Roughness-dependent tribology effects on discontinuous shear thickening

TL;DR

This study investigates how nanoscale surface roughness influences the tribological properties and shear thickening behavior of colloidal suspensions, revealing that rougher surfaces promote earlier DST onset and eliminate continuous thickening.

Contribution

It provides the first systematic experimental link between nanoscale surface roughness, tribology, and shear thickening in colloids, especially for non-spherical particles.

Findings

Rougher colloids cause earlier DST onset in shear rate and solid loading.

Surface roughness eliminates continuous shear thickening.

Interlocking asperities induce stick-slip frictional contacts.

Abstract

Surface roughness affects many properties of colloids, from depletion [1] and capillary interactions [2], to their dispersibility [3] and use as emulsion stabilizers [4]. It also impacts particle-particle frictional contacts, which have recently emerged as being responsible for the discontinuous shear thickening (DST) of dense suspensions [5-17]. Tribological properties of these contacts have been rarely experimentally accessed [6, 15, 17, 18], especially for non-spherical particles. Here, we systematically tackle the effect of nanoscale surface roughness by producing a library of all-silica, raspberry-like colloids [19] and linking their rheology to their tribology. Rougher surfaces lead to a significant anticipation of DST onset, both in terms of shear rate and solid loading. Strikingly, they also eliminate continuous thickening. DST is here due to the interlocking of asperities, which we have identified as "stick-slip" frictional contacts by measuring the sliding of the same particles via lateral force microscopy (LFM). Direct measurements of particle-particle friction therefore highlight the value of an engineering-tribology approach to tuning the thickening of suspensions.