Martin Oettel, Universität Mainz
Gravity in a Petri Dish?
Interfacially trapped, micrometer-sized colloidal particles interact via long-ranged capillary attraction which is analogous to two-dimensional screened Newtonian gravity with the capillary length as the screening length. Using Brownian dynamics simulations, density functional theory, and analytical perturbation theory, we study the collapse of a finitely-sized patch of colloids. Whereas the limit of infinite capillary length corresponds to the global collapse of a self-gravitating fluid, for intermediate values of the capillary length we predict theoretically and observe in simulations a ringlike density peak at the outer rim of the disclike patch, moving as an inbound shock wave. For smaller values the dynamics crosses over to spinodal decomposition showing a coarsening of regions of enhanced density which emerge from initial fluctuations. Experimental realizations of this crossover scenario appear to be well possible for colloids trapped at water interfaces and having a radius of around 10 micrometer.