Wetting over pre-existing liquid films

Wetting of a liquid over another, pre-existing liquid film governs several natural phenomena and technical applications, such as coating and oil recovery. The dynamics of this everyday process are poorly understood due to the lack of space and time-resolved techniques, which can discriminate between the two liquids. Here we image a water front moving on a micrometer-thick film of a solid supported silicone oil using laser scanning confocal microscopy. The silicone oil forms a meniscus around the water front. We resolve the spreading dynamics within the meniscus in 3D using tracer microparticles. Capillary suction induces local thinning of the oil film adjacent to the meniscus. When moving the water front forward, viscous forces deform the oil meniscus, giving rise to a wavelike film profile with local backflows. For high velocities, the film profile can be modeled within the Landau-Levich-Bretherton framework. The theory fails to predict the film profile at low velocities where strong capillary-suction-induced backflows occur.