Inertia, coarsening and fluid motion in binary mixtures

Symmetric binary fluids, quenched into a regime of immiscibility, undergo p hase separation by spinodal decomposition. In the late stages, the fluids a re separated by sharply defined, but curved, interfaces: the resulting Lapl ace pressure drives fluid flow. Scaling ideas (of Siggia and of Furukawa) p redict that, ultimately, this flow should become turbulent as inertial effe cts dominate over viscous ones. The physics here is complex: mesoscale simu lation methods (such as lattice Boltzmann and dissipative particle dynamics ) can play an essential role in its elucidation, as we describe. Likewise, it is a matter of experience that immiscible fluids will mix, on some lengt hscale at least, if stirred vigorously enough. A scaling theory (of Doi and Ohta) predicts the dependence of a steady state domain size on shear rate, but assumes low Reynolds number (inertia is neglected). Our preliminary si mulation results (three-dimensional, so far only on small systems) show lit tle sign of the kind of steady state envisaged by Doi and Ohta; they raise instead the possibility of an oriented domain texture which can continue to coarsen until either inertial effects, or (in our simulations) finite size effects, come into play.