Showing papers by "Sarith P. Sathian published in 2020"

Kapitza resistance at water–graphene interfaces

Abstract: 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.

Non-isothermal flow of an electrolyte in a charged nanochannel.

Abstract: Electrokinetic flows are generally analyzed, assuming isothermal conditions even though such situations are hard to be achieved in practice. In this paper, the flow of a symmetric electrolyte in a charged nanochannel subjected to an axial temperature gradient is investigated using molecular dynamics simulations. We analyze the relative contribution of the Soret effect, the thermoelectric effect, and the double layer potential in the electrical double layer for various surface charges and temperature gradients. We find the flow driven by thermal gradient analogous to electroosmotic flow. The thermophoretic motion of the electrolyte is significant for negative surface charge than the positive surface charge. The vibrational spectrum of graphene is calculated to delineate the effect of the surface charge polarity on the observed thermophoretic motion of the electrolyte. A unique structure of interfacial water layer is observed for the positive and negative surface charges. We attribute the presence of these structures to the differences in water-carbon interactions existing for various surface charge polarity. For an applied thermal gradient in the range 2.6 K/nm to 8 K/nm, we observe a continuous net flow with average velocities reaching up to 9.4 m/s inside the channel for a negative surface charge of -0.101 C/m2. The results indicate that in a charged graphene-based nanochannel, temperature gradients can be employed to induce streaming current, depending on the relative influence of the Soret effect and the double layer potential.

The effect of temperature on water desalination through two-dimensional nanopores.

Abstract: Two-dimensional (2D) materials such as graphene, molybdenum sulfide, and hexagonal boron nitride are widely studied for separation applications such as water desalination. Desalination across such 2D nanoporous membranes is largely influenced by the bulk transport properties of water, which are, in turn, sensitive to the operating temperature. However, there have been no studies on the effect of temperature on desalination through 2D nanopores. We investigated water desalination through hydrogen functionalized graphene nanopores of varying pore areas at temperatures 275.0 K, 300.0 K, 325.0 K, and 350.0 K. The water flux showed a direct relation with the diffusion coefficient and an inverse relation with the hydrogen-bond lifetime. As a direct consequence, the water flux was found to be related to the temperature as per the Arrhenius equation, similar to an activated process. The results from the present study improve the understanding on water and ion permeation across nanoporous 2D materials at different temperatures. Furthermore, the present investigation suggests a kinetic model, which can predict the water and ion permeation based on the characteristics of the nanopore.

Phonon coupling induced thermophoresis of water confined in a carbon nanotube.

Abstract: The controlled transport of water through nanoscale devices is an important requirement in the design and development of various nanofluidic systems. Molecular dynamics simulations are performed to investigate the phonon coupling induced thermophoretic transport of water through a carbon nanotube (CNT). Phonon coupling is believed to have a significant role in the transport of heat at the liquid-solid interface. The thermally induced vibrational modes of water-filled and empty CNTs are examined at various thermal gradients. The spatial asymmetry along the length of a CNT due to the imposed thermal gradient contributes to the diffusion enhancement of water confined in the CNT, but does not have a strong correlation with the applied thermal gradient. Analysis shows that the vibrational modes present in the center-of-mass oscillations of CNTs do not play any significant role in the development of the thermophoretic force on water. The low-frequency phonon vibrational modes of CNTs are suppressed due to the phonon coupling between water and the CNT. Also, we observed that the spectral heat current across the water-CNT interface dominates at frequencies below 5 THz, which is the same frequency range as radial breathing modes observed in the vibrational spectrum of CNT. This observation leads us to the conclusion that the coupling of radial breathing phonon modes contributes significantly to thermophoresis. This study substantiates the existence of phonon coupling at the water-CNT interface and quantifies the accumulated heat transfer across the interface.

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

Abstract: 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.

Variation of momentum accommodation coefficients with pressure drop in a nanochannel

Abstract: Accommodation coefficients (ACs) are the phenomenological parameters used to evaluate gas-wall interactions. The gas transport through a finite length nanochannel will confront the variation of properties along the length of the channel. A three-dimensional molecular dynamics simulation has been carried out to examine this streamwise inhomogeneity of flow characteristics in a nanochannel. The rarefaction of the flow to the downstream direction is a crucial behavior in a pressure-driven nanochannel flow. This is manifested as the variation in velocity and temperature along the length of the channel. Subsequently, the interactions between the gas and wall particles will get reduced considerably. Moreover, the characteristics near the wall are examined in detail. A nonhomogeneous behavior in density and velocity profile near the wall is reported. Further, the momentum accommodation coefficient (MAC) in both the tangential and normal directions is examined along the lengthwise sections of the channel. The results show a significant variation of tangential and normal MACs along the length. Further, three channels with different length-to-characteristic dimension ($L/H$) ratios are considered to investigate the effect of $L/H$ ratio. All three channels are subjected to the same pressure drop along the length. It is observed that the MACs and slip length show distinct behavior for different ($L/H$) ratios. The work establishes that the variation of MAC along the length of the channel has to be considered in modeling the nano- and microtransport systems.