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Simulationshave been performed using the free-energy binary-liquidlattice Boltzmann method with sufficient resolution that the criticalcapillary number for coalescence was determined for collisions betweendroplets in simple shear with a small initial offset in the sheargradient direction. The simulations were used to study the behaviorof the interacting interfaces and the film between them during collisionsover a wide range of capillary numbers with emphasis on near-criticalconditions. From these three-dimensional simulations with deforminginterfaces, several features of the evolution of the film betweenthe drops were observed. The critical film thickness was determinedto be similar to the interface thickness, a power law described thedependence of the minimum film thickness on the capillary number incollisions without coalescece, and an inflection point was found inthe dynamics of the minimum distance between drops that eventuallycoalesce. The rotation of the film and the flow in it were also studied,and a reversal in the flow was found to occur before coalescence.The mobility of the phase field was therefore important in the continuedthinning of the film at the points of minimum thickness after theflow reversal. A comparison of the critical capillary number and criticalfilm thickness in the simulations with the values for experimentsin confined simple shear indicated that the effective physical radiusof the simulated droplets was on the order of several micrometers.The results are significant for simulations of droplet interactionsand emulsion flows in complex geometries and turbulence because theydemonstrate the necessary scale of the computations and how parameters,such as the interface thickness and phase field mobility, should beselected for accurate results. [ABSTRACT FROM AUTHOR]

Simulatingdroplet coalescence is challenging because small-scale(tens of nanometers) phenomena determine the behavior of much larger(micrometer- to millimeter-scale) droplets. In general, liquid dropletscolliding in a liquid medium coalesce when the capillary number isless than a critical value. We present simulations of droplet collisionsand coalescence in simple shear flow using the free-energy binary-liquidlattice Boltzmann method. In previous simulations of low-speed collisions,droplets coalesced at unrealistically high capillary numbers. Simulationsof noncoalescing droplets have not been reported, and therefore, thecritical capillary number for simulated collisions was unknown. Bysimulating droplets with radii up to 100 lattice nodes, we determinethe critical capillary number for coalescence and quantify the effectsof several numerical and geometric parameters. The simulations wereperformed with a well-resolved interface, a Reynolds number of one,and capillary numbers from 0.01 to 0.2. The ratio of the droplet radiusand interface thickness has the greatest effect on the critical capillarynumber. As in experiments, the critical capillary number decreaseswith increasing droplet size. A second numerical parameter, the interfacediffusivity (Péclet number) also influences the conditionsfor coalescence: coalescence occurs at higher capillary numbers withlower Péclet numbers (higher diffusivity). The effects of thevertical offset between the droplets and the confinement of the dropletswere also studied. Physically reasonable results were obtained andprovide insight into the conditions for coalescence. Simulations thatmatch the conditions of experiments reported in the literature remaincomputationally impractical. However, the scale of the simulationsis now sufficiently large that a comparison with experiments involvingsmaller droplets (≈10 μm) and lower viscosities (≈10–6m2/s, the viscosity of water) may be possible.Experiments at these conditions are therefore needed to determinethe interface thickness and Péclet number that should be usedfor predictive simulations of coalescence phenomena. [ABSTRACT FROM AUTHOR]

Recent advances in the collection and interpretation of surface-sensitive vibrational spectroscopic measurements have made it possible to study the orientation of peptides and proteins in situ in a biologically relevant environment. However, interpretation of sum frequency generation (SFG) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) vibrational spectroscopy is hindered by the fact that orientation cannot be inferred without some prior knowledge of the protein structure. In this work, molecular dynamics simulations were used to study the interfacial orientation and structural deformation of the short β-sheet peptide tachyplesin I at the polystyrene/water interface. By combining these results with ATR-FTIR and SFG measurements, reasonable agreement was found with the simulation results, suggesting that tachyplesin I lies parallel to the surface, although the simulation results imply a broader distribution of peptide twist angles than could be characterized using available experimental measurements. The interfacial structure was found to be deformable even when disulfide bonds were preserved, and these local deviations from a purely extended β-sheet conformation may be of importance to future developments in the interpretation of SFG and ATR-FTIR spectra. [ABSTRACT FROM AUTHOR]

