Aging in the Flow Dynamics of Dense Suspensions of Contactless Microparticles

TL;DR

The paper reveals aging in dense, contactless silica-particle piles resting before flow in an intermediate $Pe_{\mathrm g}$ regime. Using microfluidic rotating drums, the authors show that longer resting times $t_w$ delay flow onset and reduce creep, with a logarithmic dependence on $t_w$—effects stronger for thermally driven creep below $\theta^*$ than for gravity-driven avalanches, where aging is partially erased by flow. Crucially, aging occurs without changes in packing fraction or crystallization, and redispersing the suspension fully rejuvenates the piles, indicating the memory is not permanent. The proposed mechanism involves slow, surface-level electrostatic rearrangements (charge-regulation) and local energy landscape broadening due to ionic redistribution, highlighting an aging phenomenon that straddles colloidal and granular physics with implications for industrial processing of dense suspensions.

Abstract

This study demonstrates that the free-surface flow dynamics of dense piles of contactless silica microparticles depend on the resting period prior to flow. Microfluidic rotating drum experiments reveal that longer resting times lead to delayed flow onsets and reduced flow velocities, both evolving logarithmically with the resting time. These aging-like effects are more pronounced for thermally driven creep flows in piles with initial tilting angle below the athermal angle of repose, in contrast to piles initially tilted above this repose angle, where gravity-driven flows tend to gradually erase aging effects. Moreover, we show that the packing fraction does not change during the resting period, and that aging occurs in both monodisperse and polydisperse piles, indicating that crystallization is not required for the time-dependent behavior to appear. Remarkably, vigorous agitation that re-disperses the particles fully restores the piles to their initial state, demonstrating that the observed effects are not due to sample degradation. These findings evidence a form of aging in quiescent suspensions intermediate between colloidal and granular media, where thermal fluctuations, still significant relative to particle weight, progressively stabilize the system, making it more resistant to flow and deformation.

Figures (13)

• Figure 1: Schematic representation of the rotating-drum experiments. (a) Drum design with inner walls roughness ($\approx 5µm$), molded in PDMS to hold a suspension of silica microparticles in a vertical position. (b) Horizontal video-microscopy setup for flow observation at the drum-scale. (c) Characterization of flow dynamics by tilting a pile to an initial angle $\theta_\mathrm{start}$ and monitoring the relaxation of the angle $\theta$ over time $t$.
• Figure 2: Flow dynamics of gravitationally sedimented piles with low $Pe_\mathrm{g}$, after being tilted to $\theta_{\mathrm{start}} = \ang{5}$. Two key parameters are defined: the start time $t_\mathrm{s}$, when the pile reaches 90% of the initial angle, and the mean creep speed $\langle \dot{\theta}_\mathrm{c} \rangle$, representing the average rate of angle decrease along the logarithmic decay.
• Figure 3: Effect of the waiting time $t_\mathrm{w}$ (the time the pile remains at rest before tilting) on the flow dynamics of gravitationally sedimented piles with low $Pe_\mathrm{g}$, after being tilted to $\theta_{\mathrm{start}} = \ang{5}$. The inset illustrates the rejuvenation of a pile at $t_\mathrm{w}$ = 72h after shaking the sample and tilting again at $t_\mathrm{w}$ = 30min.
• Figure 4: Characteristic parameters of the creep flow dynamics of gravitationally sedimented piles with low $Pe_\mathrm{g}$, after being tilted to $\theta_{\mathrm{start}} = \ang{5}$: (a) start time of the flow $t_\mathrm{s}$, and (b) mean creep speed $\langle \dot{\theta}_\mathrm{c} \rangle$, as functions of $t_\mathrm{w}$. The dashed lines serve as a guide to the eye.
• Figure 5: Flow dynamics of gravitationally sedimented piles with low $Pe_\mathrm{g}$, after being tilted to $\theta_{\mathrm{start}} = \ang{30}$. Two flow regimes are defined: a fast avalanche regime and a slow creep regime, characterized by the start time $t_\mathrm{s}$, the mean avalanche speed $\langle \dot{\theta}_\mathrm{a} \rangle$, the mean creep speed $\langle \dot{\theta}_\mathrm{c} \rangle$, and the critical angle $\langle \dot{\theta}_\mathrm{c} \rangle$, which marks the transition between them.