On the shear-thinning of alkanes

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.