It was observed that efficiency loss is not only mass-dependent but that it also depends on the dust properties. The small positive effect of the tiny dust layer which slightly increases in surface roughness on the module performance was also observed. The results that were obtained enable the development of a reliable model for the degradation of the efficiency of the PV module caused by dust deposition. The novelty consists in the model, which is easy to apply and which is dependent on the dust mass, for low and moderate naturally deposited dust concentration  and which is applicable to the majority of cases met in an urban and non-urban polluted area can be used to evaluate the dust deposition-related derating factor (efficiency loss), which is very much sought after by the system designers, and tools used for computer modelling and system malfunction detection.The maximisation of the efficiency of the photovoltaic system is crucial in order to increase the competitiveness of this technology. Unfortunately, several environmental factors in addition to many alterable and unalterable factors can significantly influence the performance of the PV system. Some of the environmental factors that depend on the site have to do with dust, soiling and pollutants. In this study conducted in the city centre of Kraków, Poland, characterised by high pollution and low wind speed, the focus is on the evaluation of the degradation of efficiency of polycrystalline photovoltaic modules due to natural dust deposition. The experimental results that were obtained demonstrated that deposited dust-related efficiency loss gradually increased with the mass and that it follows the exponential. The maximum dust deposition density observed for rainless exposure periods of 1 week exceeds 300 mg/m Solar energy is one of the major sources of renewable energy; it is free and non-permeable and clean and has various applications—direct and indirect ones. With the current development, it is reasonable to convert solar energy directly into the most desirable type of energy—electrical energy using a photovoltaic cell. The efficiency of photovoltaic (PV) modules systematically increases and at the same time the manufacturing prices gradually decrease. Therefore, in recent years, a significant increase in the solar energy investment is observed Primer . The perIn any case, the above factors involve the scattering of solar radiation and reduction of solar radiation intensity that can reach the photovoltaic module surfaces. The PV module performance decrease influenced by dust constitutes a local phenomenon and may differ significantly from region to region , respectively. At the same time, an ash layer deposition with a deposited density of 0.6 and 2.1 g/m2 causes only a 0.15 and 0.4% performance reduction, respectively. On the other hand, red soil and limestone with a deposition of density 0.1 g/m2 cause a decrease of as much as 0.5% in efficiency.As was demonstrated by Kaldellis and Kapsali , the imp2 of dust accumulated on the photovoltaic panel surface reduces the short-circuit current by 15–21%, open-circuit voltage by 2–6% and the module efficiency by 15–35%, whereas Hottel and Woertz  was done by Bashir et al.  and Ali Generally speaking, one may infer from this survey of literature the following conclusions: the environmental conditions and the geographical site with seasonal variations have a direct influence on the amount of dust and physical properties such as morphology, composition, gradation and mechanical properties. The roughness percentage and physical and chemical properties of the photovoltaic module surface (front side) have a significant influence on the dust deposit, causing a decrease in efficiency. The effect of air pollution is significant in urban areas due to the high population density, vehicles and growth in the industrial activities, specifically dust particles which are produced by the combustion of fossil fuels. Dust deposition on the front module surface can significantly reduce the amount of solar energy eventually absorbed by the module and changes in module output voltage and current are expected. The dust deposition has a very local effect and is strongly correlated with the air pollution. Due to this fact, it is very difficult to create a general model for dust deposition and dust-related PV system efficiency reduction. The properties of deposited dust and its impact on photovoltaic module performance are a complex task and they depend primarily on very localised environmental conditions. Much of the information available in the literature applies only to the specific location in which the experimental work was conducted. The important observation from literature led us to a critical point that large areas of the world still were not covered by the research. Additionally, a large number of research use artificial dust particles which are not adequate to the natural dust composition or use significantly different from natural artificial laboratory conditions. It is only through a systematic study of the dust accumulation at different locations of the Earth that the dust deposition effect on the PV power degradation can be better understood.In this study carried out in Kraków, Poland, which is one of the most polluted European cities, the experimental investigation concerning the impact of natural dust deposition effect on the PV module front cover surface in the urban air-polluted area on the of photovoltaic system performance was studied. Most of the field studies report energy yield loss vs. exposure time without the detailed information about dust concentration density and this does not enable direct linking of a local pollution condition with the module performance degradation. In the present research, the mass of deposited dust and the photovoltaic module output power were experimentally measured in a natural high-pollution city environment during the heating and non-heating seasons for the various environmental conditions. It was intended to characterise dust accumulation on the photovoltaic module surface through a systematic approach.The main objective of the present study is to develop a practical model for the simulation of the degradation of system efficiency (derating factor) caused by natural dust deposited on the front surface of the PV module. On the basis of an experimental measurement of natural dust deposition carried out by the authors in this study as well as on the basis of the scanty literature data for natural dust, an attempt was made to develop a reliable model which takes into account the major effects that dust deposition has on the performance of a PV system.2 and representative for many geographical regions) is applicable to the majority of cases met in an urban and non-urban polluted area and it can be used to evaluate the dust deposition-related derating factor which is very much sought after by the system designers, and tools used for computer modelling and system malfunction detection.The proposed theoretical dust mass-dependent model for low and moderate naturally deposited dust concentration . During the experiment, the panels were adjusted at tilt angle β = 15o and azimuth γ = 20o West. In Fig. Pmax on every single module. The second string consists of 11 PV modules of the same type but all modules in this string are optimised at once using single-MPPT devices integrated with a PV inverter. The electrical parameters from the individual tracked PV modules and from the second string were acquired every minute and the average value was recorded for analysis every 15 min. The experimental procedure was carried out for all environmental conditions  in the period 1.05–11.11.2017. For the current analysis, only the data from selected modules of the first string P_1–P_10 were presented and the data for the second string . On the basis of the recorded measurements, the power output Pmax of the polluted and clean PV modules was determined. At the same time, the dust mass deposited on the polluted module front surface was specified for each examined case. It is assumed that all modules experienced the same instantaneous  insolation level, ambient temperature or wind speed, as well as every single module is polluted uniformly, which means that all polycrystalline cells in the module are covered by the same dust mass.The influence of dust and soiling on the natural outdoor exposure of a PV system was conducted experimentally on the roof of the Faculty of Electrical Engineering, Automatics, Computer Science and Biomedical Engineering, AGH University of Science and Technology, in the city centre of Kraków, Poland  characterised by high air pollution, high traffic intensity and relatively low wind speed. A PV system consists of two independent module strings connected in series. Strings are built by means of Sharp ND-RJ260-type polycrystalline photovoltaic modules with a nominal power of 260 W and Mb and after Ma sampling is the total mass of dust deposition on the surface of the polluted module Md. Finally, the dust deposition density md was estimated as follows:The modules P_1 and P_3 were cleaned regularly and considered as reference modules, while the other modules were made dusty because they collected dust particles from polluted air in a natural manner. For the efficiency analysis, the dusty and clean modules were used for the purposes of reference in order to calculate efficiency loss. Afterwards, the polluted modules were also cleaned. In Fig. The meteorological data  were obtained by means of the Faculty of Physics and Applied Computer Science—the Vaisala WXT520 automatic meteorological station placed nearby was used. The concentration of PM10/PM2.5 was obtained from the Voivodeship Inspectorate of Environmental Protection in Kraków at an urban background station . Total suspended particles (TSP) were collected at the PV system site. A low-volume sampler was placed on the roof next to the PV modules. Samples were collected on quartz fibre filters  of a 47-mm diameter. On weekdays, the filters were replaced at 24 ± 2 h intervals and during weekends after around 70 h. After the data acquisition and the calculation procedure, the total errors of the measurements were determined by analysing the accuracy of the equipment.md for five consecutive weeks (in the period 1.05–30.06.2017) for two identical modules (P_1 and P_3) is presented in Fig. md was 25.8 mg/m2 while the highest was 300.0 mg/m2. Due to the stochastic nature of variations of the environmental condition including pollution, the dust deposition imparts this nature. The total weekly dust mass deposited at the PV module surface depends on the air pollution, wind speed, humidity and—what is most important—rainfalls which occurred during the period of analysis almost every week. The only week without rainfall was recorded for the week 17–23.05.2017 when the highest dust deposition density was observed. In this figure, 15 weeks, dust deposition density (in the period 30.06.2017–16.10.2017) is presented also. One may see from Fig. md and what is also worth noting is that a longer time causes a