What triggers the gravitational collapse of a colloidal gel?

n a colloidal gel the particles are arrested in a space-spanning network which can support its own weight for a finite delay time before collapsing. This delay time sets the shelf life of many products. Therefore, predicting and avoiding this delayed collapse is of great scientific interest. We study the collapse of a colloidal gel in which we induce depletion attraction by the addition of non-absorbing polymers. We distinguish between two classes of delayed collapse gels, both collapse after a delay time but the first class collapses rapidly and second class collapses slowly. At high colloid volume fractions and attraction strength we observe slowly collapsing gels. Microscopic imaging shows fluid rich droplets rising up through the gel, giving rise to volcanos as they reach the top. The gels stays intact but the pathways formed by these volcanos remain and act as channels for the fluid to drain more rapidly from the gel. At low colloid volume fraction and attraction strength we observe rapidly collapsing gels. Here the collapse is triggered when dense clusters of particles, formed at the top, fall through the gel. As the clusters fall through, the entire gel structure is destroyed by shear. Then, the sample collapses rapidly. We are investigating what initiates the formation of dense clusters at the top or fluid rich droplets at the bottom.