Effects of Electrostatic Interactions on Kapitza Resistance in Hexagonal Boron Nitride-Water Interfaces.

Electrostatic interactions in nanoscale systems can influence the heat transfer mechanism and interfacial properties. This study uses molecular dynamics simulations to investigate the impact of various electrostatic interactions on the Kapitza resistance (Rk) on a hexagonal boron nitride-water system. The Kapitza resistance at hexagonal boron nitride nanotube (hBNNT)-water interface reduces with an increase in diameter of the nanotube due to more aggregation of water molecules per unit surface area. An increase in the partial charges on boron and nitride caused the reduction in Rk. With the increase in partial charge, a better hydrogen bonding between hBNNT and water was observed, whereas the structure and order of the water molecules remain the same. Nevertheless, the addition of NaCl salt into water does not have any influence on interfacial thermal transport. Rk remains unchanged with electrolyte concentration because the cumulative Coulombic interaction between the ions and the hBNNT is significantly less when compared with water molecules. Furthermore, the effect of electric field strength on interfacial heat transfer is also investigated by providing uniform positive and negative surface charges on the outermost hBN layers. Rk is nearly independent of the practical range of applied electric fields and decreases with an increasing electric field for extreme field strengths until the electrofreezing phenomenon occurs. The ordering of water molecules toward the charged surface leads to an increase in the layering effect, causing the reduction in Rk in the presence of an electric field.

Slip and stress from low shear rate nonequilibrium molecular dynamics: The transient-time correlation function technique.

We derive the transient-time correlation function (TTCF) expression for the computation of phase variables of inhomogenous confined atomistic fluids undergoing boundary-driven planar shear (Couette) flow at constant pressure. Using nonequilibrium molecular dynamics simulations, we then apply the TTCF formalism to the computation of the shear stress and the slip velocity for atomistic fluids at realistic low shear rates, in systems under constant pressure and constant volume. We show that, compared to direct averaging of multiple trajectories, the TTCF method dramatically improves the accuracy of the results at low shear rates and that it is suitable to investigate the tribology and rheology of atomistically detailed confined fluids at realistic flow rates.

Hydrodynamic slip of alkali chloride solutions in uncharged graphene nanochannels.

Using non-equilibrium molecular dynamics simulations, we demonstrate the effect of concentration and alkali cation types (K+, Na+, and Li+) on the hydrodynamic slip of aqueous alkali chloride solutions in an uncharged graphene nanochannel. We modeled the graphene-electrolyte interactions using the potential of Williams et al. [J. Phys. Chem. Lett. 8, 703 (2017)], which uses optimized graphene-ion Lennard-Jones interaction parameters to effectively account for surface and solvent polarizability effects on the adsorption of ions in an aqueous solution to a graphene surface. In our study, the hydrodynamic slip exhibits a decreasing trend for alkali chloride solutions with increasing salt concentration. The NaCl solution shows the highest reduction in the slip length followed by KCl and LiCl solutions, and the reduction in the slip length is very much dependent on the salt type. We also compared the slip length with that calculated using a standard unoptimized interatomic potential obtained from the Lorentz-Berthelot mixing rule for the ion-carbon interactions, which is not adjusted to account for the surface and solvent polarizability at the graphene surface. In contrast to the optimized model, the slip length of alkali chloride solutions in the unoptimized model shows only a nominal change with salt concentration and is also independent of the nature of salts. Our study shows that adoption of the computationally inexpensive optimized potential of Williams et al. for the graphene-ion interactions has a significant influence on the calculation of slip lengths for electrolyte solutions in graphene-based nanofluidic devices.

Nanoconfinement Effects on the Kapitza Resistance at Water-CNT Interfaces.

The Kapitza resistance (Rk) at the water-carbon nanotube (CNT) interface, with water on the inside of the nanotube, was investigated using molecular dynamics simulations. We propose a new equilibrium molecular dynamics (EMD) method, also valid in the weak flow regime, to determine the Kapitza resistance in a cylindrical nanoconfinement system where nonequilibrium molecular dynamics (NEMD) methods are not suitable. The proposed method is independent of the correlation time compared to Green-Kubo-based methods, which only work in short correlation time intervals. Rk between the CNT and the confined water strongly depends on the diameter of the nanotube and is found to decrease with an increase in the CNT diameter, the opposite to what is reported in the literature when water is on the outside of the nanotube. Rk is furthermore found to converge to the planar graphene surface value as the number of water molecules per unit surface area approaches the value in the graphene surface and a higher overlap of the vibrational spectrum. A slight increase in Rk with the addition of the number of CNT walls was observed, whereas the chirality and flow do not have any impact.