significant difference from 9 up to 41% in the mass deposition density between identical modules . In Fig. The dust samples were collected from cleaned modules on the basis of a daily to weekly schedule and were the subject of analysis. They represent various exposure times of the dust deposited at the module surface and were collected regardless of the weather conditions during past days . The weekly dust deposition density md for two identical modules is presented in Fig. md for rainless days follows particle concentration in the air but even for rainless days for the comparable value PM10 (17–19.10 and 7–9.11), the deposition differs by up to 125%. The reasons for this are associated with other environmental parameters, for example, water condensation at module surface (on 18.10 and 8.11) or very low wind speed (on 9.11). More information about daily to weekly dust deposition rate as well as correlation with environmental variables or chemical dust composition analysis for the presented location can be found in other works of the authors  every day during the period 17.10–11.11.2017. The daily dust deposition density Pmax losses vs. time are not really correlated with the time. It became clear that the dust exposure time in the natural environment is not a relevant parameter. This type of studies may provide quite important knowledge related to the local dust deposition rate. However, in the majority of research studies of this type  and not with the time—as was reported in a large number of research studies Fig. a. Even fAccording to the authors, in order to evaluate photovoltaic module efficiency degradation due to dust effect and to perform a reliable PV system analysis, two quite independent models are required. The first model must be able to correlate the amount of dust deposited on the module surface with photovoltaic module efficiency loss and the second one which correlates the amount of dust with locally available weather or environmental conditions . Model splitting will allow distinguishing various climate conditions, from different geographical parts of the world with diverse climatological conditions, including those of semi-arid zones or deserts and different climate zones. In order to create a theoretical model of dust mass deposition, an experimental measurement of air pollution and local weather conditions as well as dust particle properties is highly desirable for different world locations, which is far beyond the scope of this paper.In the next section, the effect of the dust deposition on the photovoltaic module efficiency under natural conditions is presented together with the dust deposition-related theoretical model for predicting photovoltaic system efficiency loss (derating factor) caused by dust deposited on the module surface.PPV has to be calculated on the basis of the module specification and solar radiation prediction or historical local real measurement. The PV module power depends on the total solar irradiance incident on the PV surface  GT but it also depends on several additional factors such as dust and soiling, losses, shading, age, snow cover and temperature. In the literature, a large number of formulas for PV power output PPV of varying complexity were proposed (Duffie and Beckman YPV (W) is the rated capacity of the PV module (power output under STC conditions), ηder (%) is the PV module derating factor, GT (W/m2) is the total incident solar irradiance, αP (%/°C) is the PV temperature coefficient for the power, TC (°C) is the PV module instantaneous temperature. TC,STC (°C) and GT, STC (W/m2) are the PV module temperature and solar irradiance at standard test conditions and AC (m2) is the module area and ηPV (%) is the module efficiency.In order to properly design and optimise the photovoltaic or photovoltaic base hybrid system, the PV module-generating electrical power η (%) are the module derating factors caused by the dust and soil ηdust, shading ηshade and age ηage.The PV module derating factor is introduced taking into account additional factors affecting the module such as dust and soiling, shading, age and snow cover and can be decomposed into few key derating components and written as follows:ηdust to the amount of natural dust deposition on PV module surface. When we explored a large number of papers, it turned out that only four papers . However, as far as the same module series and batch line are concerned, this tolerance is usually not higher than 1%.In order to evaluate the dust mass deposition-dependent theoretical model for derating factors Pmax from modules is continuously fluctuating, but what is more important is that the difference in acquired power output Pmax between the analysed modules is not constant. This effect can be better seen in Fig. As can be seen in Fig. Pmax normalisation procedure is implemented and the module correction factor is evaluated on the basis of the energy yield E of the PV module for the period of time (1 day):In Fig. κ for modules P_1–P_4 is presented in Table The calculated mean correction coefficient In Fig. md = 25.8 mg/m2  and this amount is responsible for the decrease in energy production of about 2.1%. For the other days (and masses), the observed effect is also clearly perceptible. For all the days, a significantly lower electrical energy production was observed for module P_S and the effect is primary caused by the system optimisation and secondarily by the dust deposited on the surface .In Fig. entclass1pt{minimaDuring the testing period, in the majority of cases, the power output delivery by the modules decreases due to dust accumulation. Since, generally speaking, the power rating can be different for the different module types and sizes, in order to calculate module performance loss, normalised results should be taken into consideration. Typically, in the literature  power output of the clean and dusty modules while ηc  and ηd represent the efficiency of the clean and dusty modules calculated from the following equation:When we implement Eq.  to Eq.  and 4),,4), it bm in Eq. :Most of the investigations presented in the literature were conducted in arid and semi-arid areas and they were performed using artificial dust. Only a few studies deal with natural dust. The key and unique data available in the literature  in the urban environment and, with the help of literature data, also for other environments and module types. The key aim is to evaluate the relation between the derating factor ust (Eq. ) and theure Fig. a.md ranged from 0.0258 to 0.300 g/m2. The effect of dust deposition md in the periods May–June and October–November on the efficiency loss is presented in Fig. 2. This phenomenon is not observed for higher deposition.According to the experimental results which were obtained, the measured dust deposition density md. The theoretical model . In order to extend the model range, all current experimental measurement points as well as experimental measurement for natural dust available in the literature  with thA and B are the model constant and md is the mass deposition density, g/m2.where 2 , coefficients are different.This trend line can be even extended up to 5.0 g/m2 Fig. c by mean2 Fig.  and , it is hardly possible to obtain natural dust concentration higher than 5.0 g/m2.The ranges up to 1.0 or 5.0 g/mηPV loss having nominal ηo STC module efficiency (clean one), the following practical relationship can be used:In order to calculate dusty module efficiency 2, it is as low as 0.92, while for polycrystalline, it is about 0.95 and 0.97. This is more than what was observed for polycrystalline loss in efficiency which puts into question the benefits of the greater efficiency of monocrystalline photovoltaic modules.Ali et al.  providedmd can be introduced:It was already demonstrated that efficiency loss is not only mass-dependent but also PV type-dependent as well as dust type- and dust size-dependent. In order to take into account the dust type and type effect in Eq. , apparenCs and Ct are average size dust-dependent and average type dust-dependent correction coefficient. Natural dust type and size are local site-dependent and may follow seasonal fluctuation. It is important to notice that the average dust size deposited at the module surface varied with the time of deposition  which is a significant decrement, considering the fact that this mass was deposited during 1 week. The efficiency loss gradually increased with the dust mass deposited and it follows the exponential trend line. It was observed that this loss is not only mass-dependent but that it also depends on the dust properties. The samples from the same period have similar properties and the effect of mass is a key issue while the samples from different periods have different properties , and in that case, the sample mass is not the only but still a key factor. On the basis of the results which were obtained, one may state that the particles which exist in the air of polluted urban areas negatively affect the PV module performance and the level of degradation is non-negligible. The derating factor (efficiency loss) depends on the apparent mass of the dust particles deposited on the module surface and PV module type. The performance of PV module can be considerably reduced even after a small exposure time into the atmospheric air (more than two percentage point in power loss in 1 week).The dust deposition effect on photovoltaic module performance is a complex problem and it depends primarily on very localised environmental conditions. It is only through the systematical study of the natural dust accumulation in different locations of the Earth and at different seasons that the dust deposition effect on the PV system degradation can be better understood. The dust accumulation on the photovoltaic modules involves energy production loss and thus a decrease in the overall photovoltaic system efficiency. It also generates economic loss. In this study conducted in the city centre of Kraków, Poland, characterised by high pollution and low wind speed, the focus is on the evaluation of the efficiency degradation (derating factor) of polycrystalline photovoltaic modules due to natural dust deposition. The experimental measurement was conducted under variable environmental conditions and under different dust deposition exposure periods. It was observed that the power output is significantly influenced by the dust deposition and the highest efficiency decrement observed in the current experiment was 2.1%  on the module performance was observed which results in better light trapping and absorption in solar cells but still without a significant decrease in light transmissivity.The experimental results enable the creation of a simple but reliable mathematical model Eqs. –12)) fo) fo12)) el (Eqs. –(12)) wi) fo) fo1i et al.   which is much sought after by the system designers and tools used for and computer modelling of hybrid energy systems or to detect system malfunction (low performance) due to dust deposition. This relation can help to predict the effect of dust deposition on PV module performance in real environmental conditions.


