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The transport of water and protons in the cathode catalyst layer (CCL) of proton exchange membrane (PEM) fuel cells is critical for cell performance, but the underlying mechanism is still unclear. Herein, the ionomer structure and the distribution/transport characteristics of water and protons in CCLs are investigated via all-atom molecular dynamics simulations. The results show that at low water contents, isolated water clusters form in ionomer pores, while proton transport is mainly via the charged sites of the ionomer side chains and the Grotthuss mechanism. Moreover, with increasing water content, water clusters are interconnected to form continuous water channels, which provide effective paths for proton transfer via the vehicular and Grotthuss mechanisms. Increasing the ionomer mass content can enhance the dense arrangement of the ionomer, which, in turn, increases the density of charge sites and improves the proton transport efficiency. When the ionomer mass content is high, the clustering effect reduces the space for water diffusion, increases the proton transport path, and finally decreases the proton transport efficiency. By providing physics insights into the proton transport mechanism, this study is helpful for the structural design and performance improvement of CCLs of PEM fuel cells. [ABSTRACT FROM AUTHOR]

The breakup of liquid threads into droplets is crucial in various applications, such as nanoprinting, nanomanufacturing, and inkjet printing, where a detailed understanding of the thinning neck dynamics allows for a precise droplet control. Here, the role of surfactant in the breakup process is studied by many-body dissipative particle dynamics, in particular, the various regime transitions and thread profiles, shedding light on molecular-level intricacies of this process hitherto inaccessible to continuum theory and experiments. Moreover, the role of surfactant in the most unstable perturbation, the formed droplet size, and surfactant distributions have been unraveled. As surfactant concentration rises, both the wavelength and time to breakup steadily increase due to the lowering of surface tension below the critical micelle concentration (CMC) and viscous effects introduced by micelles above the CMC. These changes prior to the breakup lead to larger droplets being formed in cases with higher surfactant concentration. We also compared the thinning dynamics to existing theoretical predictions, revealing that the surfactant-laden breakup starts at the inertial regime and transitions into the thermal fluctuation regime when the concentration is increased. Thus, we illuminate the hitherto poorly investigated and intricate breakup process of surfactant-laden liquid threads driven by thermal fluctuations, contributing to a deeper understanding of this process at molecular scales. [ABSTRACT FROM AUTHOR]

With most of the focus to date having been on the coalescence of freely suspended droplets, much less is known about the coalescence of sessile droplets, especially in the case of droplets laden with surfactant. Here, we employ large-scale molecular dynamics simulations to investigate this phenomenon on substrates with different wettability. In particular, we unravel the mass transport mechanism of surfactant during coalescence, thus explaining the key mechanisms present in the process. Close similarities are found between the coalescence of sessile droplets with equilibrium contact angles above 90° and that of freely suspended droplets, being practically the same when the contact angle of the sessile droplets is above 140°. Here, the initial contact point is an area that creates an initial contact film of surfactant that proceeds to break into engulfed aggregates. A major change in the physics appears below the 90° contact angle, when the initial contact point becomes small and line-like, strongly affecting many aspects of the process and allowing water to take part in the coalescence from the beginning. We find growth exponents consistent with a 2/3 power law on strongly wettable substrates but no evidence of linear growth. Overall bridge growth speed increases with wettability for all surfactant concentrations, but the speeding up effect becomes weaker as surfactant concentration grows, along with a general slowdown of the coalescence compared to pure water. Concurrently, the duration of the initial thermally limited regime increases strongly by almost an order of magnitude for strongly wettable substrates. [ABSTRACT FROM AUTHOR]

