A molecular dynamics simulation on the effect of different parameters on thermal resistance of graphene-argon interface

Molecular dynamics simulations of argon molecules confined between two parallel graphene sheets are carried out to investigate the parameters affecting heat transfer and thermal properties. These parameters include wall–fluid interaction strength, fluid density and wall temperature. For constant wall temperature simulations, we show that the first two parameters have influence on near-wall fluid density. As a result, the heat transfer at wall–fluid interfaces and thus through argon molecules across the domain will change. Also, we demonstrate that variations in wall temperature rarely affects the density profiles of argon molecules next to the walls. Therefore, in these cases, the variations in thermal resistance at the interface is most dominantly due to wall temperature itself. To analyse the results, the density and temperature profiles and also other parameters including heat flux and temperature gradient of bulk of argon molecules, Kapitza length and argon thermal conductivity are considered. The Kapitza length describes thermal resistance at liquid–solid interface. According to the results, increasing wall–fluid interaction strength leads to greater molecular aggregation of argon molecules near the walls and, consequently, decreasing the Kapitza length. Furthermore, higher fluid density leads to greater thermal resistance at wall–fluid interactions and therefore greater temperature jumps are observed in temperature profiles.     

A molecular dynamics study on the thermal rectification effect at the solid–liquid interfaces between the face-centred cubic (FCC) of gold (Au) with the surfaces of (100), (110) and (111) crystal planes facing the liquid methane (CH4)

A molecular dynamics study on the solid–liquid (S-L) interfaces for solid wall of gold having the face-centred cubic of (100), (110) and (111) crystal planes contacting liquid methane was examined using non-equilibrium molecular dynamics simulations. An investigation on the thermal rectification effect was performed by measuring the thermal boundary conductance (TBC) at the S-L interface. Thermal rectification can be defined as the differences in the TBC at the interface between the two opposite heat flow directions; one is from the liquid to solid and vice versa. The thermal rectifications are up to 13% for (110) crystal plane, followed by 6% and 0.3% for (111) and (100) crystal planes, respectively. It was found that the TBC at the S-L interface was influenced by the magnitude of the adsorption of liquid molecules at the vicinity of the interface. The results show that due to the different temperature distribution, different magnitude of the adsorption of liquid molecules is generated for the two opposite heat flow directions. On the surface of the solid walls for (110) crystal plane, where lattice-scale corrugation exists, it was found that there exists difference in distance between the surface layers of the solid and liquid across the interface between the cases of the two opposite heat flow directions, which affects the TBC at the interface. The present results suggest that the factors that influence the thermal rectification at the S-L interface are the magnitude of the adsorption of liquid molecules and the surface structure of the solid walls that differ significantly among the three types of crystal planes.     

Molecular dynamics study on explosive boiling of ultra-thin liquid over solid substrate: considering interface wettability of Argon/MoS2

A molecular dynamics (MD) study is carried out to reveal the phenomenon about the normal and explosive boiling of ultra-thin liquid argon film absorbed on MoS₂ surface with different wetting conditions. The three-phase molecular system is composed of a solid MoS₂ wall, a liquid argon film and a vapour argon region. Initially, the three-phase simulated system is thermally equilibrated at a low temperature. Then the MoS₂ heat source is suddenly heated up to two different high temperatures those far above critical point of liquid argon, and the argon experienced a phase transition process in the NVE ensemble. The simulation results show that the good wetting properties and high heat source temperature dramatically enhance phase transition efficiency, accelerating the heat transfer rate, shortening the boiling time, and increasing the evaporation rate, and they have remarkable effects on temperature and pressure histories, density distribution during whole boiling process. Explosive boiling is more likely to occur at high superheated degree, but evaporation occurs at low superheated degree. In addition, at a high superheated degree, it can be conclude from the simulation results that the better wetting properties of the solid-liquid interface is, the shorter time of the explosive boiling is needed.     

