Tuning Interparticle Hydrogen Bonding in Shear-Jamming Suspensions: Kinetic Effects and Consequences for Tribology and Rheology.

The reversible shear-induced solidification of dense suspensions, known as shear jamming, critically depends on frictional interparticle contacts. Recently, it was shown that shear jamming can be strongly affected by molecular-scale interactions between particles, e.g., by chemically controlling their propensity for hydrogen bonding. However, hydrogen bonding not only enhances interparticle friction but also introduces (reversible) adhesion, whose interplay with friction in shear-jamming systems has so far remained unclear. Here, we present atomic force microscopy studies to assess interparticle adhesion, its relationship to friction, and how these attributes are influenced by urea, a molecule that interferes with hydrogen bonding. We characterize the kinetics of this process with nuclear magnetic resonance, relating it to the time dependence of the macroscopic flow behavior with rheological measurements. We find that time-dependent urea sorption reduces friction and adhesion, causing a reduction in the high-shear viscosity. These results extend our mechanistic understanding of chemical effects on the nature of shear jamming, promising new avenues for fundamental studies and applications alike.