Séminaire de Mécanique d'Orsay

Le Jeudi 6 décembre à 14h00 - Salle de conférences du FAST

Computational Studies of Marangoni Instabilities in Evaporating Thin Films

Stergios G. Yiantsios
Department of Chemical Engineering, Aristotle University of Thessaloniki

Marangoni phenomena have a wide range of manifestations of both fundamental and practical importance. In the paint and coating technology they are usually undesirable since they induce nonuniformities and defects. On the other hand, in evaporating polymer solutions the structures developed may have desirable properties for nanotechnological applications (organic photovoltaics, photodiodes). In this talk aspects of relevant Marangoni phenomena are highlighted by the tools of hydrodynamic stability and direct nonlinear simulations. First, a thin film is considered containing a stable dispersion of colloidal particles. Evaporative cooling drives a thermocapillary flow leading to uneven and inhomogeneous final particle coating. Numerical simulations suggest possible defect types, like craters or pinholes, or a closely packed skin with voids underneath. Next, a mechanism of instability is presented having its origin on the effects of a soluble surfactant. As the film dries thickness perturbations lead to surfactant concentration perturbations, which in turn drive fluid motion and enhance unevenness. Large Marangoni stresses and bulk surfactant solubility promote the instability, whereas low bulk solubility and increased interfacial viscosity suppress disturbance growth. Finally, the characteristics of solutocapillary instability in binary mixtures are analyzed. When surface tension increases with the concentration of a non-volatile solute, Marangoni stresses lead to the development of cellular patterns similar to the well-known hexagons of the thermocapillary instability. An additional mode also exists analogous to the respective deformational instability, which is manifested as a long-wave oscillatory behavior with fast leveling and subsequent reversal of film thickness disturbances. On the other hand, when surface tension decreases with increasing solute concentration the deformational mode still exists but now leads to monotonic disturbance growth. The above characteristics are supported by experimental observations in the literature.