Computer simulation is an important tool for scientific progress, especially when lab experiments are either extremely costly and difficult or lack the required resolution. However, all of the simulation methods come with limitations. In molecular dynamics (MD) simulation, the length and time scales that can be captured are limited, while computational fluid dynamics (CFD) methods are built on a range of assumptions, from the continuum hypothesis itself, to a variety of closure assumptions. To address these issues, the coupling of different methodologies provides a way to retain the best of both methods. Here, we provide a perspective on multiscale simulation based on the coupling of MD and CFD with each a distinct part of the same simulation domain. This style of coupling allows molecular detail to be present only where it is needed, so CFD can model larger scales than possible with MD alone. We present a unified perspective of the literature, showing the links between the two main types of coupling, state and flux, and discuss the varying assumptions in their use. A unique challenge in such coupled simulation is obtaining averages and constraining local parts of a molecular simulation. We highlight that incorrect localisation has resulted in an error in the literature. We then finish with some applications, focused on the simulation of fluids. Thus, we hope to motivate further research in this exciting area with applications across the spectrum of scientific disciplines. [ABSTRACT FROM AUTHOR]

Although Lycium barbarum is a valuable medicinal food, it is unfortunately not easy to store due to its high moisture content. In order to maximize the preservation of the medicinal value of L. barbarum, drying is increasingly used for its storage. Motivated by the need to better understand and improve the drying of L. barbarum, in this study, the VFD (vacuum freeze drying) and HD (hot air drying) are employed as drying processes, and the rehydration ratio, drying time, specific energy consumption, sensory characteristics, moisture ratio, energy consumption, moisture mobility, and amino acids and flavonoids content of dried L. barbarum are comprehensively analyzed. The results show that, after VFD, L. barbarum has lower hardness (71.53 ± 1.68), higher rehydration ratio (1.31 ± 0.02), larger volume, plump, round and smooth surface, and similar color to the fresh L. barbarum. At the same time, compared with the sample after HD, the moisture ratio of L. barbarum after VFD is lower (1.4 ± 0.1), and its content of free water is almost negligible (2.2%). In addition, the amino acids and flavonoids content of L. barbarum after VFD are closer to those of the fresh product (133.7 μmol/g, 5.69 mg/g). Based on the analysis of the drying effect on the physical and chemical properties of L. barbarum, we conclude that VFD is a promising processing technology. However, HD proves to greatly reduce the specific energy consumption and drying time when raising the temperature appropriately, whereas VFD could not achieve the same result by increasing the heating rate of VFD. Practical applications: In this paper, based on the mechanism of different drying methods, the quality of L. barbarum products after drying was compared, and the water distribution in the drying process was analyzed. It is found that using VFD technology to dry L. barbarum can improve the production efficiency of L. barbarum industry and the quality of L. barbarum products. Meanwhile, the research results can provide reference for the optimization of VFD system and the scientific formulation of process parameters. [ABSTRACT FROM AUTHOR]

As one of the most fascinating confined water/ice phenomena, two-dimensional square ice has been extensively studied and experimentally confirmed in recent years. Apart from the unidirectional homogeneous square icing patterns considered in previous studies, the multidirectional partitioned square icing patterns are discovered in this study and characterized by molecular dynamics (MD) simulations. Square icing parameters are proposed to quantitatively distinguish the partitioned patterns from the homogeneous patterns and the liquid water. The number of graphene monolayers n is varied in this study, and the results show that it is more energetically favorable to form partitioned square icing patterns when the water molecules are confined between graphite sheets (n ≥ 2) compared to graphene (n = 1). This phenomenon is insensitive to n as long as n ≥ 2 because of the short-range nature of the interaction between water molecules and the carbon substrate. Moreover, it is energetically unfavorable to form partitioned square icing patterns for a single layer of water molecules even for n ≥ 2, verifying that the interaction between layers of water molecules is another dominant factor in the formation of partitioned structures. The conversion from partitioned structure to homogeneous square patterns is investigated by changing the pressure and the temperature. Based on the comprehensive MD simulations, this study unveils the formation mechanism of the partitioned square icing patterns. [ABSTRACT FROM AUTHOR]

Droplet coalescence is ubiquitous in nature and, at the same time key to various technologies, such as inkjet printing. Herein, this study reports on the coalescence of polymer droplets with different chain lengths coalescing on substrates of different wettability. By means of molecular dynamics simulations of a coarse‐grained model, it is found that the rate of bridge growth is higher in the case of droplets with smaller contact angles (more wettable substrates) and decreases with the increase of the chain length of the polymers. Different behavior is also identified in the dynamics of the approach of the two droplets during coalescence with the substrate wettability playing a more important role compared to the chain length of the polymers. While the dynamics of the droplet are greatly affected by the latter parameters, the density profile and flow patterns remain the same for the different cases. Thus, this study anticipates that it provides further insights into the coalescence of liquid polymer droplets on solid substrates with implications for relevant technologies. [ABSTRACT FROM AUTHOR]