We present a non-calorimetric method which yields the heat absorbed during hyperthermia: it is based on accurately measuring the quality factor change of a resonant radio frequency circuit which is employed for the irradiation. The approach provides the absorbed power in real-time, without the need to monitor the sample temperature as a function of time. As such, it is free from the problems caused by the non-adiabatic heating conditions of the usual calorimetry. We validate the method by comparing the dissipated power with a conventional calorimetric measurement. We present the validation for two types of resonators with very different filling factors: a solenoid and a so-called birdcage coil. The latter is a volume coil, which is generally used in magnetic resonance imaging (MRI) under in vivo condition. The presented method therefore allows to effectively combine MRI and thermotherapy and is thus readily adaptable to existing imaging hardware.Nanomagnetic hyperthermia (NMH) is intensively studied with the prospect of cancer therapy. A major challenge is to determine the dissipated power during  One promising cancer therapeutic method is hyperthermia. It involves raising the temperature of the local environment of a tumour, which results in an induced cell death7 due to the large susceptibility of tumour cells to temperature as compared to healthy ones. In addition, local heating may enhance the efficiency of ionizing radiotherapy i.e. combining thermotherapy and conventional method is also a promising prospect. Hyperthermia using single domain magnetic nanoparticles (MNPs) is intensively studied; MNPs absorb energy from an external source that is usually an alternating magnetic field (AMF).Cancer is one of the major death causes worldwide, with several proven and being developed therapeutic methods10:cs and ms are the specific heat and the mass of the sample, respectively, mNP is the mass of the MNPs and T is its temperature. For in vitro conditions, the measurement of SAR is crucial for the design of novel materials with a capability for hyperthermia treatment. For in vivo studies, the most important quantity is the temperature of the tissue itself which depends on the absorbed power or the SAR therefore its knowledge is required for designing the thermal dosage.The central quantity in MNP based hyperthermia is the absorbed power. It is related to the specific absorption rate, SAR, or specific loss power, SLP through a normalization with the MNP mass. SAR determines the efficiency of power absorption per unit sample mass, whose knowledge is important to assess the chances of hyperthermia as clearly the uptake of MNPs is limited in the organism. The classical definition of SAR is based on calorimetric measurements and it is defined as13 or a more direct calorimetric method14. The earlier involves an electromagnetic modeling of the irradiation circuit and also the accurate knowledge of the magnetic properties of the ferrite material for the given irradiation frequency and magnitude of magnetic field. In addition, this method requires a highly homogenous AMF in a well defined geometry that calls for oversized irradiating coils and therefore a low efficiency of the input power. The calorimetric method requires to embed a non-metallic thermometer into the ferrite material itself or in the surrounding tissue. However, it is difficult to implement either of these methods in in vivo conditions and the accuracy of the modeling is limited. In addition, the conventional calorimetric method suffers from the so-called non-adiabatic condition15 as it deduces the dissipated power while attaining a finite temperature difference between the sample and its environment. However, heat loss through heat conduction, convection, radiative loss, or evaporation15 strongly limits the accuracy of this method.The conventional methods to determine the absorbed power in ferrites use either the measurement of magnetization curvesQ of the irradiation circuit. We show that a change in Q upon placing a ferrite sample in the resonator with respect to a suitable reference provides an accurate means of determining the absorbed power. In addition, the method works well for small sized irradiation coils, i.e. the required input power can be significantly lower than in conventional studies. We validate the method by a comparison of the absorbed power with the theoretically expected values and also by comparing the result on the absorbed power with the more standard calorimetric approach. We also studied the method for a birdcage coil. The importance of this type of resonator is that it allows homogeneous RF irradiation for animal models and is readily available in magnetic resonance imaging (MRI) instruments.Herein, we present a method which circumvents all these limitations as it allows to determine the power absorbed during ferrite based hyperthermia via a non-calorimetric method. It is based on monitoring of the quality factor, Q) after the ferrite sample is introduced with respect to a reference situation. In our proof of concept approach, we consider a resonator filled with water as reference, however a more realistic study should involve an phantom which is filled with an appropriate artificial tissue emulating (ATE) material17. In general, the resonator Q is the ratio of the energy stored in the resonator in the form of electromagnetic field and the power dissipated during a time period of the electromagnetic oscillation. As such, Q is an accurate measure of the power dissipated due to the different mechanisms, such as e.g. ohmic losses in the resonator material, dielectric losses in capacitive elements, radiation from an inductive element, or dissipation in a sample which is placed inside the resonator. A common method to the determine the resonator Q is to measure the power reflected from the resonator as a function of the irradiation frequency, which typically yields a Lorentzian profile, whose full width at half maximum gives Δf and Q = f0/Δf, where f0 is the resonator frequency18.The method to directly obtain the absorbed power in a hyperthermia relevant ferrite samples is based on measuring the change in the resonator quality factor . In view of this, the literature distinguishes the quality factor of an ideal, uncoupled resonator, or Q0 and that of a coupled (or loaded) resonator, or QL with Q0 can be measured with precision as for this case QL = Q0/2. In practice, a reflected power of about 1% (or in other words −20 dB) or smaller of the incoming power is usually considered as a critically coupled situation and we will also employ this threshold herein.This type of measurement inevitably requires the use of an external circuit  whose presence affects the measured Pferrite can be obtained by measuring the quality factor in the presence of the ferrite, Qferrite, and with the appropriate reference material, Qref. We emphasize that for both measurements a near critical coupling is required. We derive in the Supplementary Material that:Pinput is the power transmitted to the resonator. Eq.  to measure the absorbed power in the presence of ferrite in a sample and ii) for the measurement of SAR. In the absence of non-linear absorption effects, the power dissipated in the ferrite is proportional to the square of the RF magnetic field, which is proportional to the electromagnetic energy stored in the resonator. The latter quantity is proportional to the quality factor. We thus obtain for the resonator quality factor in the presence of the ferrite:k = 10−4 and also the absorbed power as a function of the resonator reference Q. Let σa denote the standard deviation of a = Pferrite/Pinput. This quantity solely depends on the resonator Q values with the ferrite and without, i.e. a = a according to Eq. , we obtain:Figure Q measurement is known. It is clear from Eq.  for a given sample loss. Figure Q reference resonator measurement. In practice, resonator Q values are limited to a few hundred in the radio-frequency range (1–100 MHz) due to various effects including electric dissipation and radiative losses21.This formula allows for a practical estimate of the absorbed power error in real life situations if the uncertainty of the from Eq. , that tht or a →  for a gia = Pferrite/Pinput as a function of sample mass, which is the result of the non-linear Eq. . The absorbed power was deduced from measuring the resonator quality factors for the samples according to Eq.  plays a crucial role in order to obtain the extra power dissipated due the presence of the ferrite material. The best choice of reference measurement is a sample of the same material (preferably with the same heat capacity) except for the absence of the ferrite material itself.We note that the reference measurement . The primary reason for its utility in MRI is that a birdcage coil sustains a homogeneous RF magnetic field over a relatively large sample volume26. The field direction is perpendicular to the coil axis when it is driven with a single RF signal however when driven with a quadrature RF signal, the bridcage coil produces a circularly rotating magnetic field, in contrast to e.g. a conventional