Sliding or Rolling? Characterizing single-particle contacts

TL;DR

This study uses advanced microscopy to measure forces and motions at particle contacts, revealing how surface roughness and adhesion influence rolling, sliding, and friction, which affect suspension rheology and shear thickening.

Contribution

It introduces a novel experimental method to directly measure microscopic forces and motions at particle contacts, providing new insights into contact mechanics in suspensions.

Findings

Particles spontaneously roll under sufficient traction, reducing dissipation.

Surface roughness causes load-dependent asperity interlocking and off-axis rotations.

Smooth, adhesive surfaces promote principal-axis rolling and lubrication.

Abstract

Contacts between particles in dense, sheared suspensions are believed to underpin much of their rheology. Roughness and adhesion are known to constrain the relative motion of particles, and thus globally affect the shear response, but an experimental description of how they microscopically influence the transmission of forces and relative displacements within contacts is lacking. Here we show that an innovative colloidal-probe atomic force microscopy technique allows the simultaneous measurement of normal and tangential forces exchanged between tailored surfaces and microparticles while tracking their relative sliding and rolling, unlocking the direct measurement of coefficients of rolling friction, as well as of sliding friction. We demonstrate that, in the presence of sufficient traction, particles spontaneously roll, reducing dissipation and promoting longer-lasting contacts. Conversely, when rolling is prevented, friction is greatly enhanced for rough and adhesive surfaces, while smooth particles coated by polymer brushes maintain well-lubricated contacts. We find that surface roughness induces rolling due to load-dependent asperity interlocking, leading to large off-axis particle rotations. In contrast, smooth, adhesive surfaces promote rolling along the principal axis of motion. Our results offer direct values of friction coefficients for numerical studies and an interpretation of the onset of discontinuous shear thickening based on them, opening up new ways to tailor rheology via contact engineering.