The puzzle of delayed gel collapse

In the presence of attractive interactions (Uc), colloidal particles aggregate forming a space spanning network, or gel. Sedimentation or creaming of the particles imposes a stress on the network which ultimately leads to its collapse. Strong gels (Uc > 10 kBT) show a fairly simple behaviour with collapse occurring smoothly. Weak gels (Uc < 10 kBT), in contrast, display a very unusual behaviour with a period of apparent stability, where nothing appears to be happening, followed by a sudden abrupt collapse. Rapid gel collapse is a very undesirable phenomenon which limits the shelf life of a diverse range of food, pesticide and personal care products. The microscopic mechanisms responsible for this peculiar behaviour however remain an intriguing and open question. In this talk we present measurements of anomalous collapse in a weakly attractive network induced by depletion and present a possible mechanism. We use a fluorescently labelled colloidal suspension consisting of a nearly monodisperse emulsion of PDMS in an index-matching mixture of ethylene glycol and water. The microscopic evolution of the network structure, both before and during collapse, is followed by confocal microscopy.