  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.
