Aging in the Flow Dynamics of Dense Suspensions of Contactless Microparticles

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.

💡 Research Summary

In this work the authors investigate the time‑dependent flow behavior of dense suspensions of silica microparticles that interact only through electrostatic repulsion, i.e., “contactless” particles. By employing microfluidic rotating‑drum experiments they are able to prepare well‑defined vertical piles of particles, let them rest for a controlled waiting time (t_w) ranging from minutes to several days, and then induce flow by a single rapid tilt of the drum to a prescribed initial angle (θ_start). Two gravitational Peclet numbers are explored: a low value (Pe_g ≈ 15) obtained with 1.9 µm particles and a high value (Pe_g ≈ 264) obtained with 3.9 µm particles, allowing the authors to probe the intermediate regime where thermal fluctuations are still comparable to particle weight.

The flow is monitored by tracking the free‑surface angle θ(t) with high‑speed microscopy. Two quantitative descriptors are introduced: (i) the start time t_s, defined as the time needed for the angle to reach 90 % of its initial value, and (ii) the mean creep speed ⟨·θ_c⟩, obtained as the average rate of angle decrease during the slow, logarithmic relaxation phase. The key observations are:

1. Logarithmic aging – Both t_s and ⟨·θ_c⟩ depend logarithmically on the waiting time. Longer t_w delays the onset of flow and reduces the creep speed, indicating that the system becomes progressively more resistant to deformation.

2. Peclet‑number dependence – The aging effect is most pronounced at low Pe_g, where thermal agitation can still drive particle rearrangements. When the initial tilt is below the athermal angle of repose (θ* ≈ 5.8°), the flow is entirely thermally driven and the logarithmic slowdown is evident. At high Pe_g, gravity dominates; even when the initial tilt is well above θ*, the flow gradually erases the aging signature.

3. Independence from packing fraction – Direct measurements of the bulk volume fraction before and after the waiting period show no statistically significant change. High‑resolution imaging also reveals no crystallization or permanent clustering, confirming that the observed aging does not stem from densification.

4. Reversibility – After a long rest (e.g., 72 h) the sample can be fully rejuvenated by a brief vigorous rotation that re‑disperses the particles. Subsequent tilting reproduces the flow characteristics of a freshly prepared sample with a short waiting time, demonstrating that the aging is fully reversible and does not involve irreversible chemical degradation or particle contact.

5. Aging during flow – When a pile that has rested for a short time is allowed to flow to completion and then immediately re‑tilted without re‑dispersing, the resulting dynamics match those of a pile that had rested for a much longer time. This suggests that the slow structural evolution continues even while the system is undergoing thermally driven creep.

The authors discuss possible microscopic origins. In the low‑Pe_g regime, the probability for a particle to escape its cage decreases with time, effectively raising an energy barrier that must be overcome by thermal fluctuations. Simultaneously, subtle changes in the electrostatic double‑layer (e.g., slight variations in surface charge due to ion redistribution or pH drift) could increase the repulsive force, further stabilizing the configuration. No measurable change in the macroscopic packing fraction implies that these mechanisms operate at the level of inter‑particle potentials rather than bulk compaction.

Overall, the study provides the first experimental evidence of a genuine aging phenomenon in quiescent suspensions that lie between colloidal and granular limits. It bridges the gap between classic colloidal glass aging (structural relaxation) and granular contact‑strengthening aging, showing that even in the absence of solid contacts, a dense assembly of repulsive particles can become increasingly “solid‑like” simply by waiting. The findings have practical implications for any technology that relies on the flow of dense, thermally active suspensions—such as cosmetics, pharmaceuticals, food processing, and geological sediment transport—by highlighting that storage time can significantly alter flow onset and creep rates, yet the system can be fully reset by a brief mechanical agitation.