Electropumping of nanofluidic water by linear and angular momentum coupling: theoretical foundations and molecular dynamics simulations.

In this article we review the relatively new phenomenon of electropumping in nanofluidic systems, in which nonzero net flow results when polar molecules are rotated by external electric fields. The flow is a consequence of coupling of the spin angular momentum of molecules with their linear streaming momentum. By devising confining surfaces that are asymmetric - specifically one surface is more hydrophobic compared to the other - unidirectional flow results and so pumping can be achieved without the use of pressure gradients. We first cover the historical background to this phenomenon and follow that with a detailed theoretical description of the governing hydrodynamics. Following that we summarise work that has applied this phenomenon to pump water confined to planar nanochannels, semi-functionalised single carbon nanotubes and concentric carbon nanotubes. We also report on the energy efficiency of this pumping technique by comparisons with traditional flows of planar Couette and Poiseuille flow, with the surprising conclusion that electropumping at the nanoscale is some 4 orders of magnitude more efficient than pumping by Poiseuille flow.

Improved methodology to compute the intrinsic friction coefficient at solid-liquid interfaces.

In this work, we propose an improved methodology to compute the intrinsic friction coefficient at the liquid-solid (L-S) interface based on the theoretical model developed by Hansen et al. [Phys. Rev. E 84, 016313 (2011)]. Using equilibrium molecular dynamics, we apply our method to estimate the interfacial friction for a simple Lennard-Jones system of argon confined between graphene sheets and a system of water confined between graphene sheets. Our new method shows smaller statistical errors for the friction coefficient than the previous procedure suggested by Hansen et al. Since we only use the interfacial particles, the interfacial friction calculated using our method is solely due to the wall-fluid interactions and is devoid of bulk fluid contributions. The intrinsic nature of the friction coefficient has been validated by measuring the friction coefficient at different interfaces and channel sizes and against direct non-equilibrium molecular dynamics measurements. Our improved methodology is found to be more reliable than the existing equilibrium and non-equilibrium methods and does not suffer from the well-known convergence and correlation-time ambiguities in the methods formulated along Green-Kubo-like ideas.

The phase space distribution of confined fluids under shear is not fractal.

We analyze the phase-space compression, characteristic of all deterministic, dissipative systems for an inhomogeneous boundary-driven shear fluid via nonequilibrium molecular dynamics simulations. We find that, although the full system undergoes a phase space contraction, the marginal distribution of the fluid particles is described by a smooth, volume preserving probability density function. This is the case for most thermodynamic states of physical interest. Hence, we show that the models currently employed to investigate inhomogeneous fluids in a nonequilibrium steady state, in which only walls are thermostatted, generate a non-singular distribution for the fluid.

Efficiency of Electropumping in Nanochannels.

Electropumping has been shown to be an effective means of inducing a net positive flow in fluids confined within planar nanochannels and carbon nanotubes. In this Letter, we investigate the efficiency of electropumping relative to Couette and Poiseuille flows. We apply a spatially uniform rotating electric field to a fluid confined in a functionalized nanochannel that couples the water's permanent dipole moment resulting in a net positive flow. We then induce a net positive flow in nanochannels for Couette and Poiseuille flows, matching volume flow rates to allow a direct comparison of average power dissipation per unit volume between all flow types. We show that while electropumping is less efficient than Couette flow, it is 4 orders of magnitude more efficient than Poiseuille flow. This suggests that, rather than being a mere novelty, electropumping is a far more energetically efficient means of transporting water compared to conventional pressure driven pumping.

Kapitza resistance at water-graphene interfaces.

Heat transfer across fluid-solid interfaces in nanoconfinement has received significant attention due to its relevance in nanoscale systems. In this study, we investigate the Kapitza resistance at the water-graphene interface with the help of classical molecular dynamics simulation techniques in conjunction with our recently proposed equilibrium molecular dynamics (EMD) method [S. Alosious et al., J. Chem. Phys. 151, 194502 (2019)]. The size effect of the Kapitza resistance on different factors such as the number of graphene layers, the cross-sectional area, and the width of the water block was studied. The Kapitza resistance decreases slightly with an increase in the number of layers, while the influence of the cross-sectional area and the width of the water block is negligible. The variation in the Kapitza resistance as a function of the number of graphene layers is attributed to the large phonon mean free path along the graphene cross-plane. An optimum water-graphene system, which is independent of size effects, was selected, and the same was used to determine the Kapitza resistance using the predicted EMD method. The values obtained from both the EMD and the non-equilibrium molecular dynamics (NEMD) methods were compared for different potentials and water models, and the results are shown to be in good agreement. Our method allows us to compute the Kapitza resistance using EMD simulations, which obviates the need to create a large temperature gradient required for the NEMD method.

