Effects of interfacial molecular mobility on thermal boundary conductance at solid-liquid interface.

The effects of interfacial molecular mobility on the thermal boundary conductance (TBC) across graphene-water and graphene-perfluorohexane interfaces were investigated using non-equilibrium molecular dynamics simulations. The molecular mobility was varied by equilibrating nanoconfined water and perfluorohexane at different temperatures. The long-chain molecules of perfluorohexane exhibited a prominent layered structure, indicating a low molecular mobility, over a wide temperature range between 200 and 450 K. Alternatively, water increased its mobility at high temperatures, resulting in an enhanced molecular diffusion that significantly contributed to the interfacial thermal transport, in addition to the increasing vibrational carrier population at high temperatures. Furthermore, the TBC across the graphene-water interface exhibited a quadratic relationship with the rise in temperature, whereas for the graphene-perfluorohexane interface, a linear relationship was observed. The high rate of diffusion in interfacial water facilitated additional low-frequency modes, and a spectral decomposition of the TBC also indicated an enhancement in the same frequency range. Thus, the enhanced spectral transmission and higher molecular mobility of water with respect to perfluorohexane explained the difference in the thermal transport across the interfaces considered herein.

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.

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.

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.

Thermal conductivity of graphene under biaxial strain: an analysis of spectral phonon properties.

Thermal transport in graphene is strongly influenced by strain. We investigate the influence of biaxial tensile strain on the thermal conductivity of zigzag and armchair graphene (AG and ZG) using non-equilibrium molecular dynamics simulations (NEMD). We observe that the thermal conductivity is significantly reduced under strain with a maximum reduction obtained at equi-biaxial strain. It is interesting to note that the high lateral to longitudinal strain ratios reduce the negative impact of strain on the thermal conductivity of AG and ZG. The in-plane acoustic modes are found to be the major heat carriers in unstrained graphene but are severely softened due to strain, and hence, their contribution to the conductivity drops down significantly. Strain alleviates the out-of-plane fluctuations in graphene and the group velocity of the out-of-plane acoustic mode (ZA) increases due to the linearisation of its dispersion relation. These factors result in the dominance of ZA mode in the thermal transport of strained graphene. Significant increase in the size dependence of the thermal conductivity of strained graphene is observed, which is attributed to the long-wavelength ZA phonons. The discrepancies between the results of BTE studies and NEMD are also discussed. This study suggests that biaxial strain can be an effective method to tune the thermal transport in graphene. Our findings can lead to better phonon engineering of graphene for various nanoscale applications.

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.

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.

Inducing a Net Positive Flow of Water in Functionalized Concentric Carbon Nanotubes Using Rotating Electric Fields.

Electropumping has shown great potential as an effective means of inducing a net positive flow of water in confined channels. In this paper we present the first nonequilibrium molecular dynamics study and continuum based numerical solutions that demonstrate an effective net positive flow between concentric carbon nanotubes (CNT) using electropumping. We apply a spatially uniform rotating electric field that couples to the water's permanent dipole moment. Taking advantage of the coupling between the spin angular momentum and the linear momentum we break the symmetry of the channel radius by functionalizing the inner CNT's outer surface with carboxyl groups to induce a net positive flow. We also show that our results for concentric nanotubes are consistent with our previous work where we demonstrated that an increase in functionalization beyond an optimal point in a single walled carbon nanotube resulted in a decrease in positive net flow. We then numerically solve the coupled hydrodynamic momentum equations to show that the nonequilibrium molecular dynamics results are consistent with the continuum theory.

Rotational Diffusion of Proteins in Nanochannels.

The rotational diffusion coefficient is an essential parameter in determining the mechanistic features of biomolecules in both crowded and confined environments. Understanding the influence of nanoconfinement on rotational diffusion is vital in conceptualizing dynamics of biomolecules (such as proteins) in nanopores. The control of the translational movement of biomolecules is practiced widely in nanopore experiments. However, the restrictions on the translational movement may affect other dynamic properties such as rotational diffusion. In this paper, we use a coarse-grained molecular dynamics approach to study the rotational dynamics of a sample protein under the influence of cylindrical nanopore confinement. Our simulation reveals a 2-fold reduction in magnitude from the bulk rotational diffusion coefficient value as the confinement radius reaches double the size of protein's hydrodynamic radius. However, the changes in the rotational diffusion coefficient are relatively small compared to the changes in the translational diffusion coefficient. Interestingly, the rotational anisotropy also varies considerably when pore radii approach protein dimensions. Our simulations point out that the confinement effects cause the breakdown of small angular displacement theory when the pore radius is close to the protein hydrodynamic radius.