气/液界面及固体表面硬脂酸LB膜结构性质研究

对气液/和固/液界面上硬脂酸LB膜的结构性质的研究表明,二价离子能够使气/液界面上LB膜表面压力降低,并出现一个固-固转变的过程.对此可以解释为是由二价离子富集在亚相表面,减弱了膜分子之间的库仑作用,使表面电势降低引起的.同时由于二价离子与硬脂酸分子形成复合物,单层膜的结构发生改变,导致固-固转变点的产生.对固体基质上多层LB膜的椭圆偏振研究表明,有序排列的硬脂酸LB膜具有明显的双折射性质.电镜观察发现两个固相垂直提位获得的多层膜在形貌上存在差异,低压固相膜较之高压固相膜存在明显的不均匀性.分析认为这是在膜从气/液界面向固体表面转移过程中发生重结晶引起的.     

Responses of Marine Bacteria Under Starvation Conditions at a Solid-Water Interface

Size changes during starvation of 17 marine bacterial isolates at a solid-water interface and in the liquid phase were examined. Twelve rod-shaped, hydrophilic bacteria decreased in size more rapidly at the solid surface than in the liquid phase, a result parallel to that observed previously for one of the strains at an air-water interface. On the other hand, three rod-shaped, hydrophobic bacteria diminished in size more rapidly in the liquid phase than at the solid-water interface. The rapid size decrease (defined here as the dwarfing phase) in either situation appeared to be an active process which occurred more rapidly when the cells were in an early stage of logarithmic growth at the onset of starvation. Dwarfing was reversibly inhibited by low temperature and low pH but was not inhibited by chloramphenicol. Three coccoidal bacteria showed little tendency to become smaller upon starvation in the liquid phase or at a surface.     

Direct numerical simulation of expiratory crackles: Relationship between airway closure dynamics and acoustic fluctuations

This paper investigates the relationship between airway closure dynamics and acoustic fluctuations in expiratory crackles using direct numerical simulation. A unified mathematical model is proposed to deal with flow in an airway, elastic deformation of the airway wall, surface tension driven motion of the liquid film that lines the airway, and their acoustic fluctuations because of material compressibility. Airway closure is induced by increasing the surrounding pressure, then the source of the pressure fluctuations is measured over time. Our results show that the airway closure occurs suddenly because of a bridge formation of the liquid film, and high energy transfer occurs between the kinetic energy, the surface energy of the liquid interface, and the elastic energy of the airway wall, invoking a large acoustic fluctuation that causes the expiratory crackles. Nonlinear behavior is observed in terms of the airway wall stiffness; the dynamic motion of the airway closure becomes moderate and both the energy transfer and acoustic fluctuations are dramatically reduced with an increase in airway wall stiffness.     

Cryogenic temperature effects and resolution upon slow cooling of protein preparations in solid state NMR

X-ray crystallography using synchrotron radiation and the technique of dynamic nuclear polarization (DNP) in nuclear magnetic resonance (NMR) require samples to be kept at temperatures below 100 K. Protein dynamics are poorly understood below the freezing point of water and down to liquid nitrogen temperatures. Therefore, we investigate the α-spectrin SH3 domain by magic angle spinning (MAS) solid state NMR (ssNMR) at various temperatures while cooling slowly. Cooling down to 95 K, the NMR-signals of SH3 first broaden and at lower temperatures they separate into several peaks. The coalescence temperature differs depending on the individual residue. The broadening is shown to be inhomogeneous by hole-burning experiments. The coalescence behavior of 26 resolved signals (of 62) was compared to water proximity and crystal structure Debye–Waller factors (B-factors). Close proximity to the solvent and large B-factors (i.e. mobility) lead, generally, to a higher coalescence temperature. We interpret a high coalescence temperature as indicative of a large number of magnetically inequivalent populations at cryogenic temperature.     

