Sudden collapse of a colloidal gel

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 tau(d), resists compaction. This solidlike behavior persists only for a limited time. Gels whose age t(w) is greater than tau(d) 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 a 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(tau) = h(0) - A tau(3/2), with h(0) the initial height and tau = t(w) - tau(d) 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 reestablished.