Effect of Hydrogen Bonds on the Dielectric Properties of Interfacial Water.

The dielectric constant for water is reduced under confinement. Although this phenomenon is well known, the underlying physical mechanism for the reduction is still in debate. In this work, we investigate the effect of the orientation of hydrogen bonds on the dielectric properties of confined water using molecular dynamics simulations. We find a reduced rotational diffusion coefficient for water molecules close to the solid surface. The reduced rotational diffusion arises due to the hindered rotation away from the plane parallel to the channel walls. The suppressed rotation in turn affects the orientational polarization of water, leading to a low value for the dielectric constant at the interface. We attribute the constrained out-of-plane rotation to originate from a higher density of planar hydrogen bonds formed by the interfacial water molecules.

Prediction of fluid slip in cylindrical nanopores using equilibrium molecular simulations.

We introduce an analytical method to predict the slip length (L s) in cylindrical nanopores using equilibrium molecular dynamics (EMD) simulations, following the approach proposed by Sokhan and Quirke for planar channels [39]. Using this approach, we determined the slip length of water in carbon nanotubes (CNTs) of various diameters. The slip length predicted from our method shows excellent agreement with the results obtained from nonequilibrium molecular dynamics (NEMD) simulations. The data show a monotonically decreasing slip length with an increasing nanotube diameter. The proposed EMD method can be used to precisely estimate slip length in high slip cylindrical systems, whereas, L s calculated from NEMD is highly sensitive to the velocity profile and may cause large statistical errors due to large velocity slip at the channel surface. We also demonstrated the validity of the EMD method in a BNNT-water system, where the slip length is very small compared to that in a CNT pore of similar diameter. The developed method enables us to calculate the interfacial friction coefficient directly from EMD simulations, while friction can be estimated using NEMD by performing simulations at various external driving forces, thereby increasing the overall computational time. The EMD analysis revealed a curvature dependence in the friction coefficient, which induces the slip length dependency on the tube diameter. Conversely, in flat graphene nanopores, both L s and friction coefficient show no strong dependency on the channel width.

Translational mobilities of proteins in nanochannels: A coarse-grained molecular dynamics study.

We investigated the translation of a protein through model nanopores using coarse-grained (CG) nonequilibrium molecular dynamics (NEMD) simulations and compared the mobilities with those obtained from previous coarse-grained equilibrium molecular dynamics model. We considered the effects of nanopore confinement and external force on the translation of streptavidin through nanopores of dimensions representative of experiments. As the nanopore radius approaches the protein hydrodynamic radius, r_{h}/r_{p}→1 (where r_{h} is the hydrodynamic radius of protein and r_{p} is the pore radius), the translation times are observed to increase by two orders of magnitude. The translation times are found to be in good agreement with the one-dimensional biased diffusion model. The results presented in this paper provide useful insights on nanopore designs intended to control the motion of biomolecules.

Water desalination using graphene nanopores: influence of the water models used in simulations.

Molecular dynamics simulations are widely employed to analyze water and ion permeation through nanoporous membranes for reverse osmosis applications. In such simulations, water models play an important role in accurately reproducing the properties of water. We investigated the water and ion transport across a hydroxyl (OH) functionalized graphene nanopore using six water models: SPC, SPC/E, SPC/Fw, TIP3P, TIP4P, and TIP4P/2005. The water flux thus obtained varied up to 84% between the models. The water and ion flux showed a correlation with the bulk transport properties of the models such as the diffusion coefficient and shear viscosity. We found that the hydrogen-bond lifetime, resulting from the partial charges of the model, influenced the flux. Our results are useful in the selection of a water model for computer simulations of desalination using nanomembranes. Our findings also suggest that lowering the hydrogen-bond lifetime and enhancing the rate of diffusion of water would lead to enhanced water/ion flux.

