Shear thinning and shear dilatancy of liquid n-hexadecane via equilibrium and nonequilibrium molecular dynamics simulations: Temperature, pressure, and density effects

Abstract

Equilibrium and nonequilibrium molecular dynamics (MD) simulations have been performed in both isochoric-isothermal (NVT) and isobaric-isothermal (NPT) ensemble systems. Under steady state shearing conditions, thermodynamic states and rheological properties of liquid n-hexadecane molecules have been studied. Between equilibrium and nonequilibrium states, it is important to understand how shear rates (gamma) affect the thermodynamic state variables of temperature, pressure, and density. At lower shear rates of gamma<1 x 10(11) s(-1), the relationships between the thermodynamic variables at nonequilibrium states closely approximate those at equilibrium states, namely, the liquid is very near its Newtonian fluid regime. Conversely, at extreme shear rates of gamma>1 x 10(11) s(-1), specific behavior of shear dilatancy is observed in the variations of nonequilibrium thermodynamic states. Significantly, by analyzing the effects of changes in temperature, pressure, and density on shear flow system, we report a variety of rheological properties including the shear thinning relationship between viscosity and shear rate, zero-shear-rate viscosity, rotational relaxation time, and critical shear rate. In addition, the flow activation energy and the pressure-viscosity coefficient determined through Arrhenius and Barus equations acceptably agree with the related experimental and MD simulation results.

Viscous heating in nanoscale shear driven liquid flows

“…2 exhibits significant variations in the shear rate that results in drastic increases in density and viscosity near the surfaces, which was previously shown in (Tseng et al 2008;Thomas and McGaughey 2007;Ohara and Torii 2005;Jabbarzadeh et al 1999;Khare et al 1997;Liem et al 1992). However, such effects for the weak wetting cases could be negligible.…”

Nanocontraction flows of short-chain polyethylene via molecular dynamics simulations

“…Overall, the CM model predicted rheological properties of shear flows better than 'the transferable potential for phase equilibria (TraPPE)' model [27], which is an adaptation for calculating the vapour -liquid coexistence curve and the surface tension of n-alkane phase diagrams. By contrast, in predicting thermodynamic properties, the TraPPE model is superior to the CM model [17]. Thereby, 50 CH 2 groups of short-chain PE in the present study can be directly extended, largely increasing from the 16 CH 2 groups of the n-hexadecane in our previous study [17] regarding the thermodynamic and rheological discussions of steady shear flows.…”

“…By contrast, in predicting thermodynamic properties, the TraPPE model is superior to the CM model [17]. Thereby, 50 CH 2 groups of short-chain PE in the present study can be directly extended, largely increasing from the 16 CH 2 groups of the n-hexadecane in our previous study [17] regarding the thermodynamic and rheological discussions of steady shear flows.…”

“…Our simple motivation is that an increase in the molecular length will make an enhancement in the degree of non-Newtonian flow behaviours. In this regard, based on our previous studies [17,22], n-hexadecane shear flow and propane nanojets, the objective of the present study is further extended in observing nanocontraction flow phenomena of short-chain polyethylene (PE) fluid. Our PE fluids are [CH 2 ] 50 chains, which belong to long chains in contrast to n-propane (C 3 H 8 ) and n-hexadecane (C 16 H 34 ) molecules.…”

“…Their results clearly indicated that the molecules underwent stretching, rotating and relaxing motions in the nanochannel. It is noteworthy that Tseng et al [17] presented NEMD discussions on liquid n-hexadecane in terms of the variations of shear thinning and shear dilatancy, along with different temperatures, pressures and densities. Additionally, using the Arrhenius equation and Barus equation, they assessed the flow activation energy and the pressure -viscosity coefficient, respectively, which were in good agreement with related MD and experimental studies.…”