Colloidal gelation as a nonequilibrium continuous phase transition

A new view on gelation as a second-order nonequilibrium phase transition is presented using combined experiments on critical Casimir colloidal suspensions, simulations, and analytic solutions to a simplified master kinetic equation. The critical Casimir forces provide effective, short-ranged colloidal interactions that can be tuned with temperature, allowing to study gelation over a range of moderate attractive strength (bond energy between 3 and 6k_BT). The experiments and simulations show cluster sizes and correlation lengths diverging with exponents ~1.6 and 0.8, respectively, consistent with growth exponents in percolation theory. Cluster masses exhibit power-law distributions with exponents -3/2 and -5/2 before and after gelation, respectively, as predicted by a master kinetic equation with single-bonded particle detachment. As detailed balance is violated in this process, our results indicate that the observed gelation is a nonequilibrium continuous phase transition (nonequilibrium percolation). These results suggest that the observed gelation process, in which fluid particles aggregate and percolate into rigid structures, is an analogue, mirror-image process of yielding, where emerging fluid-like particles percolate within a rigid matrix.