We have studied the self-assembly of hydrophobic nanoparticles at ionic liquid (IL)–water and IL–oil (hexane) interfaces using molecular dynamics (MD) simulations. For the 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])/water system, the nanoparticles rapidly approached the IL–water interface and equilibrated more into the IL phase although they were initially in the water phase. In contrast, when the nanoparticles were dispersed in the hexane phase, they slowly approached the IL–hexane interface but remained primarily in the hexane phase. Consequently, the IL–hexane interface was rather undisturbed by the nanoparticles whereas the IL–water interface changed significantly in width and morphology to accommodate the presence of the nanoparticles. The equilibrium positions of the nanoparticles were also supported and explained by potential of mean force (PMF) calculations. Interesting ordering and charge distributions were observed at the IL–liquid interfaces. At the IL–hexane interface, the [BMIM] cations preferentially oriented themselves so that they were immersed more in the hexane phase and packed efficiently to reduce steric hindrance. The ordering likely contributed to a heightened IL density and a slightly positive charge at the IL–hexane interface. In contrast, the cations at the IL–water interface were oriented isotropically unless in the presence of nanoparticles, where the cations aligned across the nanoparticle surfaces. [ABSTRACT FROM AUTHOR]

Hysteresis of wetting, like the Coulombic friction at solid/solid interface, impedes the motion of a liquid drop on a surface when subjected to an external field. Here, we present a counterintuitive example, where some amount of hysteresis enables a drop to move on a surface when it is subjected to a periodic but asymmetric vibration. Experiments show that a surface either with a negligible or high hysteresis is not conducive to any drop motion. Some finite hysteresis of contact angle is needed to break the periodic symmetry of the forcing function for the drift to occur. These experimental results are consistent with simulations, in which a drop is approximated as a linear harmonic oscillator. The experiment also sheds light on the effect of the drop size on flow reversal, where drops of different sizes move in opposite directions due to the difference in the phase of the oscillation of their center of mass. [ABSTRACT FROM AUTHOR]

We offer a novel process to render hydrophobic surfaces resistant to relatively small proteins during adsorption. This was accomplished by self-assembly of a well-known natural osmolyte, trimethylamine oxide (TMAO), a small amphiphilic molecule, on a hydrophobic alkanethiol surface. Measurments of lysozyme (LYS) adsorption on several homogeneous substrates formed from functionalized alkanethiol self-assembled monolayers (SAMs) in the presence and absence of TMAO, and direct interaction energy between the protein and functionalized surfaces, demonstrate the protein-resistant properties of a noncovalently adsorbed self-assembled TMAO layer. Molecular dynamics simulations clearly show that TMAO molecules concentrate near the CH3−SAM surface and are preferentially excluded from LYS. Interestingly, TMAO molecules adsorb strongly on a hydrophobic CH3−SAM surface, but a trade-off between hydrogen bonding with water, and hydrophobic interactions with the underlying substrate results in a nonintuitive orientation of TMAO molecules at the interface. Additionally, hydrophobic interactions, usually responsible for nonspecific adsorption of proteins, are weakly affected by TMAO. In addition to TMAO, other osmolytes (sucrose, taurine, and betaine) and a larger homologue of TMAO (N,N-dimethylheptylamine-N-oxide) were tested for protein resistance and only N,N-dimethylheptylamine-N-oxide exhibited resistance similar to TMAO. The principle of osmolyte exclusion from the protein backbone is responsible for the protein-resistant property of the surface. We speculate that this novel process of surface modification may have wide applications due to its simplicity, low cost, regenerability, and flexibility. [ABSTRACT FROM AUTHOR]