Prediction of Kapitza resistance at fluid-solid interfaces.

Understanding the interfacial heat transfer and thermal resistance at an interface between two dissimilar materials is of great importance in the development of nanoscale systems. This paper introduces a new and reliable linear response method for calculating the interfacial thermal resistance or Kapitza resistance in fluid-solid interfaces with the use of equilibrium molecular dynamics (EMD) simulations. The theoretical predictions are validated against classical molecular dynamics (MD) simulations. MD simulations are carried out in a Lennard-Jones (L-J) system with fluid confined between two solid slabs. Different types of interfaces are tested by varying the fluid-solid interactions (wetting coefficient) at the interface. It is observed that the Kapitza length decreases monotonically with an increasing wetting coefficient as expected. The theory is further validated by simulating under different conditions such as channel width, density, and temperature. Our method allows us to directly determine the Kapitza length from EMD simulations by considering the temperature fluctuation and heat flux fluctuations at the interface. The predicted Kapitza length shows an excellent agreement with the results obtained from both EMD and non-equilibrium MD simulations.

Computation of the equilibrium three-particle entropy for dense atomic fluids by molecular dynamics simulation.

We have computed the two- and three-particle contribution to the entropy of a Weeks-Chandler-Andersen fluid via molecular dynamics simulations. The three-particle correlation function and entropy were computed with a new method which simplified the calculation. Results are qualitatively similar to Lennard-Jones systems. We observed a numerical instability in the three-particle contribution. This phenomenon has been previously detected when the traditional method is used; thus, it is likely to be intrinsic in the computation. While the effect of statistical fluctuations can be removed through an extrapolation procedure, the discretization error due to the finite bin size is more difficult to characterize. With a correct choice of the bin size, a good estimate of the three-particle entropy contribution can be achieved at any state, even close to the freezing point. We observed that, despite the fact that the magnitude of the three-particle contribution increases significantly compared to that of the two-particle contribution as freezing is approached, the error induced from overestimation of the excess entropy by the two- and three-body terms exceeds that induced by approximating the excess entropy with the two body term alone.

Flow of water through carbon nanotubes predicted by different atomistic water models.

Nonequilibrium molecular dynamics simulations are reported to investigate the influence of different atomistic water models on the predicted flow behavior in carbon nanotubes (CNTs) with diameters between 0.81 nm and 1.9 nm. The comparison was made using rigid three-site [simplified point charge (SPC), extended SPC (SCP/E), and transferable intermolecular potential three point (TIP3P)] and four-site (TIP4P and TIP4P/2005) models. In addition, a flexible three-site model (SPC/Fw) was also investigated. The effect of different simulation conditions was determined by generating a flux across the CNT using either a pressure gradient across a membrane separating two water reservoirs or a periodic CNT with a constant force applied to each water molecule. Simulations involving the two water reservoirs indicate that the flux is strongly dependent on the choice of water model, which confirms earlier work. By contrast, this strong model dependency is not a feature of the periodic CNT simulations. Instead, the flux depends mainly on the pore diameter and the molecular density of water inside the CNT. The influence of the water model becomes very small in the periodic CNT simulations, which eliminates distorting entrance/exit effects.

Measuring heat flux beyond Fourier's law.

We use nonequilibrium molecular dynamics to explore the effect of shear flow on heat flux. By simulating a simple fluid in a channel bounded by tethered atoms, the heat flux is computed for two systems: a temperature driven one with no flow and a wall driven, Couette flow system. The results for the temperature driven system give Fourier's law thermal conductivity, which is shown to agree well with experiments. Through comparison of the two systems, we quantify the additional components of the heat flux parallel and normal to the walls due to shear flow. To compute the heat flux in the flow direction, the Irving-Kirkwood equations are integrated over a volume, giving the so-called volume average form, and they are also manipulated to get expressions for the surface averaged and method of planes forms. The method of planes and volume average forms are shown to give equivalent results for the heat flux when using small volumes. The heat flux in the flow direction is obtained consistently over a range of simulations, and it is shown to vary linearly with strain rate, as predicted by theory. The additional strain rate dependent component of the heat flux normal to the wall is obtained by fitting the strain rate dependence of the heat flux to the expected form. As a result, the additional terms in the thermal conductivity tensor quantified in this work should be experimentally testable.