The performance of the air-cooler in the refrigeration system plays a key role in improving the energy efficiency. Here, the plenum chamber coefficient was defined to study a possible way of enhancing the refrigeration system performance. In order to investigate the influence of the plenum chamber coefficient on the frosting and the cooling system of the chillers, three plenum chamber coefficients (0.74,0.97,1.2) were studied under high humidity. The temperature decreasing curve of the environmental chamber, the velocity distribution of the air cooler, and the frost accumulation under and on the air-cooler were analysed. The results show that the frost accumulation on the tube will increase with the increase of the plenum chamber coefficient and the frost accumulation on the fin will increase at first and then decrease with the increase of the plenum chamber coefficient. On the whole, in this study, frosting is the most obvious when the plenum chamber coefficient was 0.97

In order to gain a deeper knowledge of the heat and mass transfer processes in the case of starchy fruits and vegetables during microwave drying, we have conducted an experimental study based on a developed microwave apparatus for drying potato. The changes in the temperature, drying rate, moisture content, thermal parameters, moisture state and distribution of the potatoes samples during the microwave drying process were obtained. The results show that, according to the temperature changes of the samples dried at different microwave powers (600 W, 800 W, 1000 W), the microwave drying process can be divided into three stages: heating stage, constant temperature stage, and cooling stage. The moisture content reduction of the samples in the constant temperature stage accounted for more than 90% of the total reduction. Meanwhile, as the microwave drying process goes by, the thermal conductivity and thermal diffusivity of the samples both decrease first and then increase, while the volumetric heat capacity presents an opposite trend. Moreover, the range and amplitude of transverse relaxation time of the samples reduce over the drying time, which confirms the existence of massive, outward moisture-migration in the dried samples. The loss of the weakly bound water in the moisture-migration, which is closely related to the starch, has a significant effect on the heat and mass transfer in the dried samples. [ABSTRACT FROM AUTHOR]

The performance of an electronic radiator filled with metal foam with a porosity of 96% was studied. The effect of the factors including the flow rates, the pores per linear inch (PPI) and the numbers of fins was analyzed. The results show that the electronic radiator with metal foam reflects a stronger ability of the heat transfer compared to the electronic radiator without metal foam. With the increase in the flow rate between 10 L/h and 60 L/h, the heat transfer coefficient of both of the two electronic radiators will be improved, but it is also dependent on the number of fins. In this study, we find that the heat transfer coefficient first increases and then decreases with the number of fins. The optimum number is three. As for the effect of the PPI, the higher the PPI, the larger the heat transfer coefficient, while the pressure drop always increases with the flow rates' increase, the pores per linear inch (PPI) and the numbers of fins. [ABSTRACT FROM AUTHOR]

A decline in natural snow due to global warming increases the reliance of ski resorts on artificial snow. To improve the formation rate and snow amount of artificial snow under any environmental conditions, it is necessary to use atomization technology that maintains the size of liquid drops of the snow-making spray field below a critical size, thus maximizing the number of freezing droplets into ice crystals. Here, we study the effects of the gas–liquid mass-mixing ratio (GLR) and the spacing (d) between two nozzles on the characteristics of the snow-making spray field produced by atomization technology for GLR of 0.05, 0.10, 0.15, and 0.2 and d of 5, 10, 15, and 20 cm. The measurement of the morphology, velocity, and particle-size distribution of the snow-making spray field is conducted by a laser particle-size measuring instrument, particle image velocimetry, and a high-speed camera. Our results show that both the spray cone and the intersection angles increase with the increase in the GLR in a two-nozzle arrangement, while the degree of interference between the nozzles increases with the decrease in spacing, d. We also find that the inward offset of the spray is the main reason for the increase in the intersection angle and the interference area and the deviation of the spray midline. In addition, we compare the characteristics of the spray field produced by the single and the two-nozzle systems. We conclude that the flow-field velocity of the single nozzle is smaller than that of the two nozzles and the velocity gradient of the liquid drops in the two-nozzle flow field varies greatly. The maximum vorticity (Max = 147) in the case of the two-nozzle arrangement is much larger than that of the single nozzle. Moreover, by analyzing the two-nozzle flow field at different spacings, we find that the particle-size distribution is most uniform for GLR = 0.10 and d = 15 cm. Thus, our study elucidates the characteristics of spray fields suitable for snow-making toward achieving higher quality and more efficient snow-making processes. [ABSTRACT FROM AUTHOR]

