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Compositional dependency of double-diffusive layers during binary alloy solidification: Full-field measurements and quantification

Density variations arising from thermal and compositional gradients in multi-component fluids can lead to natural convection flows. The double-diffusive layer is one such flow phenomenon, commonly observed in oceanic and phase change systems. The solidification of high Prandtl number fluids offers a suitable platform to study multi-diffusive convection owing to continuously evolving temperature and compositional fields. In this work, an experimental investigation was conducted to study the influence of transport phenomena on the double-diffusive layer formation, by performing full-field measurements of concentration and flow velocities during bottom-cooled solidification of a hyper-eutectic aqueous mixture. Using a Mach-Zehnder interferometer, the first-ever real-time, quantitative observations of solutal mixing, plume formation, and the evolution of the double-diffusive layers by forming a stepped compositional distribution have been reported. In addition, the associated flow velocities were measured using the particle image velocimetry technique which clearly characterizes the compositional and thermal natural convection patterns along the vertical and horizontal directions, respectively. The study revealed a life-cycle for the existence of the double-diffusive layers, wherein they undergo onset, development, and disappearance depending on the initial composition, and identified critical Rayleigh numbers for each of these stages. The experimental observations were further supported with analytical scale estimates of the critical length, time, and velocities of the system. The quantitative results elucidate the conditions, including a newly hypothesized threshold composition difference, which led to the formation as well as the disappearance of the layers.Density variations arising from thermal and compositional gradients in multi-component fluids can lead to natural convection flows. The double-diffusive layer is one such flow phenomenon, commonly observed in oceanic and phase change systems. The solidification of high Prandtl number fluids offers a suitable platform to study multi-diffusive convection owing to continuously evolving temperature and compositional fields. In this work, an experimental investigation was conducted to study the influence of transport phenomena on the double-diffusive layer formation, by performing full-field measurements of concentration and flow velocities during bottom-cooled solidification of a hyper-eutectic aqueous mixture. Using a Mach-Zehnder interferometer, the first-ever real-time, quantitative observations of solutal mixing, plume formation, and the evolution of the double-diffusive layers by forming a stepped compositional distribution have been reported. In addition, the associated flow velocities were measured usi...