Preferential orientation of an immunoglobulin in a glycolipid monolayer controlled by the disintegration kinetics of proteo-lipidic vesicles spread at an air-buffer interface

The insertion of immunoglobulin (IgG) in a glycolipid monolayer was achieved by using the ability of new proteo-glycolipid vesicles to disintegrate into a mixed IgG-glycolipid interfacial film after spreading at an air-buffer interface. The interfacial disintegration kinetics was shown to be directly dependent on the initial vesicle surface density and on the buffer ionic strength. The presence of the immunoglobulin in the glycolipid film was displayed by an increase of the lateral compressibility (Cs) during monolayer compression. Cs magnitude modifications, due to the antibody effect on the monolayer packing, decreases as the spread vesicle density increases. At interfacial saturation, the lateral compressibility profile becomes similar to that of a control monolayer without antibody. However, the careful analysis of the mixed monolayer after transfer by Langmuir-Blodgett technique (ATR-FTIR characterisation, enzyme immunoassociation) clearly demonstrated that the antibody was still present in such conditions and was not completely squeezed out from the interface as compressibility changes could have meant. At nonsaturating vesicle surface density, IgG molecules initially lying in the lipid matrix with the Y-shape plane parallel to the interface move to a standing-up position during the compression, leading to lateral compressibility modifications. For a saturating vesicle surface density, the glycolipid molecules force the IgG molecules to directly adopt a more vertical position in the interfacial film and, consequently, no lateral compressibility modification was recorded during the compression.     

Imaging the ordered arrays of water-soluble protein ferritin with the atomic force microscope

Individual water-soluble molecules of the protein ferritin have been imaged on a silicon surface in pure water at room temperature with the atomic force microscope (AFM). The ferritin molecules formed an ordered monolayer by binding to a charged polypeptide monolayer of poly-1-benzyl-L-histidine (PBLH) spread at the air-water interface. The film, fully wetted with water, was horizontally transferred onto an alkylated silicon wafer for AFM imagings. The hexagonal arrangement of ferritin molecules was imaged with high reproducibility on the whole surface of the film, since the forces between cantilever and the sample could be kept sufficiently smaller than 10^-10 N, mainly due to a “self-screening effect” of the surface charges of the ferritin-PBLH layer. This is the first observation of two-dimensional ordered arrays of water-soluble protein molecules directly confirmed by AFM with molecular resolution.     

Supramolecular assembly using helical peptides

We investigated supramolecular assemblies of various hydrophobic helical peptides. The assemblies were formed at the air/water interface or in aqueous medium. The hexadecapeptide, Boc-(Ala-Aib)₈-OMe (BA16M), was reported to take α-helical structure by X-ray analysis. Several derivatives were prepared, which have the repeating sequence of Ala-Aib, Lys(Z)-Aib or Leu-Aib, or have the terminal chemically modified. CD spectra of the peptides indicated helical conformation in ethanol solution. The surface pressure-area isotherms of the peptide monolayers showed an inflection at the surface area corresponding to the cross section along the helix axis, and the monolayers were collapsed by further compression. All the helical peptides oriented their helix axis parallel to the air/water interface on the basis of the results of transmission IR spectra and RAS of the monolayers transferred onto substrates.A small mound was observed in the isotherm of BA16M and other derivatives, which was ascribed to the phase transition from the liquid state to the solid state. One mol% of FITC-labeled peptide was mixed into the monolayers to visualize the phase separation of the solid and liquid states at the surface pressure of the coexisting region. Various shapes of the dark domain were observed at the top of the mound in the isotherms by fluorescence microscopy. The helical peptides formed two-dimensional crystals at the air/water interface when they were compressed to the solid state.An amino-terminated helical peptide, HA16B, was suspended in an aqueous medium by a sonication method and transparent dispersion was obtained. The dynamic light scattering measurement of the dispersion revealed the particle size of 75 nm with a narrow size distribution. The molecular assembly of the helical peptide in water was called “Peptosome”, because it takes a vesicular structure.     

