mechanical miniaturization such as small MEMS accelerometers and gyroscopes, LiPo batteries and efficient micro-mechanical motors, have made it possible to develop flight vehicles at the so-called nano-scale; nano air vehicles or NAVs with a wingspan small enough to fly indoors.These NAVs can be used for indoor missions such rescue, surveillance, security, inspection of calamities in contaminated spaces or any other first responder applications.NAVs designed for indoor use face certain design constraints concerning size, weight and agility. The size is restricted by the limited space available indoors. Regardless of the design concept and the driving mechanism of the NAV and weight optimisation of its components, the wingspan of any flight vehicle always sets a natural limit to the maximum airborne weight of the NAV. Finally the NAV should have sufficient agility to avoid all kinds of obstacles and fly swiftly through corridors, rooms and windows.http://www.delfly.nl/micro.html).In general, three types of NAVs can be distinguished: fixed wing NAVs, rotary wing NAVs, like helicopters or multi-copters, and flapping wing NAVs (or FNAVs), which flap their wings in order to fly just like hummingbirds and insects do. With the particular constraints of indoor flight, fixed wing NAV are usually inappropriate as their agility is very limited and more in particular, they are unable to hover. Both rotary and flapping wing NAVs offer viable alternatives. While rotary wing NAVs are well established, the Nano Hummingbird  is stillSmall hummingbirds are able to perform all kinds of remarkable flying manoeuvres. They can fly forwards, backwards, sideways, hover and they can even transition swiftly between these flight regimes. They are the living proof that flapping wing propulsion is a promising choice for agile artificial NAVs.Several authors have investigated the mechanisms and aerodynamic phenomena with small birds and insects in different flight regimes. The complexity required to artificially generate the wing motion of hummingbirds has limited the successful development of operational FNAVs up to now to a single development .Selasphorus rufus). The Rufous hummingbird is the only small hummingbird species of which the wing motion is already thoroughly studied . The angle of attack variation is described relatively to the body orientation by the chord angle and is about 92° during upward (backward) wing motion and about −38° during downward (forward) wing motion.Measurements on the wing motion of the Rufous hummingbird  have revThe total force generated by the flapping wing is resolved into two components, which are defined with respect to the average stroke plane. By definition, thrust is the component of force perpendicular to the average stroke plane. A literature study reveals that three features of the wing motion have a significant influence on the thrust generated by a flapping wing: the flapping frequency , the winTo maximize the thrust generated by an artificial flapping wing, three parameters should be set at a high value; the flapping frequency, wingbeat amplitude and the speed of the wing rotation. At the same time, a relatively high angle of attack should be maintained during forward and backward.Drosophila (The phase between the stroke (sweeping) motion of the wing and the pitching motion of the wing may also be of importance because it may have a small but significant influence on the thrust generated by a flapping wing. If a larger part of the wing pitch occurs before the stroke reversal, it is called an advanced wing pitch, while if a larger part of the wing pitch occurs after the stroke reversal it is a delayed wing pitch. Research on osophila  shows thosophila .Drosophila.The development of an artificial flapping wing mechanism that generates the wing motion of hummingbirds and insects is particularly challenging as a high frequency and at the same time large amplitude motion should be generated with lightweight components which are inevitable fragile. Much of the experimental research on flapping wings is restricted to dynamic scale models like Ellington's model  or DickiAn important observation is that the measured stroke angle, deviation angle and pitch angle are well-repeated over several successive cycles, which means that the mechanism operates accurately. The main parameters for the wing motion are summarised in The measured stroke angles closely approximate the desired harmonical pattern. However a small undesired asymmetry between forward and backward stroke can be distinguished. This asymmetry may be the result of the asymmetry of the friction between the stroke cam and cables described above. The stroke amplitude tends to increase with increasing flapping frequency. We assume this is mainly caused by an elastic deformation of the cable.Although no deviation of the wing from the stroke plane was intended, the measurements do show a deviation, but with a relatively small amplitude. This deviation is due to the flexibility of the leading edge of the wing and manufacturing tolerances of the rotational joints.The variation with time of the pitch angle correlates well with the