  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.