Effect of airway surface liquid on the forces on the pharyngeal wall: Experimental fluid–structure interaction study

Obstructive sleep apnoea syndrome (OSAS) is a breathing disorder with a multifactorial etiology. The respiratory epithelium is lined with a thin layer of airway surface liquid preventing interactions between the airflow and epithelium. The effect of the liquid lining in OSAS pathogenesis remains poorly understood despite clinical research. Previous studies have shown that the physical properties of the airway surface liquid or altered stimulation of the airway mechanoreceptors could alleviate or intensify OSAS; however, these studies do not provide a clear physical interpretation. To study the forces transmitted from the airflow to the liquid-lined compliant wall and to discuss the effects of the airway surface liquid properties on the stimulation of the mechanoreceptors, a novel and simplified experimental system mimicking the upper airway fundamental characteristics (i.e., liquid-lined compliant wall and complex unsteady airflow features) was constructed. The fluctuating force on the compliant wall was reduced through a damping mechanism when the liquid film thickness and/or the viscosity were increased. Conversely, the liquid film damping was reduced when the surface tension decreased. Based on the experimental data, empirical correlations were developed to predict the damping potential of the liquid film. In the future, this will enable us to extend the existing computational fluid–structure interaction simulations of airflow in the human upper airway by incorporating the airway surface liquid effect without adopting two-phase flow interface tracking methods. Furthermore, the experimental system developed in this study could be used to investigate the fundamental principles of the complex once/twice-coupled physical phenomena.     

Molecular dynamics simulation of thermal conductivity of an argon liquid layer confined in nanospace

Molecular dynamics simulations were used to study the thermal conductivity of liquid argon ultra thin films confined between two plates spaced several nanometres apart. The research focused on the dependence of the liquid argon thermal conductivity on the liquid layer thickness and the interaction between liquid and solid. The results show that the thermal conductivity of liquid argon ultra thin films confined between two plates depends on the distance between the two plates and the existence of solid-like liquid layering at the liquid–solid interface and the average migration frequency of all liquid molecules. Stronger interactions between the liquid and the solid resulted in a larger number of atoms in the solid-like liquid layer along the surface and hence smaller thermal resistance between the liquid and the solid. However, as the strength of the interaction with the solid increased, the thermal conductivity was reduced due to fewer atoms near the hot solid boundary and less molecular migration.     

Fabrication of Ag Nanoparticles Embedded in Al:ZnO as Potential Light-Trapping Plasmonic Interface for Thin Film Solar Cells

Incident photon conversion efficiency of the absorbing materials at either side of a thin film solar module can be enhanced by integrating a plasmonic interface. Silver nanoparticles represent a good candidate that can be integrated to a thin film solar cell for efficient light-trapping. The aim of this work is to fabricate plasmonically active interface consisting of Ag nanoparticles embedded in Al:ZnO that has the potential to be used at the front surface and at the back reflector of a thin film solar cell to enhance light-trapping and increase the photoconversion efficiency. We show that Ag can readily dewet the Al:ZnO surface when annealed at temperatures significantly lower than the melting temperature of Ag, which is beneficial for lowering the thermal budget and cost in solar cell fabrication. We find that such an interface fabricated by a simple dewetting technique leads to plasmonic resonance in the visible and near infrared regions of the solar spectrum, which is important in enhancing the conversion efficiency of thin film solar cells.     

Self-assembled hydrophobin protein films at the air-water interface: structural analysis and molecular engineering

Hydrophobins are amphiphilic proteins produced by filamentous fungi. They function in a variety of roles that involve interfacial interactions, as in growth through the air-water interface, adhesion to surfaces, and formation of coatings on various fungal structures. In this work, we have studied the formation of films of the class II hydrophobin HFBI from Trichoderma reesei at the air-water interface. Analysis of hydrophobin aqueous solution drops showed that a protein film is formed at the air-water interface. This elastic film was clearly visible, and it appeared to cause the drops to take unusual shapes. Because adhesion and formation of coatings are important biological functions for hydrophobins, a closer structural analysis of the film was made. The method involved picking up the surface film onto a solid substrate and imaging the surface by atomic force microscopy. High-resolution images were obtained showing both the hydrophilic and hydrophobic sides of the film at nanometer resolution. It was found that the hydrophobin film had a highly ordered structure. To study the orientation of molecules and to obtain further insight in film formation, we made variants of HFBI that could be site specifically conjugated. We then used the avidin-biotin interaction as a probe. On the basis of this work, we suggest that the unusual interfacial properties of this type of hydrophobins are due to specific molecular interactions which lead to an ordered network of proteins in the surface films that have a thickness of only one molecule. The interactions between the proteins in the network are likely to be responsible for the unusual surface elasticity of the hydrophobin film.     