Droplet evaporation on rough substrates is ubiquitous in nature and has numerous technological applications, such as coating. To study the evaporation characteristics of nanodroplets on substrates with different rough structures, molecular dynamics simulations have been performed. Based on the Wenzel wetting model, three systems with different rough structures were constructed, in which the ratios of contact area (RCA) between substrates and nanodroplets were 33.3%, 50%, and 66.6%, respectively. The results indicate that the evaporation pattern of the nanodroplet in the system with RCA = 33.3% is the mixed mode, the evaporation pattern of the nanodroplet in the system with RCA = 50% is the constant-contact-radius mode, and the evaporation pattern of the nanodroplet in the system with RCA = 66.6% is the constant-contact-angle mode. Meanwhile, when the temperature of substrates (T) is suddenly raised from 0.67 to 0.83 ε/kB, followed by a corresponding rise in the temperature of nanodroplets, we find that temperature and evaporation rates of the nanodroplets increase with the increase in RCA in this period. Changes in RCA that lead to the changes in substrate wettability affect the heat transfer between nanodroplets and substrates. In particular, the more the substrate wettability, the higher the heat transfer efficiency. Moreover, the wettability of the substrate in the systems with RCA = 50% and 66.6% promotes the heat and mass transfer in the three-phase contact line region of the nanodroplets during the evaporation process, while the non-wettability of the substrate in the system with RCA = 33.3% suppresses the local heat and mass transfer. This work offers further knowledge of droplet evaporation on rough substrates, which, in turn, provides a reference for the process optimization of its technological applications. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols can consist of inorganic and organic substances, including surfactants at a significant concentration. Importantly, the latter can reduce the surface tension at the liquid–vapor surfaces, where they preferentially adsorb due to their amphiphilic structure. As a result, processes such as droplet coalescence, development of precipitation, and ultimately cloud lifetime may depend on the presence of surfactants in the aerosols. Here, we present a numerical model for cloud droplet formation, which is based on the Lagrangian particle-based microphysics-scheme superdroplet method and takes into account the presence of the surfactant in the droplets. Our results show that surfactant facilitates cloud formation by increasing the number and size of activated droplets, which concentrate at the bottom of the cloud, while the largest droplets are concentrated at the top of the cloud. This indicates a circulation of droplets that involves activation and growth processes from the bottom of the cloud toward the top. Moreover, our conclusions are independent of the particular approach used for modeling the diffusion of Eulerian variables due to the subgrid-scale turbulence. We anticipate that our results will enrich our understanding of the role of surfactants in the behavior of atmospheric aerosols and, importantly, will pave the way for further developments in the numerical modeling of systems with surfactants at macroscopic scales. [ABSTRACT FROM AUTHOR]

