On the shear-thinning of alkanes

TL;DR

This paper demonstrates that shear thinning in alkanes can be quantitatively modeled using a simplified Prandtl model with three parameters, revealing an elementary instability and specific pressure-dependent viscosity behaviors.

Contribution

It shows that shear thinning in alkanes can be explained by a Prandtl model with three parameters, linking microscopic dynamics to macroscopic rheology.

Findings

Quantitative agreement between Prandtl model and alkane shear viscosity data.

Identification of an elementary instability causing shear thinning.

Pressure dependence of viscosity near cavitation and at high pressure.

Abstract

The approximate power-law dependence of the apparent viscosity of liquids on shear rate is often argued to arise from a distribution of energy barriers. However, recent work on the Prandtl model, which consists of a point mass being dragged by a damped, harmonic spring past a sinusoidal potential, revealed a similar dependence of the friction on velocity as that of many liquids. Here, we demonstrate that this correlation is not only qualitative but can also be made quantitative over a broad temperature range using merely three dimensionless parameters, at least for alkanes, in particular hexadecane, at elevated pressure p. These and other observations made on our all-atom alkane simulations at elevated pressure point to the existence of an elementary instability causing shear thinning. In addition, the equilibrium viscosity shows power law dependence on p near the cavitation pressure but an exponential dependence at large p, while the additional parameter(s) in the Carreau-Yasuda equation compared to other rheological models turn out justifiable.