Surface rheology and phase transitions of monolayers of phospholipid/cholesterol mixtures

The dynamic surface elasticity and the surface dilational viscosity of three binary phospholipid/cholesterol mixtures were determined with axisymmetric drop shape analysis on a harmonically oscillating pendent drop. Dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and dioleoylphosphatidylcholine were used to explore the rheological properties and phase transitions of mixtures of saturated and unsaturated phospholipids with cholesterol. The growth rates for surface dilational viscosity and dynamic elasticity are parallel for all film pressures studied. Characteristic breaks and plateaus could be found for these growth rates, indicating phase transitions. For dipalmitoylphosphatidylcholine/cholesterol and dimyristoylphosphatidylcholine/cholesterol mixtures, phase diagrams with six regions separated by phase boundaries were found, which are in good agreement with phase transitions reported in the literature for static measurements of isotherms and isobars on a Langmuir film balance and from fluorescence microscopy. Some phase boundaries were only found by dynamic, but not by static, elasticity measurements. Imaging methods revealed phase separations produced by the formation of condensed stoichiometric complexes leading to micron-sized and mostly circular domains. The effects of these complexes on monolayer rheology in liquid/liquid phases is described. Furthermore, liquid/solid and solid phase transitions are discussed.     

Molecular Dynamics Simulation of Behaviours of Non-Polar Droplets Merging and Interactions with Hydrophobic Surfaces

This paper presents a molecular dynamics simulation of the behaviours of non-polar droplets merging and also the fluid molecules interacting with a hydrophobic surface. Such behaviours and transport phenomena are popular in general microchannel flow boiling and two-phase flow. The droplets are assumed to be composed of Lennards-Jones type molecules. Periodic boundary conditions are applied in three coordinate directions of a 3-D system, where there exist two liquid droplets and their vapour. The two droplets merge when they come within the prescribed small distance. The merging of two droplets apart from each other at different initial distances is tested and the possible larger （or critical） non-dimensional distance, in which droplets merging can occur, is discussed. The evolution of the merging process is simulated numerically by employing the Molecular Dynamics （MD） method. For interactions with hydrophobic solid wall, a system with fluid confined between two walls is used to study the wetting phenomena of fluid and solid wall. The results are compared with those of hydrophilic wall to show the unique characteristics of hydrophobic interactions by microscopic methods.     

The binary phase diagram of lecithin and cholesteryl linolenate

The condensed binary phase diagram of cholesteryl linolenate-egg yolk lecithin has been determined by polarizing light microscopy, differential scanning calorimetry and X-ray diffraction. On increasing the temperature lecithin forms rectangular, cubic and hexagonal liquid-crystalline structures into which varying amounts of cholesteryl linolenate are incorporated. As more cholesteryl linolenate is incorporated, the transition temperatures between different phases are lowered. The rectangular and cubic structures incorporate only small amounts of cholesteryl linolenate; the molar ratios, lecithin to cholesteryl linolenate, being 11:1 and 16:1, respectively. However, the hexagonal phase, in which the phosphorylcholine groups of the lecithin molecules form the core of the rod-like assembly of molecules, incorporates up to approximately 25% cholesteryl linolenate by weight, corresponding to a molar ratio 3:1. At higher concentrations, cholesteryl linolenate forms an excess phase and may be present as crystals, smectic or cholesteric liquid crystals, or as liquid oil, depending on the temperature. At higher temperatures, a large zone of a single isotropic liquid phase exists in which large amounts of lecithin are solubilized by the cholesterol ester. Up to 40% cholesteryl linolenate by weight, the transition temperatures between different phases are influenced by approximately 1% water (by weight) associated with egg lecithin.It is probable that the incorporated apolar cholesterol ester molecules are associated primarily with the apolar hydrocarbon chain region of the different lecithin structures. The resultant decrease in the observed transition temperatures would suggest an overall chain-disordering role for the incorporated cholesteryl linolenate molecules. The influence of cholesteryl linolenate on the thermodynamic stability of the different lecithin structures, together with the models suggested for the molecular orientations of cholesterol esters in the different liquid crystalline structures, may be relevant to the role of these lipids in more complex biological systems, particularly serum lipoproteins.     