The liquid–vapor interface remains a key point for improving the heat transfer efficiency of heat exchangers. In this paper, we report experimental results on the evaporation of ethanol in capillary tubes of different internal cross-sections as the bulk meniscus recedes inside the tube, since the chosen conditions allow for the development of a liquid film along tube's internal corners due to the presence of capillary forces. To achieve our aims, we used 3D video microscopy to monitor the behavior of the meniscus during evaporation and, also, calculated the capillary pressures for three channels with different cross-sectional shapes each, namely circular, square and equilateral triangle cross-sections, for three solid–liquid contact angles, i.e., 30°, 60°, and 90°. At the same time, we have combined infrared thermal imaging technology with particle imaging velocimetry (PIV) visualization technology to analyze the severe Marangoni convection at the included angle. To this end, the interfacial temperature distribution of Marangoni convection was measured by means of an infrared camera and the flow pattern by means of PIV, respectively, through the plane in various views. For capillary tubes with included angle, the temperature gradient at the meniscus reaches its maximum at the included angle. Moreover, both temperature and convective gradients exist in the liquid film with included angle. When the angle of the capillary tube is smaller, the capillary effect will increase, leading to the formation of a thicker film. Our investigation constitutes an "extreme" experiment of Marangoni flow at the corners of the capillary tube. Due to the capillary forces, liquid films can rise in the corners of polygonal tubes up to the entrance of the tubes. Furthermore, the wetting force of ethanol at the angle of the triangle tube is much larger than that in the case of the square tube. Finally, the Marangoni vortex flows from the relatively high-temperature region in the center of the liquid towards the included angle, where evaporation is faster and the recharge of material and energy is more concentrated. We anticipate that our study sheds light into the meniscus shape and Marangoni flow in capillary tubes of with cross-sections of different shapes, which is of fundamental importance for various applications and in particular microfluidic systems. [ABSTRACT FROM AUTHOR]

Droplet evaporation is fascinating, ubiquitous, and relevant for a wide spectrum of applications, such as printing, drug testing, coating, and biomedical diagnosis. In this paper, the effects of substrate temperature (30 °C, 47 °C, 64 °C, 81 °C) and 20 nm Al2O3 nanoparticles mass fraction (0.05%, 0.1%, 0.2%) on the nanoparticles distribution mechanisms and deposition patterns during the evaporation of Al2O3-H2O nanofluid droplet were experimentally investigated. An experimental device was designed and realized. The self-assembly and Marangoni forces were first highlighted and compared. Two coffee-ring and inner-ring patterns were then identified and analyzed. The evaporation process characteristics were finally determined and discussed. [ABSTRACT FROM AUTHOR]

The aggregation of Aβ42 peptides is considered as one of the main causes for the development of Alzheimer's disease. In this context, Zn2+ and Cu2+ play a significant role in regulating the aggregation mechanism, due to changes in the structural and the solvation free energy of Aβ42. In practice, experimental studies are not able to determine the latter properties, since the Aβ42–Zn2+ and Aβ42–Cu2+ peptide complexes are intrinsically disordered, exhibiting rapid conformational changes in the aqueous environment. Here, we investigate atomic structural variations and the solvation thermodynamics of Aβ42, Aβ42–Cu2+, and Aβ42–Zn2+ systems in explicit solvent (water) by using quantum chemical structures as templates for a metal binding site and combining extensive all‐atom molecular dynamics (MD) simulations with a thorough solvation thermodynamic analysis. Our results show that the zinc and copper coordination results in a significant decrease of the solvation free energy in the C‐terminal region (Met35‐Val40), which in turn leads to a higher structural disorder. In contrast, the β‐sheet formation at the same C‐terminal region indicates a higher solvation free energy in the case of Aβ42. The solvation free energy of Aβ42 increases upon Zn2+ binding, due to the higher tendency of forming the β‐sheet structure at the Leu17‐Ala42 residues, in contrast to the case of binding with Cu2+. Finally, we find the hydrophobicity of Aβ42–Zn2+ in water is greater than in the case of Aβ42–Cu2+. [ABSTRACT FROM AUTHOR]

We use a standard bead-spring model and molecular dynamics simulations to study the static properties of symmetric linear multiblock copolymer chains and their blocks under poor solvent conditions in a dilute solution from the regime close to theta conditions, where the chains adopt a coil-like formation, to the poorer solvent regime where the chains collapse obtaining a globular formation and phase separation between the blocks occurs. We choose interaction parameters as is done for a standard model, i.e., the Lennard-Jones fluid and we consider symmetric chains, i.e., the multiblock copolymer consists of an even number n of alternating chemically different A and B blocks of the same length NA = NB = N. We show how usual static properties of the individual blocks and the whole multiblock chain can reflect the phase behavior of such macromolecules. Also, how parameters, such as the number of blocks n can affect properties of the individual blocks, when chains are in a poor solvent for a certain range of n. A detailed discussion of the static properties of these symmetric multiblock copolymers is also given. Our results in combination with recent simulation results on the behavior of multiblock copolymer chains provide a complete picture for the behavior of these macromolecules under poor solvent conditions, at least for this most symmetrical case. Due to the standard choice of our parameters, our system can be used as a benchmark for related models, which aim at capturing the basic aspects of the behavior of various biological systems. [ABSTRACT FROM AUTHOR]

