Sudden collapse of a colloidal gel

TL;DR

This study investigates the sudden collapse of metastable colloidal gels, revealing a universal height decay law and linking microscopic rearrangements to macroscopic failure.

Contribution

It introduces a simple mathematical model describing gel height during collapse and connects microscopic dynamics to the macroscopic failure process.

Findings

Gel height follows h(t) = h0 - A t^{3/2} during early collapse

Collapse is triggered after a characteristic lag time exceeding the gel's age

Internal stress buildup from local rearrangements causes the sudden collapse

Abstract

Metastable gels formed by weakly attractive colloidal particles display a distinctive two-stage time-dependent settling behavior under their own weight. Initially a space-spanning network is formed that for a characteristic time, which we define as the lag time $\taud$, resists compaction. This solid-like behavior persists only for a limited time. Gels whose age $\tw$ is greater than $\taud$ yield and suddenly collapse. We use a combination of confocal microscopy, rheology and time-lapse video imaging to investigate both the process of sudden collapse and its microscopic origin in an refractive-index matched emulsion-polymer system. We show that the height $h$ of the gel in the early stages of collapse is well described by the surprisingly simple expression, $h(\ts) = \h0 - A \ts^{3/2}$, with $\h0$ the initial height and $\ts = \tw-\taud$ the time counted from the instant where the gel first yields. We propose that this unexpected result arises because the colloidal network progressively builds up internal stress as a consequence of localized rearrangement events which leads ultimately to collapse as thermal equilibrium is re-established.