Lane formation in driven colloidal mixtures: is it continuous or   discontinuous?
  Binary mixtures of oppositely charged colloids driven by an electric field are shown to exhibit a nonequilibrium transition towards lane formation if the driving force is increased. Brownian dynamics computer simulations and real-space experiments are employed to study hysteresis effects in an order parameter measuring the extent of lane formation upon increasing and decreasing the driving force. Both from simulation and experiment, we find that lane formation due to electrical fields is continuous. However, simulations show a discontinuous transition if the driving force is gravity. 
Martin Rex, C. Patrick Royall, Alfons van Blaaderen, Hartmut L\"owen