Editorial

Thermal Forces in Simple and Complex Fluids

This topical issue of The European Physical Journal E deals with mass transport effects driven by thermal gradients, or, as we shall call them, with "thermal forces". In simple fluid mixtures, coupling of heat and mass diffusion is due to the Ludwig-Soret effect, also known as thermal diffusion. The Soret effect dramatically lowers the thermal convection threshold, since concentration gradients relax much more slowly than temperature gradients, due to the disparate values of the mass diffusion coefficient and of the thermal diffusivity. Therefore, thermal diffusion plays an important role in naturally occurring processes like thermohaline convection ("salt fingering") in oceans, crystal growth, component segregation in metallic alloys, volcanic lava and the Earth crust. More recently, its has been shown that thermal diffusion is a very convenient process in the generation of giant non-equilibrium fluctuations in fluid mixtures.

Thermophoresis is a closely akin process that takes place both in aerosols and in liquid suspensions, consisting in the drift of dispersed particles induced by thermal inhomogeneities. Thermophoresis of airborne particles has an important role in atmospheric physics and ambient pollution, and can seriously affect semiconductor manufacturing. In macromolecular solutions and colloidal suspensions, these "thermal forces" are generally much stronger than in simple mixtures or in gases: for instance, recent experiments on DNA solutions have shown that thermophoresis may concur with thermal convection in leading to patterns where the local macromolecular concentration is amplified up to a thousandfold.

Although known since a long time and clearly framed in terms of non-equilibrium thermodynamics concepts, both the Soret effect and particle thermophoresis in liquids still lack a general microscopic picture. For instance, in most cases the denser component of a binary mixture drifts to the cold, but examples of reverse behaviour are common, and no model is so far able to give a general prediction of the direction of thermodiffusive motion. Efforts in understanding the physical origin of the Soret effect in model systems like colloidal systems will probably be of great relevance. At the same time, analyzing Soret-driven hydrodynamic effects in complex fluids, and exploiting thermophoresis as a macromolecular separation/concentration method (as already done by Thermal Field-Flow Fractionation) are promising research topics.

This renovated interest in Soret-related phenomena is mirrored by the rather diversified list of contributions that appear in the present issue. These papers have been presented at the International Meeting IMT6 - Thermal Forces held in Villa Cipressi (Varenna, Italy) in July 2004, as the sixth edition of a series of conferences run under the auspices of the European Group of Research in Thermodiffusion (EGRT). The conference falls on the hundredth anniversary of Charles Soret death. Very duly, a commemorative contribution by Jean Platten and Pierre Costesèque, on behalf of EGRT, opens this issue.

In detail, the contributions cover the following topics. Soret effects in simple fluid mixtures are discussed in the first two papers: subtle problems related to thermal diffusion in multicomponent mixtures are addressed by Saghir et al., while convection-free measurements of the Soret coefficient in porous media are described by Costesèque et al. The paper by Parola and Piazza presents a general theoretical approach to particle thermophoresis in liquids, while Enge and Köhler give a detailed analysis of thermal diffusion in polymer blends. Blums reviews coupled magnetic and Soret effects in ferrofluids, while Duhr et al. describe microfluidics methods for measuring DNA thermophoresis, and Steinbach et al. design an optical trap for studying particle thermophoresis in gases under microgravity conditions. The second part of the issue is dedicated to thermal diffusion effects on convection. The role of Soret-driven concentration fluctuations in oscillatory convective patterns and their contribution to the buoyancy forces driving convection are elucidated in the paper by Jung et al. , while transient oscillations in colloidal suspensions are experimentally investigated by Mazzoni et al. Mercader et al. discuss the Eckhaus instability of travelling periodic roll patterns, and non-equilibrium fluctuations in the Rayleigh-Bénard convection is addressed by Ortiz de Zárate et al. Finally, Razi et al. discuss the effects of vibrations on the onset of Soret-driven convection under normal- or low-gravity conditions.

The IMT6 Conference has been made possible thanks to the support by INFM (Istituto Nazionale per la Fisica della Materia) and CNR (Consiglio Nazionale delle Ricerche). Support by Università di Milano, Dipartimento di Fisica, and by Politecnico di Milano, Dipartimento di Ingegneria Nucleare is also gratefully acknowledged.