Molecular dynamics simulations and experimental measurements of density and viscosity of phase change material based on stearic acid with graphene nanoplatelets

Abstract

Understanding the thermophysical properties of nano-enhanced phase change materials (NEPCMs) is crucial for developing thermal energy storage technologies. Thermal conductivity of NEPCMs is the most studied property, but investigations on density and viscosity are scarce. In this paper, the viscosity and density of pure stearic acid (SA) and stearic acid with 2 wt.%, 4 wt.%, and 6 wt.% graphene nanoplatelets (GNPs) of 6–8 nm thickness have been investigated from 343 K to 373 K at atmospheric pressure. The SA with GNP concentrations of 4 and 6 wt.% exhibits non-Newtonian behaviour, meaning that viscosity depends on shear rate. The viscosity and density for SA with 2 wt.% GNPs were measured, and the uncertainties for each property were calculated. Two empirical equations were used to correlate the viscosity and density data along the isotherms. Molecular dynamics simulations were performed to compute the density and viscosity and understand the molecular interaction of the GNP +SA system. A GNP nanoparticle (18-layer graphene nanoplate) embedded in 2123 SA molecules was simulated in a temperature range from 353 K to 378 K at a pressure of 0.1 MPa. The viscosity and density properties of a pure SA liquid and the GNP + SA system are compared with the experimental data. The orientation of the SA molecules for the pure SA and in the presence of GNP is investigated using the radial distribution function. The simulated density and viscosity exhibit the same trend as the experimental data. The simulations demonstrated that the GNP reorganises SA molecules on its surface, indicating a higher linear alignment of aliphatic chains of SA and, as a result, a greater local density of SA around the nanoplatelet.