simplified model proposed above. During stroke reversal, which at all flapping frequencies lasts about 10-15% of a flapping cycle, the pitch angle reverses rapidly and almost linearly in time while in between two stroke reversals the measured pitch angle does not change much. The pitch angle reaches a peak value just at the end of each stroke after which it undergoes two or three oscillations before the next stroke reversal takes place.A small and unintentional difference in pitch angle during forward stroke and pitch angle during backward stroke is measured. This asymmetry may be the result of small asymmetries in the hand-made prototype. On the other hand, pattern repetition is quite precise as each cycle has a variation that is almost identical to other cycles. Hz to 35 Hz the stroke amplitude increases with increasing flapping frequency, while at 40 Hz the stroke amplitude is approximately equal to the stroke amplitude at 35 Hz.The pitch angle during both forward and backward stroke decrease with increasing flapping frequency. The high speed images used to record the wing motion show that this is mainly caused by the elastic deformation of the wing.As mentioned above, the exact stroke amplitude and pitch kinematics are also affected by the imposed flapping frequency. To quantify the influence of the main kinematic parameters on the thrust independently, more measurements are needed. mm corresponds to a stroke angle of 56°.The chord angle of Tobalske's measurements is related to the pitch angle of the stroke-cam mechanism. Taking a body angle of 51° and a tracking stroke plane angle of 14°  we find  Hz) nearly equals the flapping frequency of the hovering Rufous hummingbird (44 Hz) ( Hz) measured on a Rufous hummingbird lifting maximal load (The flapping frequency generated by the stroke-cam mechanism (40 (44 Hz) , but it mal load .The wing we developed and used in our experiments does not show a significant difference in camber between forward and backward stroke in contrast to a real hummingbird wing which does show a significant difference in camber between forward and backward stroke. As a consequence the wing motion necessary to hover is symmetrical in forward and backward stroke, in contrast to the motion of a real hummingbird.Due to the wing symmetry and the symmetry in wing kinematics of forward and backward stroke, the stroke plane of the stroke-cam flapping mechanism has to be oriented horizontally to hover. The Rufous hummingbird slightly tilts it stroke plane (14°) during hovering .The stroke-cam flapping mechanism generates a stroke amplitude of 175°, considerably larger than the stroke amplitude of 112° measured on a hovering Rufous hummingbird , but sma Hz (−56° vs 53°)  . This as vs 53°) . g) is larger than the one generated by a Rufous hummingbird [3.97 g for each wing . The mass of the two motors combined would be 3.28 g, or 32% of the maximum load lifting capability. This robot would have a wingspan of about 125 mm (two times a 55 mm wingtip-to-shoulder distance and a distance between the two shoulders of about 15 mm).This section compares the characteristics of an artificial bird that uses the stroke-cam mechanism to different species of birds in nature. A hummingbird-like robot would use a set of two stroke-cam mechanisms and two motors as presented in this paper. The load-lifting capability of this robot would be 10.22Ocreatus underwoodii  has a comparable load-lifting capability of 9.99 g. This species has a flight muscle mass  of 0.633 g, which is 21% of its body mass (3.052 g) (The 3.052 g) . It is cIn addition it should be mentioned that at this point no means to perform flight manoeuvres are yet implemented. Implementing this ability would probably increase the total motor mass.This paper presents the concept of a drive system for a hummingbird-like robot with flapping wings in hovering flight, with the Rufous hummingbird having served as a reference. The core of the development is the stroke-cam mechanism which generates a reciprocating motion of the wing. The paper further compares the artificial wing kinematics to the real bird.The stroke amplitude and angle of attack at midstroke are found to depend on the flapping frequency.At this moment we only studied hovering kinematics. In the future it would be interesting to study forward flight and to study how flight manoeuvres could be performed and compare these results with the forward flight kinematics and flight agility of hummingbirds.By only taking into account the features of the wing morphology and motion which have a considerable influence on the thrust as described above, the wing motion can be simplified by leaving some particular features which do not contribute much to the level of thrust which is generated. As a consequence, mechanical design of the flapping mechanism is somewhat simplified. mm and the average chord length is 18 mm. The leading edge and veins are pultruded carbon composite rods, all attached to each other near the shoulder joint. The shoulder joint allows the wing to pitch around its leading edge. The wing membrane is a polyester film with a thickness of 0.015 mm. This design results