solenoid or a surface coil, where the magnetic field is linearly polarized27.The so-called birdcage coil is a well-known type of RF resonators which are used extensivelyk = 1, mode sustains the most homogeneous RF magnetic field, therefore it can be used for MRI. The Q factor of this mode is indeed sensitive for the presence of a ferrite sample as Fig. The clear advantage of this resonator for thermotherapy purposes is that these are readily available as imaging coils and in fact it could be used for irradiation straight on. To test the applicability of our method for this type of resonator, we constructed a so-called low-pass birdcage resonator with 8 legs, which is shown in Fig. η. For the presented study, the sample was as small as filling only 2% of the birdcage volume, although the same volume would fill about 50% of the solenoid volume. Correspondingly, the sample induced Q factor change is much smaller. However, the most important property of our method is that it provides a sample or η independent information on the absolute value of the absorbed power. This is clearly demonstrated in Fig. Q factor change, using the method and the formula above. The result is robust and it shows that the dissipated power can be very accurately determined for the birdcage configuration when the filling factor is inevitably small.This type of coil is less sensitive to the same amount of ferrite sample than the solenoid due to the lower filling factor, σa (where we defined above a = Pferrite/Pinput) depends on the error of Q measurement. It was shown previously that the relative error of the Q measurement, σ(Q)/Q, is independent of the Q value and only depends on the applied method18. In our case, it is σ(Q)/Q ≈ 10−3. This, combined with our studies on the birdcage resonator, allows to give the limit of detection for the dissipated power for the given ferrite sample (Fe3O4) for the birdcage resonator. We employed a ferrite sample of 130 mg that gave a = Pferrite/Pinput = 0.030(1).We discussed above, that the statistical error of the dissipated power, a = 22σa. As a result, for the present birdcage coil, a sample of about 6 mg is the limit of detection for the absorbed power. This ferrite amount is enormous even for a practical in vitro study. The amount of required ferrite could be reduced either with the use of a ferrite with an SAR larger than that of Fe3O4 or with the refinement of the sensitivity of the present method. One has to consider that we employed a commercial Fe3O4 sample and magnetic nanoparticle materials with improved absorption, and as large as ~30 times larger SAR28, are available. This could substantially reduce the limit of absorbed power detection of our method to sample amounts as low as 0.2 mg. In vivo thermotherapy studies in mice employ a typical sample amount of 0.1–2 mg of magnetic nanoparticles30. This means that our method provides a sensitivity of the absorbed power measurement which could be used for in vivo thermotherapy studies.According to Eq. , we obtain-situ without replacing the sample. In such cases, calorimetry is known to have about ±5% error in determining the dissipated power15.Our above estimate of the accuracy of the dissipated power measurement considers stochastic error sources only, and disregards systematic errors related to e.g. reproducibility when replacing the samples. However, the present method definitely overperforms conventional calorimetric studies when small absorbed power variations are to be monitored We presented a non-calorimetric method which yields the absorbed power during nanomagnetic hyperthermia. This method, when combined with more conventional chemo- or radiotherapy, has growing significance in battling malignant tissues. It is based on the accurate measurement of the quality factor of resonators, that is used to sustain the irradiating radio frequency magnetic field. It represents an improvement over existing methods for similar purposes as these are either invasive or rely on an inaccurate modeling. The method allows to obtain the dissipated power in real-time and is not limited by the so-called non-adiabatic conditions of conventional calorimetric methods. We validate our method by comparing the determined dissipated power, with data obtained with the more conventional calorimetric approach. The method allows for an alternative measurement of the specific absorption rate. We also show that besides conventional solenoids, the method performs well on birdcage coils; these are volume coils which sustain a very homogeneous RF field and are important in magnetic resonance imaging. We envisage that our method paves the way for the use of existing imaging hardware for RF based hyperthermia.3O4. The producer supplied volume fraction is 3.6% and its density is We used a commercial, water-based ferrite solution (Ferrotec EMG 705) which contains magnetite, Fementclass2pt{minim31 which consists of a solenoid for the test-tube studies and a so-called birdcage coil which is employed25 in magnetic resonance imaging. High power/voltage trimmer capacitors (Voltronics Corp.) are used to match the circuits to 50 Ohm for efficient power transmission. The birdcage coil also contains fixed chip capacitors which set its resonant frequency.We studied two kinds of resonant circuits: a conventional tank circuitf0, and the linewidth, Δf  of the resonance curve. The quality factor is obtained as Q = f0/Δf. This also allows to average the reflected signal for a number of measurements, which enables to determine the mean and variation of the respective parameters.Quality factors were determined using RF reflectometry with a 180 degree hybrid junction (ANZAC HH107) duplexer. Figure 20 the quality factor of the ideal, strongly undercoupled (Q0 and that of the coupled (or loaded) resonator, QL. When the resonator is critically coupled (β = 1), i.e. there is no reflection from it (S11 = 0), QL = Q0/2. The losses in the resonator can be due to dielectric, ohmic, and radiative losses. The latter two dominates in our case as the capacitors have a high quality factor (about 1000). The loaded quality factor or QL is typically between 20–100 for a RF circuit, as in our case.The literature of resonators distinguishesQferrite and Qref, for the loaded quality factor of the resonator with and without the ferrite sample, respectively. This means that upon placing the sample under study into the resonator, the matching has to be readjusted to achieve zero reflection (S11 = 0). The power which is incident on the resonator, Pinput is divided between the loss in the resonator and the loss in the sample as, Pferrite, therefore we obtain for the latter quantity:We use the terminology, Pferrite = Pinput/2 when Qferrite = Qref/2, Pferrite ≈ 0 when Qferrite ≈ Qref, and Pferrite ≈ Pinput when Equation  returns The choice of a reference sample is crucial in our case. We found that the most appropriate reference measurement can be performed using a sample containing pure water only. The reason is that water itself can give rise to a slight change in the resonator properties.33.We studied nanoparticle based hyperthermia around the 20–30 MHz frequency range. Most hyperthermia studies use a somewhat lower frequency. Our choice was motivated by the frequency range of MRI  and the availability of well developed RF resonator techniques. In addition, we present a proof of concept i.e. the choice of frequency is somewhat arbitrary. We also note that an ongoing research focuses on the use of magnetic nanoparticles as an MRI contrast agent in addition to hyperthermiaWe used a synthesized signal generator (Siglent SDG1025) followed by a power amplifier (Amplifier Research 306781) for irradiating the sample. The output power was calibrated with a power meter (Mini-Circuits PWR-SEN-6GHS). Calorimetry was performed using an optical thermometer (Luxtron FOT LAB Kit) with an accuracy of 0.01 K and temperature values were read out in every second. This allowed to determine Supplementary Information


