The interaction between drops and solidification front in presence of Marangoni effect

The behaviour of drops in non-isothermal liquids with solidification front has been experimentally and numerically investigated using transparent model fluids and optical diagnostic techniques. The experiments have been performed using Cyclohexane (melting point equal to 6.5 °C) as external matrix and Fluorinert drops. In presence of a temperature gradient, the surface tension driven flows at the drop liquid interface affect the drop/solidification front interaction. Preliminary experiments have been carried out to measure the interfacial tension of the liquid–liquid system under investigation at different temperatures and to study the fundamental behaviour of a single drop close to the melting front, in presence of a temperature gradient. Heating the system from above and cooling from below, the melting front exhibits a shape deformation as a consequence of the surface tension-driven convection around the drop. A systematic analysis has been carried out to investigate the phenomenon with droplets of different sizes. The convective flows have been visualised using particle tracers with average size of 30μm. The results of the numerical simulations at the experimental conditions are in sufficient agreement with the experimental results.The second part of the paper is focused on the investigation of the interaction between free droplets and a melting front, translating a horizontal cuvette between a fixed temperature gradient, with relative speed in the range 0.5–5μm/s. The experiments show that when the relative distance between the droplets and the melting front is of the order of the drop radius, the droplets engulfment in the solid is prevented by a layer of liquid between the drops and the solidification front. This experimental evidence supports the assumption that a “pushing force” is induced by Marangoni effect when the droplets approach the solidification front. Numerical simulations provide further explanation and correlations with the experimental results.