Thermal diffusion phenomena in thick fluid layers

Various salt solutions are characterized by a strong tendency of the salt to diffuse from lower to higher temperature zones. This phenomenon, commonly known as thermal diffusion (Soret effect), was studied extensively either for the determination of its own characteristics in various solutions, or in the context of double diffusive convection, where its inclusion introduces peculiar effects. All the experiments reported so far considered a very thin (about few mms) horizontal layer of initially uniform solution exposed to a carefully controlled vertical temperature difference. The existence of a vertical temperature gradient at the impervious horizontal boundaries, gave rise to the establishment of a solute gradient whose magnitude and sign depended on the thermal diffusion properties of the solution. In many situations, however, much thicker fluid layers are encountered and therefore, in this work, thermal diffusion processes were studied experimentally by heating a large tank of salt solution from below while its upper free surface was exposed to room conditions. It is shown that depending on the initial solute distribution, a solution of salt with a large enough negative Soret effect, heated from below, can develop into (at least) two different states. If the process starts with a two-layer stratified solution, a double diffusive system with uniform gradients in equilibrium is established, but if initially the solution is uniform, the resulting system consists of a very thick mixed layer, topped by a relatively thin zone with large gradients of temperature and solute.