Aged people all over the world are prone to fall down accidentally and be injured with fracture, such as the rib fracture. To protect the elderly, the safety vest has been developed to protect them from being injured when falling down. To effectively protect the elderly, more analysis on the protective capacity of a safety vest under different situation are needed.Herein, a finite element model based on the computed tomography CT scanning data of a Chinese old female was built, and then used to simulate the process of falling down at different velocities. Analysis and comparison were done on the maximum shear stress, kinetic energy curves and internal energy curves with and without safety vest. The maximum shear stress indicated that the Abbreviated Injury Scale (AIS) 2+ injury risks of rib were 8%, 100% and 100% at the velocities of 1.5 m/s, 2.0 m/s and 2.5 m/s, respectively. The corresponding risks were lowered to 0%, 0% and 60% by the vest, respectively. Furthermore, the vest could absorb the internal energy resulted by the deformation of the thoracic osseous tissue by about 20%, thus decreasing the shear stress and the injury risk.It is concluded that the safety vest decreases the injury risk when the elderly fall down, thus protects them from being injured. Population aging has been a common problem all over the world. In 2008, the portion of the population older than 65 in the world was about 13%, and by 2056, it is predicted to be 24–25% –9, 11. BSince falling down in the elderly usually results in bad consequence, they are afraid and restricted to do exercise, lowering their life of quality. More seriously, they might suffer from injury or death by falling down accidently –14. HayeThere have been several researches working on developing devices to protect the elderly from being injured when falling down –21. AlmoIn the equestrian sports the riders can fall occasionally, and for the riders older than 50, the most frequent injury was rib fractures , 23. To The purpose of this study was to assess the risk of the rib fracture for the Chinese elderly when falling down, and the protective capacity of the safety vest by a finite element method to better describe the structure of the Chinese population, we constructed a new finite element model of the thoracic osseous tissue based on the computed tomography (CT) scanning data of a Chinese old female, instead of using the available finite element model like the total human model for safety (THUMS). Through our finite element model, the falling process was simulated to assess the risk of the rib fracture. Lastly, through comparing the computed results with and without safety vest, its protective capacity was then analyzed.Due to the fact that more women fell, and the incidence of the fracture is greater among women, we chose the CT scanning data from a Chinese female to construct our finite element model as shown in Fig. The CT scanning data was based on a 65-year-old Chinese female. The slice thickness was 0.620 mm and a total of 545 slices were scanned using a Philips/CT 256 scanner as shown in Fig. Many attempts have been made to use hexahedral elements to mesh the thoracic osseous tissue –27. HoweEach part of the geometry was meshed with tetrahedral elements as the cortical bone. Some researchers used the validated finite element models of the thorax with the element size of 4–5 mm –32, usinA simplified element model of a typical safety vest for horse riders was created . The maiThere were some reports on the material model and parameters of the human tissue based on the young cadaver, which has been validated by experiments –38. HereTo evaluate the protective capacity of the vest, the processes of falling down with and without vest at different velocities were simulated and compared, with the anterior part of the thorax impacting onto the floor. According to the literature , the velSix different simulations were accomplished in total, three without vest, and three with vest. During the process of falling down, the thoracic osseous tissue impacted the rigid wall at different velocities, and then rebound.Figure Figure After that, the KE began to decrease. Then, as the rebound of the system began, the KE values increased slowly. The minimum KE values were nearly zero, for instances without vest, the values were 0.194 J, 0.652 J and 0.761 J as the initial velocity increased. For instances with vest, they were 0.305 J, 0.697 J and 1.146 J. After that, as the system rebounded, the KE began to increase It is predicted that the rebound velocity would reach an approximate constant value lower than the initial value.The change of the IE values was just the opposite. At the beginning, the IE values were zero, and they began to increase when the contact between the system and the rigid wall occurred. At the maximum stress point, they reach the maximum value and then decreased. The maximum IE values for instances without vest were 7.981 J, 17.564 J and 26.032 J. As the vest was on, they were lowered to 6.271 J, 14.244 J and 21.788 J, respectively.During the biomechanical simulation of the human body, it is not realistic to describe material model and its parameters accurately. Therefore in simulations where fractures were possibly to occur. A better method was to assess the fracture and injury risk according to the stress distributions and deformations, rather than considering the fitted failure criterion. The maximum shear stress is an important factor when assessing the rib injury under impact , 31. In The high stress zones were mainly at the 1st, 4th–6th ribs where it is much easier to occur rib fractures. Li  found thThe influence of the vest on the value of KE was very limited, but that on the IE values was relatively significant. The IE indicated the elastic deformation resulted from the impact. Thanks to the vest, the IE value increased more slowly, and the maximum IE value at the three velocities were lowered by 22.42%, 18.90% and 16.30%, respectively. The decreased part was absorbed by the vest. Because the IE value indicated the deformation extent and thus the stress level, lower IE value means lower injury risk. We noted that the curve reaches a constant value rather than 0. It is because the motions of the anterior and posterior part of the system were not coherent, and the velocities of the different part of the model will not reach 0 at the same time. Therefore, the minimum value of the KE curve was not 0. In our models it was the elastic deformation and would disappear after an enough long time. But in reality there will be plastic deformation even if no fracture occurred, so this kind of energy difference will still exist and never disappear.From the curves without vest we could also see that at a relative high velocity (2.0 m/s and 2.5 m/s), there was a sudden “jump” in a short time before the maximum or minimum value reached. This might be the vibration caused by the contact between the bone and the rigid wall. This situation was not found at the low velocity of 1.5 m/s. Furthermore, the vest could be a buffer between the bone and the rigid wall and thus eliminate this vibration when it was on. In computing dynamics, it is possible to be unstable when a rigid body contact with a relative “soft” body at a high velocity. The vest was proven to be able to avoid this problem, but we still do not know whether this is valid at a higher velocity. Therefore, we think that avoiding the contact with rigid body could improve our results. Of course, the results we obtained now were still good enough under the reasonable simplifications.In conclusion, the vest does have the capacity of decreasing the injury risk when the elderly fall down. From the stress distribution we can find that the stress is localized. To improve the protective capacity of the vest, we can increase the thickness of the locations relating to the high stress, and decrease the thickness at the low stress location to make the vest more flexible.It should be noted that the results we got in this work could be used to predict the protective capacity of the safety vest even some simplifications have been used in our model. Further work will be done to optimize our model.There were several limitations that would affect the final results in this study. Firstly, to simulate the falling down process of a person, there were many factors should be considered, such as the body weight, accelerate and the angular accelerate caused by the gravity. In this study we simplified the models to have only the translation degree of freedom and a constant initial velocity, the other factors mentioned were not considered. In our future work we will consider these factors in our models progressively. Secondly, our models were constructed according to one single Chinese old female. Therefore, to be more accurate, more comparisons based on different objectives containing different properties are needed. Thirdly, during the falling process the organs and other soft tissues within the torso can also serve as the buffer, and therefore decrease the stress on the rib. We planned to add these soft tissue to improve the models. Lastly, the tied contact between the rib and the vertebra was just the approximate of the truth, leading to the virtual high stress at this location. Using better method to describe the joint will improve the result.In this study we constructed the finite element model based on the CT scanning data of a Chinese old female, and then used this model to simulate the process of falling down at different velocities. Analysis and comparisons were done between the results with and without a safety vest. The maximum shear stress indicated that the AIS2+ injury risks of rib were 8%, 100% and 100% at the velocities of 1.5 m/s, 2.0 m/s and 2.5 m/s. Those risks were lowered to 0%, 0% and 60%, respectively by the protection from a safety vest. Furthermore, the vest could absorb the internal energy resulted from the deformation of the thoracic osseous tissue by about 20%, thus reducing the shear stress and the injury risk. It is believed that the safety vest could decrease the injury risk when the elderly fall down, and thus protect them from being injured. The result provided a way of studying how to protect the Chinese elderly during the process of falling down.


