At temperatures close to absolute zero, a phase transition called Bose–Einstein condensation takes place, in which quantum mechanics 'takes over' and rules the behaviour at macroscopic scales. Superfluidity is a consequence of Bose–Einstein condensation, and occurs in liquid helium, ultra-cold atomic gases and neutron stars. The striking property of a superfluid is that it can move without any viscous effects. Since it suffers no friction, a superfluid can flow freely through infinitesimal holes, move around a closed loop forever, and climb up the walls of its container. Research in this area is largely concerned with nonlinear, dissipative and stochastic effects in atomic Bose–Einstein condensates, with particular emphasis on solitons and vortices, quantised vortices in superfluid liquid (quantum turbulence), and the role of superfluidity in neutron stars. A problem which is receiving increasing attention is the observed similarity between quantum turbulence and turbulence in classical fluids. See the website of the Joint Quantum Centre for more information.
In the area of classical fluids, solitons and nonlinear waves are topics of ongoing research. Dispersion is a common phenomenon in wave propagation, in which perturbations of different wavelengths move at different speed. An example of dispersion is the rainbow, where the different colours which make up white light propagate differently inside droplets of rain and can be seen separately. Solitons are remarkable solutions of wave equations in which nonlinear terms balance out the effects of dispersion exactly, so that solitons keep their shape as they travel, like particles. Another topic of current interest is nonlinear waves on shallow water, an example of which is a tsunami. Current work is also concerned with turbulence, vortex filaments, and instabilities in Taylor–Couette flow.