Convection in mushy layers

As a molten alloy or any multi-component liquid is cooled and solidified the growing solid phase usually forms a porous matrix through which the residual liquid can flow. The reactive two-phase medium comprising the solid matrix and residual liquid is called a mushy layer. Buoyancy forces, owing primarily to compositional depletion as one or more of the components of the alloy are extracted to form the solid phase, can drive convection in the layer. In this review, I present an account of various studies of buoyancy-driven convection in mushy layers, paying particular attention to the complex interactions between solidification and flow that lead to novel styles of convective behavior, including focusing of the flow to produce chimneys: narrow, vertical channels devoid of solid. I define an 'ideal' mushy layer and argue that chimneys are an inevitable consequence of convection in ideal mushy layers. The absence of chimneys in certain laboratory experiments is explained in terms of nonideal effects.