Printing techniques are becoming increasingly prevalent in modern manufacturing. However, its biggest drawback is the limit in printing resolution. In this paper, we present solvodynamic printing as a novel printing system which aims to improve print resolution by incorporating an additional immiscible carrier solvent into the ink delivery system. The resolution is improved due to the solvent-solvent interactions between the ink and the carrier solvent which alter the contact angle of the ink on the substrate and limit the printed feature size. We demonstrate the proof of concept of solvodynamic printing by printing silver nanoparticle inks on a polyethylene naphthalate substrate. Silver nanoparticle tracks with widths of 35.2 ± 7.0 μm were achieved using a 300 μm nozzle. This is equivalent to 11.7 ± 2.3% of the nozzle diameter. The result shows great potential in solvodynamic printing as not many modern printing techniques can achieve such nozzle to feature size ratios. Traditionally, high resolution patterning is done via lithographic techniques which can achieve resolutions in the nanometer range6. However, such techniques are usually multi-step, require expensive equipment and low in throughput7. As a result, additive ink-based printing techniques like inkjet, flexographic and gravure printing have become increasingly used and sparked the development of the field of printed electronics. Such techniques have substantially lower costs and are compatible with large area high throughput processes10. In this article we use solvent-solvent interactions to create high resolution features for printed electronic applications using additive patterning techniques.The patterning of functional materials has become an important aspect of manufacturing technology as it is essential in the fabrication of devices like microelectronic integrated circuits, microelectromechanical systems, optoelectronic devices and DNA microassays11. During operation, ink droplets are ejected from the printhead onto the desired location on the substrate and therefore do not require the use of masks or templates. It is also a purely additive and non-contact process, which minimizes material waste and substrate damage12. Although many electronic devices have been successfully fabricated by inkjet printing, its application is limited by its current resolution of around 20–30 µm13. Therefore, it cannot be used in many applications which require sub-micron feature sizes.Among the family of additive printing techniques used in printed electronics, inkjet printing is one important method which has gained popularity due to its simplicity and low cost13. To further improve printing resolution, several different solutions have been developed. One such method is to pre-patterning the substrate structurally or chemically to alter the wetting behaviour of the ink droplets on different parts of the substrate15. However, the pre-patterning step generally requires another complex lithographic or mask-based high-resolution patterning technique prior to the printing step. This seems to undermine the overall advantage of using printing as a high throughput and simple patterning technique in the first place. Another way to improve the resolution is to reduce the dimensions of the ink droplets dispensed during printing. One such technique is electrodynamic inkjet printing which uses an additional electric field applied between the substrate and printhead to pull tiny droplets or jets from the printhead16. Electrodynamic inkjet printing has achieved resolutions as low as several microns. However, the addition of the electrical system adds to the complexity of the technique. Additional requirements on the ink’s electrical property must also be met as it affects the overall performance of the printing.The resolution of inkjet printing is limited by the printhead nozzle diameter, the rheology of droplet formation and ink spreading on the substrate17. We hypothesize that the incorporation of an immiscible carrier solvent into the ink delivery system can reduce the dimension of the ink ejected from printheads and alter the ink substrate interactions, leading to improvements in resolution. We name this technique “Solvodynamic printing” as the improvement in resolution arises from the molecular interactions between the ink and the immiscible carrier solvent. Here we demonstrate the proof-of-concept of solvodynamic printing by applying it to printing silver nanoparticle inks and focus on improving print resolution from a 300-micron nozzle.In this paper, we present a simple and novel solution to improve the printing resolution. The inspiration of this method is drawn from microfluidic microreactors, which synthesize materials within tiny droplets carried by an immiscible carrier solvent through a micron-scale channel18. Microfluidic chips are currently already used for many applications like fluid mixing, material synthesis and other biomedical applications22.In order to achieve good control over the fluid behaviour between two immiscible fluids, we employ the use of microfluidic chips in our printer setup. Microfluidic chips are devices with narrow channels used to manipulate the flow of small amounts of fluidsFigure 23. This effectively reduces its stream width below the physical limits of the channels and nozzles. We hypothesize that this would have a beneficial effect on print resolution.The presence of the carrier solvent confines the ink to flow through a smaller part of the microfluidic channel and therefore creates a flow focusing effect on the ink stream24. By increasing the carrier solvent pump rate (qc) relative to the ink pump rate (qi), the shear force exerted by the carrier solvent stream on the ink stream increases and deforms it to flow within a narrower region. We define the relative pump rate, Q on the resultant width of the ink stream. With increasing Q, the ink stream becomes narrower. A detailed explanation on the control of the ink flow within the microfluidic chip can be found in the Supplementary Information.The flow focusing effect is easily controlled by the relative pump rates of the two fluids25.The incorporation of the carrier solvent also alters the ink wetting behaviour on the substrate after deposition. Given that the ink is denser than the carrier solvent, the carrier solvent will be able to surround the ink droplet on the substrate as shown in Fig. 26. The contact angle is determined by the interactions between three sets of interfacial energies as shown in Fig. 27:σsa, σsi and σia are the substrate-air, substrate-ink and ink-air interfacial energies and θi is the resultant contact angle of the ink.The wetting behaviour of the ink, which is characterized by its contact angle, is important in printing as it is related to the area of contact between the ink and the substrate and therefore the dimensions of the resultant printed patternθ* is the new equilibrium contact angle of the ink when completely surrounded by the carrier solvent. σsc and σic are the interfacial tensions between the carrier solvent with the substrate and with the ink respectively.In solvodynamic printing, the wetting medium is changed from air to the carrier solvent. This changes two of the interfacial energies in the system and therefore alters the equilibrium contact angle of the ink. The Young’s equation can be modified to give:Therefore, with the appropriate choice of carrier solvent, the contact angle can be increased which reduces the degree of spreading leading to higher printing resolution. Intuitively, this means that by choosing an appropriate carrier solvent which has more favourable carrier-substrate interactions than ink-substrate interactions, the three-phase contact line will shift to reduce the ink-substrate interface area, leading to higher print resolution. An example of this is shown in Fig. θ and the cross-sectional area of the segment, denoted by A as follows:Based on the concepts introduced in the previous section, we set up a basic mathematical model to relate the achievable resolution of solvodynamic printing to the printing parameters. We first make a basic assumption that the printed ink lines are infinitely long and have uniform truncated circular cross sections as in Fig. A, can then be expressed in terms of ink loading per unit length along the printed line by considering the conservation of volume of ink:vs is the substrate’s velocity beneath the nozzle. Combining (3) and (4), we get a combined expression (5) which relates the printing resolution to the printing parameters. An additional fitting parameter k is added to take into account any systematic mismatch between the modelled results and experimental results.The cross-sectional area, This preliminary set up will be combined with experimental results in the subsequent sections to obtain a final semi-empirical model.In this section, we set out to understand how different printing parameters affect the resultant resolution of solvodynamic printing. The three main parameters of interest are the ink stream width within the microfluidic channel, ink surface tension and substrate surface energy.d, affects the width of the final printed line, denoted by w. By controlling the relative pump rate Q, silver nanoparticle ink streams with widths, d ranging from 34 to 200 µm were used to print silver lines on the substrate. Figure d. Figure Q = 0 and d = 200 µm). Without the use of carrier solvent, a printed line width of approximately 300 µm was achieved. As Q was increased and d decreased, we observed that the printed lines became thinner. This shows the proof-of-concept of solvodynamic printing as the use of carrier solvent improved the printing resolution. The value of w decreased to a minimum before increasing again at lower values of d.Our main hypothesis regarding solvodynamic printing is the improvement in resolution by focusing the flow of ink into a narrower stream. Therefore, we first study how the width of the ink stream within the microfluidic channel, denoted by 28. As a result, the width of the printed lines remains fixed even after the carrier solvent has evaporated. The uneven cross section profile of a printed line as shown in Fig. 29. Conversely, the profile of the control line in Fig. It is noted that since hexane is much more volatile than water, we would expect the carrier solvent to evaporate before the ink fully dries. In addition, the ink should only start drying after the surrounding carrier solvent has evaporated. If this was the case, the carrier solvent should have had no effect on improving the printing resolution. Therefore, it is likely that the contact line has been pinned by the silver nanoparticles upon the initial deposition. This is a common phenomenon in liquids with solutes or dispersed particles, which accumulate at the three-phase contact line due to inherent surface roughness or heterogeneity of the substratew. Therefore, we start by understanding the relation between the ink stream width, d and the contact angle of the ink, θ. We recall that the ink stream width is controlled by