Showing papers by "Sarith P. Sathian published in 2019"

Water flow in carbon nanotubes: the role of tube chirality.

Abstract: We investigated the effects of the chirality of carbon nanotubes (CNTs) on water transport using molecular dynamics simulations. For the study, we considered CNTs with similar diameter and varying chiralities, obtained by altering the chiral indices (n,m) of the nanotubes. The tubes with an armchair (n = m) structure show the maximum streaming velocity, flux, flow rate enhancement and slip length, whereas the corresponding values are lower for chiral (n≠m) tubes, and are the lowest in zigzag (m = 0) CNTs. The difference in flow rates with varying tube structures can be primarily attributed to the alteration in potential energy landscape experienced by the water molecules, leading to changes in the friction coefficient at the fluid-solid interface. The water molecules experienced the least resistance to flow in an armchair tube, while the force exerted by the CNT surface on the water molecules increased monotonically with the change in the CNT type to chiral and then to zigzag. The chirality effects on water transport are, however, found to decrease with an increase in tube diameter. Furthermore, an analysis of the influence of the CNT type on ion (Na+ or Cl-) transport in water-filled CNTs showed the interaction energy of ions with water to be much higher than that with the CNT surface, demonstrating minimal dependence of ion transport on the chiral structure. Hence, the tube chirality should be considered an ineludible factor in controlling the water transport through CNTs and in the designing of novel devices in nanotechnology.

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

Abstract: 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 Kapitza resistance at fluid-solid interfaces.

Abstract: 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.

Tunable thermoelectric properties of SnS2 under high pressure at room temperature

Abstract: In this paper, we study the structural, electronic, vibrational, thermoelectric and elastic properties of tin disulfide (SnS2) using first principles density functional theory calculations in the pressure range 0 ≤ p ≤ 5 GPa. The variation of lattice constant along c-axis is found to be higher than that along the a-axis which significantly affects the properties. The electronic band gap is observed to decrease with the applied pressure. The Raman shift of Eg and A1g modes increases with applied pressure. Furthermore, SnS2 remains dynamically stable up to 5 GPa. Thermoelectric properties such as thermopower (S), electrical conductivity (σ), power factor (S2σ) show anisotropy. While the in-plane direction is more dominant at ambient pressure, the out-of-plane is more dominant with the increase in pressure. The calculated power factor is higher in the hole concentration than the electron concentration in the defined pressure range at room temperature. This suggests that SnS2 could be an excellent candidate material of p-type thermoelectric under high pressure conditions.

Rotational Diffusion of Proteins in Nanochannels

Abstract: 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.

The Effect of Water Models on Desalination Through Graphene Nanopores

Abstract: Nanoporous carbon materials are extensively studied for various separation applications. Among them, water desalination by means of Reverse Osmosis (RO) stands out due to it’s large socio-economic relevance. Many studies are carried out in this area both computationally and experimentally. In computational studies the water simulated using different water models are prone to produce inconsistent results. In this study water desalination through hydrogen functionalized graphene nanopore is studied using different water models (SPC, SPC/E, TIP3P, TIP4P/2005). Up to 81% difference was observed in the flux estimates among the models. The water permeation rate was found to be closely related to the bulk transport properties of the simulated water.

Stokes-EInstein-Debye Relation: A Check of Validity for Proteins in Nanoconfinements

Abstract: Combined kinetic theory-hydrodynamics treatment has been proven effective in the prediction of biomolecule dynamics, generally if a single biomolecule is present in the bulk solvent. But the validity of such a theory in many physiological conditions is controversial. In the present study, a sample protein surrounded by other large biomolecules is approximated as the protein in a cylindrical nanopore. The hydrodynamic radius of the protein is chosen as an indicator to check whether one of the widely used kinetic theoryhydrodynamics relation namely Stokes-Einstein-Debye relation, is genuine for confined conditions of the protein. It has been found that Stokes-Einstein-Debye relation cannot be satisfied by the protein if confinement dimensions are very close. The reason for the violation can be attributed to van der Waals interaction between pore and the protein.

Effect of Uniaxial Strain on Thermal Conductivity of Graphene

Abstract: In this study, we investigate the effect of uniaxial strain on the thermal conductivity of zigzag graphene (ZG) and armchair graphene (AG). It is observed that the thermal conductivity reduces with strain in ZG and AG. For a given strain, the maximum reduction in thermal conductivity is observed in AG. The tensile strain causes a larger deformation of bonds in AG whereas a minor variation in bond angles is observed. This causes a high reduction of thermal conductivity in AG. DOS (Density of States) and Spectral Energy Density (SED) curves are calculated to study the phonon transport in graphene. It is found that due to strain, high frequency phonon modes undergo a red shift whereas blue shift is observed in the out-of-plane phonon modes. The phonon group velocity and lifetimes of in-plane acoustic modes get reduced because of strain and hence their contribution to the thermal conductivity reduces. But in the ZA modes, strain increases the phonon group velocities and phonon lifetimes. For unstrained graphene, in-plane acoustic modes contribute nearly 60-70% of thermal conductivity in ZG and AG. However, when graphene is strained their contribution sharply reduces and the contribution by ZA mode increases and is nearly 40-60% of the thermal conductivity. But the contribution by TA modes to the thermal conductivity is unaffected by strain. Phonon Mean Free Path (MFP) calculations indicate that MFP of phonon in strained graphene increases due to increase in the wavelength of low frequency ZA phonons. The MFP of phonons is found to be higher in AG than in ZG in both strained and unstrained conditions.