Rigid separation may play a key role in discontinuous shear thickening

While most complex liquids demonstrate decreasing shear rate with a decreasing viscosity, some, like concentrated cornstarch suspensions, show shear thickening behavior. Discontinuous shear thickening (DST), versus continuous shear thickening, is thought to relate to frictional contact between particles, which can trigger granular behavior. The sensitivity of this phenomenon, however, has yet to be well characterized. New research, reported in Physics of Fluids, investigates the shear thickening phenomenon by studying non-Brownian suspensions under steady shear flow.

Shear thickening presents in two distinct forms, either continuous or discontinuous, depending on the concentration of suspended solids. Unlike the smooth, continuous shear thickening (CST) that occurs at moderate concentrations, DST exhibits an abruptly increasing viscosity at some characteristic shear rate.

To investigate the dependence of suspension viscosity on shear rate, the researchers simulated suspended particles with point-of-contact forces between them. Their contact model contained three key parameters for particle interactions: a critical separation distance (meaning touching particles could not approach closer than a nominal contact separation), the coefficient of friction, and a critical load.

Despite accessing relatively high 2-D concentrations, the simulations only produced moderate, continuous shear thickening. This was surprising as the contact model resembled, in most respects, previous DST models, but led to the conclusion that the fixed contact separation destroys DST.

The researchers believe the true mechanism of DST, therefore, most likely relies on the particle surface layer being compressed during contact. “The question of the detailed mechanism of shear thickening has been reopened by our surprising results,” said co-author Helen Wilson.

Source: “Frictional shear thickening in suspensions: The effect of rigid asperities,” by Adam K. Townsend and Helen J. Wilson, Physics of Fluids (2017). The article can be accessed at https://doi.org/10.1063/1.4989929 .