Colloidal Suspensions can have Non-Zero Angles of Repose below the Minimal Value for Athermal Frictionless Particles

TL;DR

This study explores how colloidal suspension piles can have non-zero angles of repose below the minimal value for frictionless particles, revealing a crossover between thermal and athermal regimes.

Contribution

It demonstrates that colloidal suspensions can have finite repose angles influenced by particle size and thermal agitation, bridging thermal and granular behaviors.

Findings

Small particles always flatten, with zero repose angle.

Larger particles stop flowing at a finite angle of repose.

Repose angle increases with particle size but stays below 5.8°.

Abstract

We investigate the angle of repose ${\theta}_r$ of dense suspensions of colloidal silica particles ($d = 2$ $\mu m$ to $7$ $\mu m$) in water-filled microfluidic rotating drums experiments, to probe the crossover between the thermal (colloidal) and athermal (granular) regimes. For the smallest particles, thermal agitation promotes slow creep flows, and piles always flatten completely regardless of their initial inclination angle, resulting in ${\theta}_r = 0$. Above a critical particle size, piles of colloids stop flowing at a finite angle of repose, which increases with particle size but remains below the minimal value expected for athermal frictionless granular materials: $0 < {\theta}_r < {\theta}_{ath} \approx 5.8{\deg}$. We quantify the arrest dynamics as a function of the gravitational P\'eclet number $Pe_g$, which characterizes the competition between particle weight and thermal agitation. Our measurements are consistent with a recent rheological model [Billon et al., Phys. Rev. Fluids 8, 034302, 2023], in which the arrested state stems from a crossover between glass and jamming transitions as the granular pressure in the pile increases relative to the thermal pressure.