A liquid drop placed over a solid surface with a wettability gradient self-propels to minimize its surface energy. It can also climb an inclined plane if the applied gradient strength is high enough. We investigate the motion of liquid drops over an inclined gradient surface using a unique 3D computational technique. The technique combines diffuse interface in a smoothed particle hydrodynamics simulation to study the internal fluid structure and the contact line dynamics. Simulation results reveal that drop motion is dependent on its volume, surface inclination, and the strength of the wettability gradient. It has been demonstrated that, depending on these parameters, a drop can experience upward or downward motion or can remain stationary on the inclined plane. Finally, drop mobility maps which give an idea about the regimes of uphill and downhill movement of a drop over gradient surfaces have been proposed. [ABSTRACT FROM AUTHOR]

We study anisotropic wetting in systems governed by Lennard−Jones interactions. Molecular simulation is used to obtain the macroscopic contact angle a fluid adopts on face-centered-, body-centered-, and simple-cubic lattices with the (100), (110), or (111) face in contact with the fluid. Several amorphous substrates are also examined. Substrates are modeled as a static collection of particles. For a given set of calculations, the atomistic density of the substrate and the particle−particle interactions (surface−fluid and fluid−fluid) remain fixed. These constraints enable us to focus on the extent to which substrate structure influences the contact angle. Three substrate−fluid interaction strengths are considered, which provide wetting conditions that span from near-dry to near-wet. Our results indicate that the manner in which particles are organized within the substrate significantly influences the contact angle. For strong substrates (near-wet case), a change in the substrate structure can change the cosine of the contact angle by as much as 0.5. We also examine how well certain structural and energetic features of the substrate−fluid system serve as suitable metrics for predicting the variation of the contact angle with substrate topography. Three parameters are considered: the density of atoms within the crystalline plane closest to the fluid, a measure of the effective strength of the substrate−fluid interaction, and the roughness of the solid−liquid interface. The effective strength of the substrate potential shows the strongest correlation with the contact angle. This energy-based parameter is defined in a general manner and therefore could serve as a useful tool for describing the anisotropic wetting of solids. In contrast, the metrics based on planar density and interface roughness are found to correlate with contact angle data relatively weakly. [ABSTRACT FROM AUTHOR]

Atomistic modeling of insoluble monolayers is currently used to inspect their organization and electric characteristics, providing a link between theory and experiment. Extensive molecular dynamics simulations at 300 K were carried out for model films of the lipids dipalmitoylphosphatidylcholine (DPPC) and dicaprin (DC) at the air/water interface. Surface concentrations corresponding to a set of points along the surface pressure/area isotherms of the surfactants were considered. The models contained 25 or 81 lipid molecules in hexagonal arrangement and explicit aqueous media (TIP3P) treated in periodic boundary conditions. Molecular dynamics simulations based on a classical force field (CHARMM27) were carried out and key characteristics of the studied films were estimated. The dielectric properties of the films in normal and tangential direction were quantified by means of dipole moment magnitude and orientation analysis and by monolayer dielectric permittivity. The contributions of lipids and interfacial water to each component of the considered characteristics were assessed and their variations upon film compression were discussed and compared for the two monolayers and to earlier results. The dielectric permittivity tensors were analyzed. Electrostatic potential profiles across the layers and surface pressure values were used for more detailed clarification of experimental measurements. The results show dissimilar behavior of the two lipids at the air−water interface. While the average electric and dielectric properties of DPPC monolayers result from opposite surfactant and water contributions, the two subsystems are synergetic in the DC films. The anisotropy of the monolayer dipole moment and dielectric permittivity is explained by domination of a different subsystem in the various components. Tangential characteristics turn out to be more sensitive to the size of the model and to the degree of film compression. [ABSTRACT FROM AUTHOR]

Extensive molecular dynamics simulations at room temperature were carried out for model films of two dissimilar lipids (DPPC and dicaprin) at the air/water interface. To study the peculiarities of the organization patterns at different average areas per molecule, surface concentrations corresponding to five almost equally spaced points along the isotherms of the two surfactants were considered. A variable of prime interest was the density distribution in a direction normal to the interface of the monolayer components: interfacial water and surfactant on one hand and the separate moieties of the lipids on the other hand. The packing pattern and cluster size dispersion were studied by means of Voronoi tessellation and radial distribution functions. Speculations regarding structural changes upon phase-state changes during film compression were made. Individual characteristics for surfactant heads and tails as well as for interfacial water were outlined and related to the available experimental data. An analysis of the diffusion coefficients revealed the limiting factors for lipid lateral and normal diffusion. Structural arguments in support of changes in monolayer dielectric properties with the area per molecule were provided. [ABSTRACT FROM AUTHOR]

