The dynamics of phase separation is central in a wide range of applications spanning from liquid mixing, polymer blends, metallurgy and even battery materials. The process through which a thermodynamically unstable homogeneous mixture undergoes phase separation into its components, for example as a result of cooling below the mixture critical temperature, is called spinodal decomposition. While the spinodal decomposition of mixtures in unbounded systems has been extensively investigated, little attention has been paid to phase separation of mixtures constrained within a slab. In this study we investigate the dynamics of spinodal decomposition of a very viscous regular binary mixture bounded within two walls which are cooled at different temperatures. The analysis is performed via numerical simulations based on a diffuse interface modelling approach consistent with non-equilibrium thermodynamics. Simulations show that, under a temperature gradient, phase separation starts from the cooler wall forming dendritic structures growing anisotropically with time. Two remarkably different dynamic patterns are identified depending on whether heat propagates slower or faster than mass. For small thermal conductivity (i.e., small Lewis number), dendrites grow parallelly to the temperature gradient, keeping such an alignment until the steady-state. On the other hand, for large Lewis number, during the early stages phase separation proceeds within stripes oriented along iso-temperature lines, i.e., with dendrites aligned perpendicularly to the temperature gradient, which, however, gradually shift their orientation parallel to the temperature gradient as the steady-state is approached. Such a dynamic transition of dendrite orientation upon a temperature gradient when heat propagates faster than mass is found to hold also for non-equimolar mixtures and for different species thermal conductivities. These results shed light on the dynamics of phase separation in non-isotropic conditions of an imposed temperature gradient, which is the typical situation found when enhancing heat transfer in microdevices by using phase-separating mixtures. Our results are relevant for polymer melts and quasi-solid solutions as well as, with some analogy, to control the orientation of metallic dendrites during electrodeposition or to tailor lithium intercalation in phase-separating battery electrode materials.
|Titolo:||Diffusive Spinodal Decomposition of Binary Regular Mixtures under a Thermal Gradient|
BERTEI, ANTONIO (Primo)
TELLINI, BERNARDO (Penultimo)
MAURI, ROBERTO (Ultimo)
|Anno del prodotto:||2019|
|Appare nelle tipologie:||4.2 Abstract in Atti di convegno|