When colloidal particles find themselves in a temperature gradient they move in response to it, in some cases toward the hotter some toward the cooler side, depending on the specific physical chemistry of the colloid and the solvent surrounding it. This process, called thermophoresis, is generally regarded as a phoretic phenomenon: the thermal motion of a colloid is mainly driven by local hydrodynamic stresses in the surrounding liquid. However a complete and unique theoretical description of thermophoresis has been lacking.
In this EPJ E Colloquium, Burelback, Frenkel, Pagonabarraga and Eiser use the dynamic length and time scale separation in suspensions to formulate a general description of colloidal thermophoresis. Their approach allows an unambiguous definition of separate contributions to the colloidal flux and clarifies the physical mechanisms behind non-equilibrium motion of colloids. In particular, the authors derive an expression for the interfacial force density that drives single-particle thermophoresis in non-ideal fluids. The relations for the transport coefficients explicitly show that interfacial thermophoresis has a hydrodynamic character that cannot be explained by a purely thermodynamic consideration. This is a new treatment that generalises the results from other existing approaches, giving them a clear interpretation within the framework of non-equilibrium thermodynamics.