Effective Viscosity of Confined Hydrocarbons

TL;DR

This study uses molecular dynamics to analyze how confined hydrocarbon films' effective viscosity varies with shear rate and temperature, revealing a transition from solidlike to liquidlike behavior depending on molecular length and temperature.

Contribution

It provides new insights into the shear-thinning behavior and thermal softening of nanometer-thin hydrocarbon films through detailed molecular dynamics simulations.

Findings

Logarithm of effective viscosity depends linearly on log shear rate.

Longer hydrocarbons soften but do not melt within studied temperature range.

Shorter hydrocarbons melt at high temperatures.

Abstract

We present molecular dynamics friction calculations for confined hydrocarbon films with molecular lengths from 20 to 1400 carbon atoms. We find that the logarithm of the effective viscosity \mu eff for nanometer-thin films depends linearly on the logarithm of the shear rate: log(effective viscosity) = C - n log (shear rate), where n varies from 1 (solidlike friction) at very low temperatures to 0 (Newtonian liquid) at very high temperatures, following an inverse sigmoidal curve. Only the shortest chain molecules melt, whereas the longer ones only show a softening in the studied temperature interval 0 < T < 900 K. The results are important for the frictional properties of very thin (nanometer) films and to estimate their thermal durability.