the ratio of carrier solvent to ink pump rate. Therefore, variations in d is related to the ratio of carrier solvent to ink deposited. Theoretically, we know that the contact angle increases from an initial θi in air to a new equilibrium, θ* under the carrier solvent. Additional experiments were done to elucidate how this change occurs. We monitored the contact angle of ink droplets as we gradually added hexane onto them. It was observed that the contact angle did not change abruptly from θi to θ* once the carrier solvent was added. Instead, there was an initial range whereby the contact angle increased gradually with the amount of hexane added until the maximum of θ*, beyond which no further changes occurred with further addition of carrier solvent. Unfortunately, the exact way which this increase occurred was difficult to observe. Therefore, we constructed linear approximations from the θi and θ* for each combination of inks and substrates to model the behavior between d and θ. This is depicted in Fig. d. The initial increase in θ corresponds to the decrease in w observed when d decreases from 200 μm to 125 μm as the term θ.From the model Eq. , we can w at lower values of d, we considered several other possible factors. We first noticed that a lower d corresponded to a higher ink velocity since the ink stream had a smaller cross-sectional area. Therefore, we considered the effects of additional spreading or splashing during the ink deposition due to a higher ink velocity from the nozzle by using a model by Ukiwe et al.30. However, the calculated variations in w were too small to explain the large variation observed from the experiments. In addition, the low Re and We of the system meant that the ink velocities were not high enough to overcome the viscosity and surface tension forces to generate this effect31. Next, we also considered the possibility of the ink droplet forming a more elliptical or paddle-shaped cross section under additional pressure from the additional hexane33. This was also dismissed upon further investigation as numerical simulations did not produce sufficient variation in w.To model the increase in qi, another parameter in the model equation. This is related to the discharge coefficient, Cd of the printer. The discharge coefficient is the ratio of the actual fluid discharge to the theoretical discharge of a fluidic system34. This mainly applies to systems which involves continuous stream or jets of fluids. The presence of the nozzle and constrictions from the connectors on the microfluidic chip reduces the flow rate of the fluids. Prior literature has established that the discharge coefficient has a positive relation with the Reynolds number of the fluid35. Since a lower d is related to a higher ink flow rate and Reynolds number, the discharge coefficient will be higher at lower values of d. To verify this effect, we performed separate experiments to measure the amount of ink dispensed by the printer over a prolonged period at a fixed ink pump rate with different ink stream widths. Indeed, we observed that more ink was being dispensed as the ink stream width was decreased as shown in Fig. qi is no longer constant, we modify the model equation by including Cd.Finally, we realized that the amount of ink dispensed was not constant which altered the ink pump rate θ and Cd at different values of d matches well with the experimental results.The empirical data from these experiments were then included in the main solvodynamic printing model and are shown in Fig. k had to be varied when fitting the modelled data for inks with different compositions. This would be investigated in the next section.From the model, we can see that the improvement in resolution is largely caused by the increasing contact angle of the ink in the presence of the carrier solvent. A further observation was that 25. However, when the printed features are lines instead of droplets, the surface tension plays another role. Lines of ink deposited onto the substrate during printing are inherently unstable due to higher total interfacial energy between the ink and its surroundings. They tend to break up into more energetically stable droplets37. Therefore, there is an optimal surface tension for inks to achieve high resolution and good stability.The second parameter of study is the ink surface tension. Inks with higher surface tension leads to larger contact angles and therefore less wetting and higher resolution can be achievedIn our study, the ink surface tension was controlled by mixing ethylene glycol (EG) as an ink co-solvent. EG was chosen as it is water soluble but immiscible with the hexane carrier solvent, and has a lower surface tension than water. Several inks with different EG-water composition were made and their calculated surface tensions are displayed in Fig. Q = 2 and 3, which achieved high resolutions from the previous experiments. Figure w should increase. Therefore, it is likely that other parameters in the printing process were also contributing to changes in w.The different inks were then used to print lines using 38. An increase in interfacial tension tend to create a straighter interface to reduce the overall interfacial tension while a decrease in interfacial tension will create a more curved interface as shown in the insets of Fig. From our experiments on the discharge coefficient of the printer, it was found that the discharge coefficient actually decreased with increasing ink surface tension shown in Fig. k changed with the different inks used as shown in Fig. w. To include this effect into the model, we defined a new parameter which we call the instability factor as the average ratio of the lengths between each consecutive printed features to the lengths of each printed feature. The experimental variation between the instability factor, denoted as f and the ink surface tension is shown in Fig. f). This factor was then included in our model in place of the original fitting parameter k to obtain the finalised form in Eq. From the previous section, we observed that the fitting constant The last parameter studied was the substrate surface energy. Generally, liquids are able to wet surfaces with higher surface energies better. Therefore, low surface energy substrates are used to achieve high resolution features. However, similar to the case for surface tension, substrates having too low surface energy will lead to instability in the printed lines. The effect of substrate surface energy on solvodynamic printing resolution should be quite predictable as it only affects the contact angle. Increasing the surface energy of the substrate will lead to increase in wetting of the ink and lead to poorer resolution.Q = 2, which were found to be optimal in previous sections.In our study, the PEN substrate’s surface energy was controlled by subjecting it to an ozone plasma treatment for different durations. The plasma treatment was necessary as the inks could not wet the untreated substrates at all. Figure From Fig. −1 while keeping the ink pump rate as 50 μl/hr. Thinner lines of approximately 40 μm were observed but the high substrate velocity seemed to be detrimental to the consistency of the system as the variance of the printed line widths increased dramatically. Therefore, the substrate velocity was lowered together with the ink pump rate to keep the ink loading low while avoiding high substrate velocities. We eventually settled on an ink pump rate of 20 μl/hr and substrate velocity of 22.5 mms−1. At these parameters, lines of approximately 35 μm with reasonable variance were printed. Any further decrease in the ink pump rate to substrate velocity ratio could not produce continuous lines probably due to the ink loading being insufficient.To further minimize the line widths to achieve the lowest resolution, we proceeded to adjust the ink pump rate to substrate velocity ratio, To determine a reliable value of the attainable resolution of solvodynamic printing, we employed a statistical approach and examined ten line segments printed using the optimized parameters as shown in Fig. In this paper, we have shown the proof-of-concept of using solvent interactions between the ink and an additional immiscible solvent as a mean to improve printing resolution. The improvement in printing resolution was primarily due to the increase in the ink contact angle with the substrate. The relation between different printing parameters and the achievable resolution was studied and a model was developed to explain the observed results. Using this technique, lines of silver nanoparticles with widths of approximately 35.2 ± 7.0 μm were achieved using a 300 μm diameter nozzle, equivalent to 11.7 ± 2.3% of the nozzle diameter. This result is promising and shows that there is still much potential for this technique to improve. With further downsizing of the system, even higher resolution can possibly be achieved. In addition, this technique may also be compatible with other resolution-improving techniques to further boost the achievable resolution of additive printing technology.Silver nitrate, ethylene glycol, sodium chloride and polyvinylpyrrolidone (PVP) with average molecular weight of 55000 are purchased from Sigma Aldrich.39. In a typical synthesis, 0.0955 g of silver nitrate was dissolved in 0.75 ml of ethylene glycol. In a separate glass vial, 1.0 g of PVP was dissolved in 10 ml of ethylene glycol and heated to 135 °C with magnetic stirring. 20 µL of a 0.15 M aqueous sodium chloride solution was then added to the PVP solution. Once the solution is mixed homogenously, the silver nitrate solution was added and left for 1 hour. After the reaction, the nanoparticles were washed with methanol and centrifuged at 6000 rpm. This is repeated 3 times before finally redispersing the nanoparticles into water to form an aqueous ink.Silver nanoparticles used in this study were synthesized via the polyol processTwo syringe pumps (Harvard Apparatus Model ‘11’ Plus 70-2208) were used to inject the silver nanoparticle ink and carrier solvent into a microfluidic Y-junction chip (Cellix Ltd) which has a rectangular channel with width of 200 µm and depth of 50 µm with a hydrophobic wall coating. The fluids exited the chip through a 300 µm inner diameter outlet pin which also acted as a nozzle. The pin was positioned 100 µm above a polyethylene naphthalate (PEN) substrate which was placed on a computer-controlled translational stage (Thorlabs DDSM50). The substrates were treated with ozone-plasma prior to the printing using UV-Ozone cleaner (Ossila). After the printed silver nanoparticle lines were dried on a hotplate at 110 °C for 10 mins.40.The dimensions of the printed features and the ink stream width within the microfluidic channel were characterized by a digital microscope (DinoLite). The cross section profile of the printed lines were obtained using a Veeco DekTak 6M stylus profiler. The ink surface tension and substrate surface energy were determined by taking contact angle measurements and analyzing them using the Equation of States approach developed by Kwok and NeumannSupplementary Information


