Colloidal Suspensions Can Have Nonzero Angles of Repose below the Minimal Value for Athermal Frictionless Particles.

We investigate the angle of repose θ_{r} of dense suspensions of colloidal silica particles (d=2-7  μm) in water-filled microfluidic rotating drum 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 θ_{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<θ_{r}<θ_{ath}≈5.8°. We quantify the arrest dynamics as a function of the gravitational Péclet number Pe_{g}, which characterizes the competition between particle weight and thermal agitation. Our measurements are consistent with a recent rheological model [A. Billon et al., Phys. Rev. Fluids 8, 034302 (2023)PLFHBR2469-990X10.1103/PhysRevFluids.8.034302], 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.