Wetting prevention by thermal marangoni effect. Experimental and numerical simulation

Abstract

The behaviour of small liquid drops, hanging from a circular disk and approaching a flat wall at a different temperature is studied experimentally and numerically. If the pendant drop and the solid surface are at the same temperature and if the liquid wets the solid, the drop spreads over the surface forming a liquid bridge in times of the order of milliseconds. If the upper disk is heated and/or the solid surface is cooled, then the drop does not wet the wall, even if pressed against the surface, but it is deformed in a completely reversible way, similarly to an elastic material (e.g. like a rubber balloon). To investigate this unusual and intriguing phenomenon, a systematic experimental programme has been carried out on silicone oils (with different viscosities) and on diesel oils. At the same time the problem was studied numerically under the assumption that a thin air film exists between the drop and the solid surface. This film is formed by the entrainment of the surrounding air caused by the Marangoni flow directed, along the liquid surface, from the upper disk towards the contact zone. If suitable conditions are established, the pressure in the air film balances the pressure necessary to deform the liquid drop, preventing the wetting of the solid surface. The experimental results agree with the proposed numerical model. In particular the computed equilibrium air film thicknesses are compared with the thicknesses measured with a background illumination system and with an interferometric technique; a good agreement is found between numerical and experimental results, for different liquids and different geometrical configurations.