To investigate long length scale structures using neutron scattering, real space techniques have shown certain advantages over the conventional methods working in reciprocal space. As one of the real space measurement techniques, spin echo modulated small angle neutron scattering (SEMSANS) has attracted attention, due to its relaxed constraints on sample environment and the possibility to combine SEMSANS and a conventional small angle neutron scattering instrument. In this report, we present the first implementation of SEMSANS at a pulsed neutron source and discuss important corrections to the data due to the sample absorption. These corrections allow measurements made with different neutron wavelengths and SEMSANS configurations to be overlaid and give confidence that the measurements provide an accurate representation of the density correlations in the sample. SAS works in reciprocal space and measures the deflection of collimated radiation away from its initial trajectory8. Hence it becomes more and more challenging as the scattering angle decreases. For neutrons, the neutron spin provides us with another option to label the neutron’s trajectory change with high precision. In a given magnetic field, the neutron spin will execute a motion called Larmor precession9, and the accumulated Larmor phase is given byγ is the gyromagnetic ratio of the neutron, m is its mass, h is the Planck constant, λ is the neutron wavelength and FI is the field integral along the neutron trajectory. Using a neutron spin analyzer, the projection of the polarization vector that is parallel to the analyzing direction can be selected and analyzed, yielding the cosine of the Larmor phase Small angle scattering (SAS), using either X-ray or neutrons, has made important measurements of the structures of many materials including polymers10, called neutron spin echo (NSE). It is typically used to label the neutron energy change in quasi-elastic scattering with high resolution. To label the neutron momentum or trajectory change in elastic neutron scattering, two methods have been implemented, spin echo small angle neutron scattering (SESANS)11 and spin echo modulated small angle neutron scattering (SEMSANS)15. Though the similarities and difference between these two techniques have been discussed before14, some important details about data analysis, especially for time-of-flight (TOF) measurements, have not been discussed.The first technique of this type was introduced by F. Mezei in 197216. There are several ways to implement the tilted field boundary, including radio frequency (RF) flippers17, permalloy films18 and magnetic Wollaston prisms (WP)22. Taking the WP as an example, the SESANS setup is shown in Fig. The magnetic field boundary of NSE is perpendicular to the beam direction, which means the Larmor phase is not sensitive to any trajectory change to the first oder. The principle of SESANS is to introduce a tilted magnetic field boundary with different field directions on the two sides of the field boundary to introduce the sensitivity to the neutron trajectory changePb, Ps are the polarization measured with the blank to remove the instrumental errors and sample respectively. ξ is termed spin echo length, which is the length scale accessed in real space. t and k0 denote the sample thickness and incoming neutron wave vector respectively. G(ξ) is the correlation function of the sample, as given in equation . SESANS is only sensitive to the direction parallel to the gradient of the magnetic field integral  than the first one (B1). Due to the field difference between the two WPs, the Larmor phase (Φ) varies across the transverse direction (y) of the neutron beam, which will introduce a modulation of polarization vectors as B1L1 = B2L2 with L1 and L2 being the distances of the WPs to the detector, the induced Larmor phase dispersion on every single point of the detector can be minimized such that a high visibility intensity modulation can be obtained14.For SEMSANS, as shown in Fig. 14. When tracing back from a specific pixel of the detector, neutrons with different momentum transfers will be detected and thus Fourier transformed by the modulation, as shown in Fig. As and Ab respectively, is given by equation  instead are measured as equation  can be rVs and Vb can be obtained by performing a sinusoidal fit. Γ is the ratio of the shim intensity between the sample and the blank, i.e. T is obtained by integrating the neutrons over the whole detector or by using a monitor placed directly behind the sample, as shown in entclass1pt{minimaVs, Vb is to extract the amplitude of the sinusoidal fit for a given modulation. Another option is to use standard deviation (SD) of the modulation as a quick way to evaluate the data. In the following section, the analysis with standard deviation will be performed and compared with fitting.Also, the typical way of obtaining 20, instead of the usual RF flippers installed on LARMOR, were used to generate the intensity modulations on the detector. For the following discussions, there are two experiments involved, labeled as 2016 and 2018. The samples for both experiments are 1 mm thick of colloidal monodisperse spherical poly(methyl methacrylate) (PMMA) nanoparticles dispersed in deuterated d8-dodecane with a volume fraction of 40%. The surfaces of the PMMA particles are coated with a (12-hydroxystearic acid) PHSA polymer brush to prevent aggregation26. The blank used for the normalization of both experiments is air. To measure the intensity modulation, a micro-channel plate detector27 was used in TOF mode. The spatial resolution of the detector is 55 μm over an active area of 28 × 28 mm27. Two different analyzers were used during these two measurements. For 2016, a 3 He analyzer was used and placed after the sample to provide a uniform analyzing power for all the neutron trajectories. For 2018, a supermirror analyzer was used, which was placed before the sample to avoid the nonuniformity of the analyzing power. The collimation of the beam was defined with two pin holes separated by ~5 m with one upstream (20 mm × 20 mm) and one just in front of the sample (10 mm × 10 mm). For both measurements, the tuning of B1L1 = B2L2 was achieved by scanning the ratio B1 − B2) the same. Benefiting from the multiple wavelengths available at LARMOR, a wide range of spin echo length can be accessed simultaneously at a single setting of magnetic fields in the WPs.The experiments were conducted on the LARMOR beamline of the ISIS Neutron and Muon Source based at the Rutherford Appleton Laboratory in Oxfordshire, UK. Superconducting magnetic Wollaston prismsz within the yellow box for each wavelength. The 1D polarization plots for all the wavelengths are then stacked together into a 2D map for both the air and PMMA, as given in Fig. Figure 28. As one can see from Fig. T, as labeled by PSD in the figure. After being corrected by equation (nm26. This is very close to the hydrodynamic diameter of the particles (200 nm) obtained from dynamic light scattering.For the 2016 experiment, the measurements were conducted at five different magnetic field settings, which are listed in the Fig. Also, the feasibility of using the standard deviation for the calculation of the correlation function is also evaluated. As we can see from Fig. T, the same experiment was repeated in 2018 with a neutron monitor employed right after the sample. When the SEMSANS measurements were finished, the relative attenuation efficiency of the sample was measured again with respect to the blank using the monitor. The monitor is made of neutron active scintillator, which was designed to sample a high flux, homogenous neutron beam. As shown in Fig. T obtained by integrating the whole detector, the same as the previous section. Since the experiments were conducted two years later compared with Fig. T obtained using the monitor. For both cases, the curves at different field settings overlap with each well. The difference is observed in the level of the curves at large spin echo lengths. The data corrected with the detector shows higher values, which means the measured total scattering of the sample is lower if we have to correct the data with the detector.To verify the effectiveness of correcting the data by integrating the detector to obtain T measured by either integrating the whole detector or the monitor. Since the scattering angle increases as wavelength, more neutrons with long wavelength will fall off the detector of limited size than at short wavelength. Therefore the value T measured by the monitor is consistently higher especially for long wavelength neutrons and yields a lower value of σ = λ2tΔρ2ϕ(1−ϕ)ζ, where t is the sample thickness, Δρ is the difference in scattering length densities between the particles and solvent ϕ is the volume fraction and ζ denotes the characteristic length scale of the sample. This means for the measurements where this level matters, using the detector with a restricted size can result in a wrong answer. While for the measurements, where only the correlation distance of the particles matters, either method can be used. To make sure one can always get the best results, a monitor is always preferred right after the sample.Figure We have discussed the feasibility of correcting the SEMSANS data to remove the contribution from the difference of the absorption efficiency between the sample and blank. The data correction is especially critical for TOF SEMSANS due to the wavelength dependence of the absorption efficiency. By employing the correction, the correlation function measured at various settings of spin echo length can overlap with each other. Though a monitor is always preferred for the measurement of the attenuation efficiency of the sample, the same high spatial resolution detector, used for the measurement of the modulation, can also be used provided its area is large enough to capture all the small angle scattered beam. But in reality, a detector with a combination of high spatial resolution and large active size is always difficult.The correction method we have described is also applicable to any other scattering technique based on the intensity modulation, including the Talbot-Lau method and modulated intensity with zero effort (MIEZE) for quasi-elastic scattering. For the Fourier transform of both SESANS or SEMSANS, the effect of limited Q range will be presented later.2O will accelerate neutrons, causing unexpected faster neutrons29. The percentage of the thermalized neutrons will increase with neutron wavelength. This means that shorter wavelength neutrons may be preferable for experiments with samples that are good moderators such as H2O.For the correction method discussed here, we are assuming the scattering is purely elastic and the neutrons will maintain their energy. But for some samples, the neutrons might gain or lose energy by inelastic scattering with the sample, which is also known as thermalization or moderation. It has been shown that, the thermalization process of cold neutrons within a liquid such as H