Molecular dynamics investigation of heat capacity of nanogap confined liquid

Abstract

Geometric confinement with characteristic length comparable to the molecular diameter, can induce a dramatic change in the transport properties of fluids which is of great importance from technological point of view like thermal management of micro/nano electronics, energy conversion devices, micro/nano fluidics devices, energy storage system, drug delivery, understanding different biological systems etc. This study is focused on the fundamental understanding of heat capacity of liquid entrapped in such molecular scale cavity. The study starts by the modeling of heat capacity of bulk liquid. As continuum approximation is not applicable for nanoscale phenomena, molecular dynamics (MD) simulation was used for the modeling. Simplified Lenard Jones (LJ) type molecular model was used in this study. Heat capacity of the liquid was evaluated from non-equilibrium molecular dynamics (NEMD) simulation following fluctuation formula. Heat capacity of the bulk liquid obtained from the simulation was compared with that of the published literature value and found in excellent agreement. The simulation was extended for the confined liquid by placing the liquid in a nanogap confinement of varying gap thicknesses from 27.8 nm to 0.585 nm. Heat capacity of the confined liquid was observed to vary significantly and the variations follow a very complex relation with the confinement gap thickness. For a limited gap thickness, heat capacity of the nanoconfined liquid was found to be higher than the bulk liquid and beyond that, the nanoconfinement doesn’t have any significant effect on the heat capacity of the liquid. Temperature was also found to play a significant role in that complex behavior of the nanoconfined liquid. Heat capacity of the nanoconfined liquid for a certain temperature and gap thickness can be more than double of that of the bulk liquid. To dissect the underlying facts of this complex behavior, some specific behavioral changes was identified and thoroughly analyzed. It was found that configurational change due to the variation in density, contribution of ballistic and coherent phonon transport, mode of energy transfer, interfacial thermal resistance, modification of vibrational density of states, guided molecular mobility, etc. are some of many underlying key factors that contributes to this anomalous behavior of heat capacity of the nanogap confined liquid.