Pre-shear holds the key in understanding discontinuous shear thickening in dense suspensions

The phenomenon of discontinuous shear thickening (DST) in dense non-Brownian suspensions,

wherein the viscosity jumps by several orders of magnitude at a critical shear rate, has received

considerable attention in the last decade. The dominant prevailing opinion is that it is stress con-

trolled transition from a fluid-lubricated regime to a particle-contact dominated regime, modulated

by short-range repulsion between particles. Most of the experimental studies pre-sheared the suspen-

sion for long periods before making rheological measurements, but there has been little discussion

on what changes occur during pre-shear. Here we provide clear evidence the rheology of the sus-

pension evolves during pre-shear. We show that DST occurs only when the strain exceeds a critical

value, and for smaller strain there is only continuous shear thickening (CST). At even larger strain,

the relation between the shear stress and shear rate becomes non-monotonic, yielding a ‘S-shaped’

rheological response. Beyond the DST transition, the stress becomes independent of shear rate,

resembling the response of a dry granular material. We also propose a model that captures the evo-

lution of the suspension micro-structure with strain, via a system parameter obtained using DEM

simulations. Finally, we propose a flow diagram that captures both CST and DST for different

volume fractions and explains the need of such long strains prior to DST