Soret separation and thermo-osmosis in porous media

Temperature-driven flows in nanochannels: Theory and simulations

The motion of a fluid induced by thermal gradients in the absence of external forces is known as thermo-osmosis. The physical explanation of this phenomenon stems from the emergence of gradients in the tangential pressure due to the presence of a confining surface. The microscopic origin of the effect has recently been elucidated in the framework of linear response theory. Here, by use of conservation laws, we provide an explicit solution of the equations governing the fluid flow at stationarity in slab geometry, expressing the thermo-osmotic coefficient as the integrated mass current-heat current correlation function (which vanishes in the bulk). A very simple expression for the pressure gradient in terms of equilibrium properties is also derived. To test the theoretical predictions in a controlled setting, we performed extensive nonequilibrium molecular dynamics simulations in two dimensions. Few simple models of wall-particle interactions are examined, and the resulting pressure drop and velocity profile are compared with the theoretical predictions both in the liquid regime and in the gas regime.

Impact of the Interfacial Kapitza Resistance on Colloidal Thermophoresis

Thermal gradients impart thermophoretic forces on colloidal particles, pushing colloids toward cold or hot regions, a phenomenon called thermophoresis. Current theoretical approaches relate the Soret coefficient to local changes in the interfacial tension around the colloid, which lead to fluid flow around the colloid surface. The Kapitza resistance, a key variable in the description of interfacial heat transport, is an experimentally accessible property that modifies interfacial thermal fields. Here, we introduce a theoretical approach that describes colloid thermophoretic forces by incorporating explicitly Kapitza resistance effects. Our formulation can be used to monitor the dependence of thermophoresis on interfacial thermal resistance. We show that the resistance modifies the thermal field around the colloids and identify experimental conditions where the Kapitza resistance influences the thermophoretic forces. We validate our theoretical approach by implementing a nonequilibrium molecular dynamics model of a colloid suspended in a solvent.