Membrane surface pressure can account for differential activities of membrane-penetrating molecules

It is a common observation that cis-unsaturated and branched chain fatty acids, which are usually liquid, affect membrane function differently from saturated and trans-unsaturated fatty acids, which are usually solid. We also found that the former are much more potent than the latter in inhibiting viral hemolytic activity. A search for the origin of this difference revealed a correlation between inhibition and equilibrium surface pressure (the surface pressure at the air/water interface of a solution of the substance in question). Using a simple but rigorous thermodynamic analysis, we show that penetration of a lipid bilayer is correlated with equilibrium surface pressure of the penetrating molecule. We therefore conclude that an important reason for the difference in effects of liquid and solid fatty acids on membranes is the greater penetrability of the former relative to the latter. We suggest that attributing such effects to fluidity changes in the membrane should await demonstration of actual intramonolayer residence of the fatty acid in the membrane. The thermodynamic analysis is readily generalized and, in the absence of specific interactions between penetration and bilayer molecules, provides a convenient method for predicting membrane penetration by virtually any type of exogenous molecule.     

Release of phospholipids from colloidal particles to polymeric surfaces

This study presents a small-scale polymerization of high molecular weight methyl methacrylate/n-butyl acrylate (MMA/n-BA) colloidal particles that are synthesized in an aqueous environment in the presence of phospholipid hydrogenated soybean phosphatidylcholine (HSPC) molecules that also serve as the particle stabilizing agents. When such particles coalesce to form polymeric films, they release phospholipids, which, in turn, form organized structures near the film-air (F-A) interface. Diffusion and mobility of phospholipid molecules are affected not only by their compatibility with colloidal particles but also by electrolyte environments of colloidal dispersions. When Na(+), K(+), and Ca(2+) counterions are added to MMA/n-BA aqueous colloidal dispersions stabilized with HSPC, and such films are coalesced, different degrees of diffusion of HSPC to the F-A interface exist, depending on the counterion, and conformational changes of HSPC result. For example, in the presence of Ca(2+), HSPC molecules collapse entropically to form random surface layers, as opposed to smaller Na(+) and K(+), which force amphiphilic HSPC ends to align preferentially parallel to the film surface. These studies show that it is possible to design stimuli-response colloidal systems triggered by chemical environments of active molecules on colloidal polymer particles.     

Measuring surface dynamics of biomolecules by total internal reflection fluorescence with photobleaching recovery or correlation spectroscopy.

The theoretical basis of a new technique for measuring equilibrium adsorption/desorption kinetics and surface diffusion of fluorescent-labeled solute molecules at solid surfaces has been developed. The technique combines total internal reflection fluorescence (TIR) with either fluorescence photobleaching recovery (FPR) or fluorescence correlation spectroscopy (FCS). A laser beam totally internally reflects at a solid/liquid interface; the shallow evanescent field in the liquid excites the fluorescence of surface adsorbed molecules. In TIR/FPR, adsorbed molecules are bleaching by a flash of the focused laser beam; subsequent fluorescence recovery is monitored as bleached molecules exchange with unbleached ones from the solution or surrounding nonilluminated regions of the surface. In TIR/FCS, spontaneous fluorescence fluctuations due to individual molecules entering and leaving a well-defined portion of the evanescent field are autocorrelated. Under appropriate experimental conditions, the rate constants and surface diffusion coefficient can be readily obtained from the TIR/FPR and TIR/FCS curves. In general, the shape of the theoretical TIR/FPR and TIR/FCS curves depends in a complex manner upon the bulk and surface diffusion coefficients, the size of the iluminated or observed region, and the adsorption/desorption/kinetic rate constants. The theory can be applied both to specific binding between immobilized receptors and soluble ligands, and to nonspecific adsorption processes. A discussion of experimental considerations and the application of this technique to the adsorption of serum proteins on quartz may be found in the accompanying paper (Burghardt and Axelrod. 1981. Biophys. J. 33:455).