The "laning transition" in colloidal dispersions is a non-equilibrium phase transition in which oppositely driven species form lanes as they pass one another, much like pedestrians on a busy pavement. Originally found in computer simulations, without hydrodynamic coupling the formation of lanes is a first order phase transition. Experiments showed that, in the case of hydrodynamic coupling between the colloids, that the laning transition between oppositely charged colloids is in fact second order. This has since been confirmed with computer simulation by the Duesseldorf Group. The movies show oppositely charged colloids driven by potentials of 50V, 113V and 200V between electrodes at millimetre separation.

Vissers T, Rex M, Imhof, A, Loewen H, Royall CP and van Blaaderen A, 'Lane Formation in Driven Colloidal Mixtures', Soft Matter. 7 2352-2356 (2011). PDF download.