Competition between thermophoretic separation and thermal convective mixing

In binary fluid systems under temperature differences, thermophoretic separation and thermal convective mixing are two crucial mechanisms that affect the processes of transport. The thermophoretic effect, also known as the Soret effect, describes the migration behavior of molecules in the fluid with a temperature gradient. Thermophilic or thermophobic molecules tend to migrate to regions of higher or lower temperature, respectively. Thermal convective mixing is triggered by a Rayleigh–Bénard-type hydrodynamic instability as a macroscopic flow caused by buoyancy induced by temperature differences, promoting the mixing of different components within the fluid. The competition between the separation caused by thermophoresis and the mixing produced by convection has significant influence on the flow fields. A dimensionless number, namely the Soret–Rayleigh number SR, which is the ratio of the dimensionless Soret factor ST and Rayleigh numbers Ra, is introduced to quantitatively describe this competition. The impact of different SR and Rayleigh numbers Ra on the concentration field is studied by energy-conserving dissipative particle dynamics in the present study. The results indicate that the component separation exhibits a non-monotonic variation with Ra increase. For a specific SR, there exists an optimal Ra with Rac∼SR−1.53, that achieves the lowest separation at long time scales. This result will have potential implications for the design and application of micro-mixers.