By molecular dynamics simulation of a coarse-grained bead-spring-type model for a cylindrical molecular brush with a backbone chain of Nb effective monomers to which with grafting density σ side chains with N effective monomers are tethered, several characteristic length scales are studied for variable solvent quality. Side chain lengths are in the range 5 <= N <= 40, backbone chain lengths are in the range 50 <= Nb <= 200, and we perform a comparison to results for the bond fluctuation model on the simple cubic lattice (for which much longer chains are accessible, Nb <= 1027, and which corresponds to an athermal, very good, solvent). We obtain linear dimensions of the side chains and the backbone chain and discuss their N-dependence in terms of power laws and the associated effective exponents. We show that even at the theta point the side chains are considerably stretched, their linear dimension depending on the solvent quality only weakly. Effective persistence lengths are extracted both from the orientational correlations and from the backbone end-to-end distance; it is shown that different measures of the persistence length (which would all agree for Gaussian chains) are not mutually consistent with each other and depend distinctly both on Nb and the solvent quality. A brief discussion of pertinent experiments is given. [ABSTRACT FROM AUTHOR]

When two types of side chains (A,B) are densely grafted to a (stiff) backbone and the resulting bottlebrush polymer is in a solution under poor solvent conditions, an incompatibility between A and B leads to microphase separation in the resulting cylindrical brush. The possible types of ordering are reminiscent of the ordering of block copolymers in cylindrical confinement. Starting from this analogy, Leibler's theory of microphase separation in block copolymer melts is generalized to derive a description of the system in the weak segregation limit. Also molecular dynamics simulation results of a corresponding coarse-grained bead-spring model are presented. Using side chain lengths up to N = 50 effective monomers, the ratio of the Lennard-Jones energy parameter between unlike monomers ([variant_greek_epsilon]AB) and monomers of the same kind ([variant_greek_epsilon]AA = [variant_greek_epsilon]BB) is varied. Various correlation functions are analyzed to study the conditions when (local) Janus-cylinder-type ordering and when (local) microphase separation in the direction along the cylinder axis occurs. Both the analytical theory and the simulations give evidence for short-range order due to a tendency toward microphase separation in the axial direction, with a wavelength proportional to the side chain gyration radius, irrespective of temperature and grafting density, for a wide range of these parameters. [ABSTRACT FROM AUTHOR]

The elastic network (EN) is a prime model that describes the long-time dynamics of biomolecules. However, the use of harmonic potentials renders this model insufficient for studying large conformational changes of proteins (e.g. stretching of proteins, folding and thermal unfolding). Here, we extend the capabilities of the EN model by using a harmonic approximation described by Lennard-Jones (LJ) interactions for far contacts and native contacts obtained from the standard overlap criterion as in the case of Gō-like models. While our model is validated against the EN model by reproducing the equilibrium properties for a number of proteins, we also show that the model is suitable for the study of large conformation changes by providing various examples. In particular, this is illustrated on the basis of pulling simulations that predict with high accuracy the experimental data on the rupture force of the studied proteins. Furthermore, in the case of DDFLN4 protein, our pulling simulations highlight the advantages of our model with respect to Gō-like approaches, where the latter fail to reproduce previous results obtained by all-atom simulations that predict an additional characteristic peak for this protein. In addition, folding simulations of small peptides yield different folding times for α-helix and β-hairpin, in agreement with experiment, in this way providing further opportunities for the application of our model in studying large conformational changes of proteins. In contrast to the EN model, our model is suitable for both normal mode analysis and molecular dynamics simulation. We anticipate that the proposed model will find applications in a broad range of problems in biology, including, among others, protein folding and thermal unfolding. [ABSTRACT FROM AUTHOR]