in a stiff wing that, unlike a real hummingbird wing, has no difference in camber between forward and backward stroke.Three angular degrees of freedom are sufficient to fully describe the motion of a stiff robotic hummingbird wing relative to its body as shown in ϕ measures the orientation of the wing in its back and forth stroking (sweeping) motion, it is defined as the angle between the x-axis and the projection of the wing's leading edge onto the stroke plane. The deviation angle parameter β measures the deviation (plunging) of the wing out of the stroke plane and it is defined as the angle between the wing's leading edge and its projection onto the stroke plane. The pitch angle parameter α measures the wing rotation about its leading edge. The pitch angle is defined in a plane perpendicular to the leading edge as shown in The stroke angle parameter The stroke range is defined as the angle between the minimum and maximum stroke angles and the deviation range as the angle between the minimum and maximum deviation angles.The deviation can be neglected and does not need to be actively controlled.The stroke plane can be taken horizontally.The stroke angle varies approximately harmonically in time.The variation of the pitch angle in time can be approximated as in To simplify the design of the flapping mechanism we take the following assumptions to obtain simplified wing kinematics:To quantify the extent to which an advanced wing rotation occurs, we define the supination/pronation before wing reversal as the percentage of the total duration of the supination/pronation that occurs before stroke reversal. A supination/pronation before wing reversal above 50% depicts an advanced wing rotation.The stroke amplitude needs to be sufficiently large; at least 112° like that of a Rufous hummingbird . A largeThe stroke angle needs to vary harmonically to minimise peaks in acceleration and jerk of the wing load. Hz)  , but preThe mass of the total system including the frame, flapping mechanism, motor, energy source and driving electronics must not exceed the thrust which is generated by the flapping wing in order to be used in an FNAV.The mechanism should be robust enough to be able to work properly during several minutes of flight.The design of a lightweight flapping mechanism which reproduces the simplified wing motion of a small hummingbird as described above is challenging because of some specific requirements:Many different lightweight flapping mechanisms are described in literature . They tyThe next section describes a new stroke-cam flapping mechanism. The stroke-cam mechanism generates a harmonic stroke motion with a stroke amplitude which may be as large as desired (not exceeding the 180° limit with some margin).C, two cables (green) which are fixed at both extreme ends and which slip over the stroke cam and a wing wheel (black) which rotates around its centre. At one side of the stroke cam the cables are fixed at points F1 and F2 and on the other side of the stroke cam the cables are guided through guides G1 and G2 and fixed to the wing wheel. The wings are attached to this wing wheel by means of a joint which allows for the pitching of the wing. The cables are pre-tensioned.The central element in the mechanism is a cam, which is designed with a particular cam profile. A static cable, which is mounted in the frame, slides over the cam and the rotating cam drives a pulley via the cable. The stroke-cam mechanism , shown in The kinematic parameters that can be modified to alter the flapping motion are the flapping frequency, stroke amplitude and the pitch angle during forward and backward stroke.The flapping frequency can be continuously varied during operation by changing the input voltage to the motor. Changing the flapping amplitude requires replacing the stroke cam with one of a different size. Changing the maximum pitch angle during forward and backward stroke requires repositioning the pitching blocking elements. mm through a 3 gear transmission with a total gear ratio of 14.75. The stroke cam material is ABS and it is printed with a SST 1200ES printer. The cables are nylon cables with a diameter of 0.1 mm. The tension in the cable can be adjusted with two small screws. The frame is assembled from several elements cut out of a 0.5 mm carbon composite plate which are consecutively glued together. The total mass of this setup is 3.39 g of which the wing constitutes 0.048 g and the motor constitutes 1.64 g. Hz, 30 Hz, 35 Hz and 40 Hz to examine its influence on the other kinematic parameters. The flapping mechanism is mounted on to the load cell  in a stereo set-up. One camera takes top view images and the other camera takes front view images . After tracking the wing motion the tracking records are further analysed in Matlab to generate stroke, pitch and deviation angles as shown in The recordings are done at a frame rate of 5000The thrust is measured 200 times per second by a common double beam strain-gauge load cell (model 1668S from BCM) and averaged over a period of 5 s. The load cell is connected to a Scaime CPJ measurement bridge with built-in amplifier. The measurement accuracy is 0.5 mN.