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.

Thermophoretically driven water droplets on graphene and boron nitride surfaces.

We investigate thermally driven water droplet transport on graphene and hexagonal boron nitride (h-BN) surfaces using molecular dynamics simulations. The two surfaces considered here have different wettabilities with a significant difference in the mode of droplet transport. The water droplet travels along a straighter path on the h-BN sheet than on graphene. The h-BN surface produced a higher driving force on the droplet than the graphene surface. The water droplet is found to move faster on h-BN surface compared to graphene surface. The instantaneous contact angle was monitored as a measure of droplet deformation during thermal transport. The characteristics of the droplet motion on both surfaces is determined through the moment scaling spectrum. The water droplet on h-BN surface showed the attributes of the super-diffusive process, whereas it was sub-diffusive on the graphene surface.

Hydrogen bonds and twist in cellulose microfibrils.

There is increasing experimental and computational evidence that cellulose microfibrils can exist in a stable twisted form. In this study, atomistic molecular dynamics (MD) simulations are performed to investigate the importance of intrachain hydrogen bonds on the twist in cellulose microfibrils. We systematically enforce or block the formation of these intrachain hydrogen bonds by either constraining dihedral angles or manipulating charges. For the majority of simulations a consistent right handed twist is observed. The exceptions are two sets of simulations that block the O2-O6' intrachain hydrogen bond, where no consistent twist is observed in multiple independent simulations suggesting that the O2-O6' hydrogen bond can drive twist. However, in a further simulation where exocyclic group rotation is also blocked, right-handed twist still develops suggesting that intrachain hydrogen bonds are not necessary to drive twist in cellulose microfibrils.

Water flow in carbon nanotubes: The effect of tube flexibility and thermostat.

Although the importance of temperature control in nonequilibrium molecular dynamics simulations is widely accepted, the consequences of the thermostatting approach in the case of strongly confined fluids are underappreciated. We show the strong influence of the thermostatting method on the water transport in carbon nanotubes (CNTs) by considering simulations in which the system temperature is controlled via the walls or via the fluid. Streaming velocities and mass flow rates are found to depend on the tube flexibility and on the thermostatting algorithm, with flow rates up to 20% larger when the walls are flexible. The larger flow rates in flexible CNTs are explained by a lower friction coefficient between water and the wall. Despite the lower friction, a larger solid-fluid interaction energy is found for flexible CNTs than for rigid ones. Furthermore, a comparison of thermostat schemes has shown that the Berendsen and Nosé-Hoover thermostats result in very similar transport rates, while lower flow rates are found under the influence of the Langevin thermostat. These findings illustrate the significant influence of the thermostatting methods on the simulated confined fluid transport.

Translational diffusion of proteins in nanochannels.

Hydrodynamic interactions play an important role in the transport of analytes through nanoscale devices. Of particular note is the role that no-slip boundary conditions have on the drag coefficient of confined particles and molecules. In this work, we use a coarse grained molecular dynamics model to measure the diffusion coefficients of proteins confined within cylindrical nanochannels of similar dimension. Finite-size corrected bulk diffusion coefficients are found to agree with experimental data, while in channels, a good match is found between theoretical expressions based on continuum fluid mechanics and the reduction of the translational diffusion coefficient across a range of protein to channel size ratios. These results demonstrate that it is possible to directly use molecular simulation to make quantitative predictions of the effects of hydrodynamics on diffusion at length scales of order 1 nm.

Thermophoretic transport of ionic liquid droplets in carbon nanotubes.

Thermal-gradient induced transport of ionic liquid (IL) and water droplets through a carbon nanotube (CNT) is investigated in this study using molecular dynamics simulations. Energetic analysis indicates that IL transport through a CNT is driven primarily by the fluid-solid interaction, while fluid-fluid interactions dominate in water-CNT systems. Droplet diffusion analysis via the moment scaling spectrum reveals sub-diffusive motion of the IL droplet, in contrast to the self-diffusive motion of the water droplet. The Soret coefficient and energetic analysis of the systems suggest that the CNT shows more affinity for interaction with IL than with the water droplet. Thermophoretic transport of IL is shown to be feasible, which can create new opportunities in nanofluidic applications.