Natural convection, solute trapping, and channel formation during solidification of saltwater

We present new experimental results relating to the growth and evolution of sea ice. These show, in particular, that brine initially remains trapped in the interstices of the sea ice, only draining into the underlying ocean once the depth of the sea-ice layer exceeds a critical value. A general theory for convection within mushy layers is applied to develop a hypothesis for when brine drainage occurs, which is tested against the experimental results. When ice grows from water-containing impurities, it typically forms a matrix of crystals, and the impurities are rejected into the interstices of the matrix. This is the structure of sea ice, for example, which grows at the surface of the polar oceans when they are cooled from above during winter. In sea ice, the solid matrix is composed of pure ice platelets with randomly oriented horizontal c axes.1 The impurities, principally salt but also man-made pollution and biological organisms, are all concentrated in the interstices between the ice platelets. These impurities increase the density of the interstitial liquid and thu