NANOSCALE THERMAL AND FLUID TRANSPORT PHENOMENON IN POROUS MEDIA: A MOLECULAR DYNAMICS STUDY

Abstract

The small length scales and large specific surface areas associated with the nanostructures play a key role in the molecular level thermal and fluid transport. In such nanometer length scales, the local intermolecular interaction creates temperature discontinuities between the solid-like interfaces and their neighboring fluid molecules. This phenomenon often referred to as interfacial thermal resistance (i.e., Kapitza resistance) during nanoscale thermal transport. There is also evidence that the fluid molecules are absorbed by the wall molecules promoting structural ordering of fluid at the solid/fluid interface. The local dynamic properties of this fluid layered structure are substantially different from the fluid properties at macroscale. Therefore, the continuum transport theories break down near the material interfaces at nanoscale. In this thesis, we investigate the unique transport behaviors of fluid molecules in confining nanoenvironments using Molecular Dynamics (MD) simulations. Firstly, heat transfer across an interface between a monolayer coated solid substrate and fluid has been analyzed by varying the atomic mass (mM) and interaction energy between monolayer molecules (εMM). In that case, the mutual combination of atomic mass (mM) and interaction energy (MM) of monolayer lead to a significant influence in heat transport at the interfacial region. It was found that Kapitza resistance monotonically increases with the increase of mM irrespective of εMM without any further change in the fluid-structure near the solid surface. This indicates the vibrational coupling between the molecules at the solid/fluid interface largely depend on the mass of monolayer molecules. We also investigate the pressure-driven transport mechanism of liquid argon through nanoporous graphene membrane (NPGM) using MD simulations. In this study we check the validity and limitations of the assumptions of continuum flow equation. We present a thorough characterization of the density and pressure distribution of liquid argon based on the respective flow region to elucidate the unique fluid transport behaviors. The argon velocity adjacent to the pore edge was found lower than pore center suggesting the influence of the interaction between argon and carbon molecules at the pore boundary. In that case, we consider the argon velocity closest to the pore edge as slip velocity, which provides an update in the continuum flow equation. The local viscosity was also calculated from the thin argon film flows sheared by graphene walls. Our study shows that the entrance interfacial pressure and higher local viscosity in the vicinity of graphene membrane associated with the optimized definition of wall/fluid boundary near the pore edge play a critical role for the permeation of argon through NPGM.