Macroecological patterns of sexual size dimorphism in turtles of the world.

Sexual size dimorphism (SSD) is a well-documented phenomenon in both plants and animals; however, the ecological and evolutionary mechanisms that drive and maintain SSD patterns across geographic space at regional and global scales are understudied, especially for reptiles. Our goal was to examine geographic variation of turtle SSD and to explore ecological and environmental correlates using phylogenetic comparative methods. We use published body size data on 135 species from nine turtle families to examine how geographic patterns and the evolution of SSD are influenced by habitat specialization, climate (annual mean temperature and annual precipitation) and climate variability, latitude, or a combination of these predictor variables. We found that geographic variation, magnitude and direction of turtle SSD are best explained by habitat association, annual temperature variance and annual precipitation. Use of semi-aquatic and terrestrial habitats was associated with male-biased SSD, whereas use of aquatic habitat was associated with female-biased SSD. Our results also suggest that greater temperature variability is associated with female-biased SSD. In contrast, wetter climates are associated with male-biased SSD compared with arid climates that are associated with female-biased SSD. We also show support for a global latitudinal trend in SSD, with females being larger than males towards the poles, especially in the families Emydidae and Geoemydidae. Estimates of phylogenetic signal for both SSD and habitat type indicate that closely related species occupy similar habitats and exhibit similar direction and magnitude of SSD. These global patterns of SSD may arise from sex-specific reproductive behaviour, fecundity and sex-specific responses to environmental factors that differ among habitats and vary systematically across latitude. Thus, this study adds to our current understanding that while SSD can vary dramatically across and within turtle species under phylogenetic constraints, it may be driven, maintained and exaggerated by habitat type, climate and geographic location.

Slip of Alkanes Confined between Surfactant Monolayers Adsorbed on Solid Surfaces.

The slip and friction behavior of n-hexadecane, confined between organic friction modifier surfactant films adsorbed on hematite surfaces, has been studied using nonequilibrium molecular dynamics simulations. The influence of the surfactant type and coverage, as well as the applied shear rate and pressure, has been investigated. A measurable slip length is only observed for surfactant films with a high surface coverage, which provide smooth interfaces between well-defined surfactant and hexadecane layers. Slip commences above a critical shear rate, beyond which the slip length first increases with increasing shear rate and then asymptotes toward a constant value. The maximum slip length increases significantly with increasing pressure. Systems and conditions which show a larger slip length typically give a lower friction coefficient. Generally, the friction coefficient increases linearly with logarithmic shear rate; however, it shows a much stronger shear rate dependency at low pressure than at high pressure. Relating slip and friction, slip only occurs above a critical shear stress, after which the slip length first increases linearly with increasing shear stress and then asymptotes. This behavior is well-described using previously proposed slip models. This study provides a more detailed understanding of the slip of alkanes on surfactant monolayers. It also suggests that high coverage surfactant films can significantly reduce friction by promoting slip, even when the surfaces are well-separated by a lubricant.

Effects of Confinement on the Dielectric Response of Water Extends up to Mesoscale Dimensions.

The extent of confinement effects on water is not clear in the literature. While some properties are affected only within a few nanometers from the wall surface, others are affected over long length scales, but the range is not clear. In this work, we have examined the dielectric response of confined water under the influence of external electric fields along with the dipolar fluctuations at equilibrium. The confinement induces a strong anisotropic effect which is evident up to 100 nm channel width, and may extend to macroscopic dimensions. The root-mean-square fluctuations of the total orientational dipole moment in the direction perpendicular to the surfaces is 1 order of magnitude smaller than the value attained in the parallel direction and is independent of the channel width. Consequently, the isotropic condition is unlikely to be recovered until the channel width reaches macroscopic dimensions. Consistent with dipole moment fluctuations, the effect of confinement on the dielectric response also persists up to channel widths considerably beyond 100 nm. When an electric field is applied in the perpendicular direction, the orientational relaxation is 3 orders of magnitude faster than the dipolar relaxation in the parallel direction and independent of temperature.

A molecular dynamics investigation of the planar elongational rheology of chemically identical dendrimer-linear polymer blends.