We have studied the dynamics of nanoparticles at polydimethylsiloxane (PDMS) oil−water interfaces using molecular dynamics (MD) simulations. The diffusion of nanoparticles in pure water and low-viscosity PDMS oil is found to be reasonably consistent with the prediction by the Stokes−Einstein equation. In addition, we have calculated the shear moduli and viscosities of bulk oil and water, as well as oil−water interfaces from single nanoparticle tracking and demonstrated the potential of probing nanorheology from an MD simulation approach. Surprisingly, we found that the lateral diffusion of nanoparticles as well as apparent interfacial nanorheology at the PDMS oil (low viscosity)−water interface are independent of the position of the nanoparticle at the interface. [ABSTRACT FROM AUTHOR]

We present an extensive molecular simulation study of the behavior of a flexible hydrophobic 25-mer polymer at interfaces presenting a range of chemistries from hydrophobic (−CH3) to hydrophilic (−CONH2). We quantify the free energy of adsorption, conformational equilibria, and translational and conformational dynamics of the polymer at these diverse interfaces. Water-mediated interactions drive the polymer to adsorb strongly at a hydrophobic interface and repel it from hydrophilic ones. At hydrophilic surfaces, van der Waals interactions between the polymer and the surface mitigate this water-mediated repulsion, leading to weak adsorption of the polymer. Although the polymer is strongly adsorbed to hydrophobic surfaces, it is also most dynamic there. Translational diffusion and conformational dynamics are faster at hydrophobic surfaces compared to those at hydrophilic ones. In bulk water, the polymer collapses into compact globular shapes, whereas the thermodynamic stability of folded polymers is significantly lowered at hydrophobic surfaces. The polymer spreads into pancake-like 2D conformations at hydrophobic surfaces and gradually beads up into globular shapes as the surface is made more hydrophilic. Interestingly, the binding thermodynamics and dynamics correlate with macroscopic droplet contact angles that characterize the wetting properties of the different interfaces. [ABSTRACT FROM AUTHOR]

Two independent series of calculations are performed simulating spherical and cylindrical C12E6micelles in a temperature range around the experimental sphere-to-rod transition temperature for surfactant concentrations less than 20% by weight. A comparative analysis of these systems helps to shed light on the microscopic details of the micelle sphere-to-rod transition. In agreement with theoretical models, we find that spherical and cylindrical micelles have a different oil core packing; the core radius of a cylindrical micelle is reduced by a factor of 0.87 with respect to the core radius of a spherical micelle. Despite this contraction, the specific volume of the alkyl tails is larger in a cylindrical micelle than in a spherical micelle. In both geometries, this specific volume follows the same linear increase with temperature. Density measurement experiments are also performed in order to evaluate the specific volume of the hydrophobic tail of surfactants of the C12Ejfamily with jranging from 5 to 8. We observe a good agreement between experimental data and simulation results. Our simulations also show that the spatial distribution of the head groups in the interface is more effective in screening the oil core in the cylindrical aggregate than in the spherical aggregate, reducing by a factor of 2 the oil surface per monomer exposed to water. This screening accounts for a free-energy difference of Δfs= fssph− fscyl≃ +2.5kBTper monomer and mirrors the essential role that the hydrophobic interactions have on the shape transition. We also find that the different interface packing correlates with different conformations and flexibility of the hydrophilic fragments E6, that appear as an entropicreservoir for the transition. Finally, comparing the degree of hydration of a spherical micelle at T= 283 K with that of a cylindrical micelle at T= 318 K, we observe an amount of dehydration in agreement with reported experimental data across the sphere-to-rod transition. However, for aggregates of fixed shape, we find a much smaller amount of dehydration with temperature, suggesting that the shape transition is not a consequence of the measured temperature dehydration but rather the opposite. [ABSTRACT FROM AUTHOR]