On thermal stress and thermal creep convection in vapour crystal growth under microgravity

In recent years a new impetus for studying vapour crystal growth fluid-dynamics has come from the utilization of space microgravity environment, where low-gravity conditions are expected to reduce gravity-driven disturbances and, therefore, to enhance the final properties of the crystals, resulting from the complex physicochemical phenomena occurring in the vapour phase. However, some "gas-kinetic" effects, usually masked by gravity on Earth, such as thermal and concentration stresses (volume-driving actions), thermal and concentration creep (surface-driving actions) have been demonstrated to become important sources of convection under the conditions encountered in low-gravity crystal growth. The new sources of convection arising from non-linear irreversible thermodynamic effects (non-Navier-Stokes fluid-dynamics) and slip boundary conditions have been studied both theoretic ally and numerically; 15 classes of convection, and a priori conditions for the existence and characterization of all possible classes of convection in terms of the problem's data, have been identified. Then, numerical results are obtained with the new set of field equations (Burnett equations with slip conditions) and compared, for different gravity levels, with the corresponding solution of the Navier-Stokes approximation. On the base of the previous results, we present the idea of an experimental research, for the International Space Station, dealing with the new transport mechanisms. The authors intend to study the occurrence, in a reduced gravity environment, of new types of convective flows induced by thermal stresses and thermal creep, in order to definitively validate the theoretical and numerical results and to obtain, for the first time, relevant experimental data on the phenomenon.