The structure and rheology of model polymer blends under planar elongational flow have been investigated through nonequilibrium molecular dynamics simulations. The polymeric blends consist of linear polymer chains (187 monomers per chain) and dendrimer polymers of generations g = 1 - 4. The number fraction, x, of the dendrimer species is varied (4%, 8%, and 12%) in the blend melt. We study the effect of extension rate, dendrimer generation, and dendrimer number fraction on pair distribution functions for different blend systems. We also calculate the extension-rate dependent radius of gyration and ratios of the eigenvalues of the gyration tensor to study the elongation-induced deformation of the molecules in the blend. Melt rheological properties including the first and second extensional viscosities are found to fall into the range between those of pure dendrimer and pure linear polymer melts, which are correlated with the mass fraction and generation of the dendrimers in the blend.

Molecular dynamics study of nanoconfined water flow driven by rotating electric fields under realistic experimental conditions.

In our recent work, J. Chem. Phys. 2013, 138, 154712, we demonstrated the feasibility of unidirectional pumping of water, exploiting translational-rotational momentum coupling using nonequilibrium molecular dynamics simulations. Flow can be sustained when the fluid is driven out of equilibrium by an external spatially uniform rotating electric field and confined between two planar surfaces exposing different degrees of hydrophobicity. The permanent dipole moment of water follows the rotating field, thus inducing the molecules to spin, and the torque exerted by the field is continuously injected into the fluid, enabling a steady conversion of spin angular momentum into linear momentum. The translational-rotational coupling is a sensitive function of the rotating electric field parameters. In this work, we have found that there exists a small energy dissipation region attainable when the frequency of the rotating electric field matches the inverse of the dielectric relaxation time of water and when its amplitude lies in a range just before dielectric saturation effects take place. In this region, that is, when the frequency lies in a small window of the microwave region around ∼20 GHz and amplitude ∼0.03 V Å(-1), the translational-rotational coupling is most effective, yielding fluid velocities of magnitudes of ∼2 ms(-1) with only moderate fluid heating. In this work, we also confine water to a realistic nanochannel made of graphene giving a hydrophobic surface on one side and ß-cristobalite giving a hydrophilic surface on the other, reproducing slip-and-stick velocity boundary conditions, respectively. This enables us to demonstrate that in a realistic environment, the coupling can be effectively exploited to achieve noncontact pumping of water at the nanoscale. A quantitative comparison between nonequilibrium molecular dynamics and analytical solutions of the extended Navier-Stokes equations, including an external rotating electric field has been performed, showing excellent agreement when the electric field parameters match the aforementioned small energy dissipation region.

Porous aromatic frameworks impregnated with fullerenes for enhanced methanol/water separation.

Molecular simulation techniques have revealed that the incorporation of fullerenes within porous aromatic frameworks (PAFs) remarkably enhances methanol uptake while inhibiting water uptake. The highest selectivity of methanol over water is found to be 1540 at low pressure (1 kPa) and decreases gradually with increasing pressure. The adsorption of water is very small compared to methanol, a useful material property for membrane and adsorbent-based separations. Grand canonical Monte Carlo (GCMC) simulations are utilized to calculate the pure component and mixture adsorption isotherms. The water and methanol mixture simulations show that water uptake is further inhibited above the pure component results because of the dominant methanol adsorption. Molecular dynamics (MD) simulations confirm that water diffusivity is also inhibited by strong methanol adsorption in the mixture. Overall, this study reveals profound hydrophobicity in C60@PAF materials and recommends C60@PAFs as suitable applicants for adsorbent and membrane-based separations of methanol/water mixtures and other alcohol/water separation applications.

Shear rheology and structural properties of chemically identical dendrimer-linear polymer blends through molecular dynamics simulations.

We present nonequilibrium molecular dynamics (NEMD) simulation results for the miscibility, structural properties, and melt rheological behavior of polymeric blends under shear flow. The polymeric blends consist of chemically identical linear polymer chains (187 monomers per chain) and dendrimer polymers of generations g = 1-4. The number fraction x of the dendrimer species is varied (4%, 8%, and 12%) in the blend melt. The miscibility of blend species is measured, using the pair distribution functions gDL, gLL, and gDD. All the studied systems form miscible blend melts under the conditions investigated. We also study the effect of shear rate γ̇ and dendrimer generation on inter-penetration between blend species for different blend systems. The results reveal that shear flow increases the interpenetration of linear chains toward the core of the dendrimers. We also calculate the shear-rate dependent radius of gyration and ratios of the eigenvalues of the gyration tensor to study the shear-induced deformation of the molecules in the blend. Melt rheological properties including the shear viscosity and first and second normal stress coefficients obtained from NEMD simulations at constant pressure are found to fall into the range between those of pure dendrimer and pure linear polymer melts.