Electrowetting-Induced Oil Film Entrapment and Instability

Abstract

We investigate the spreading at variable rate of a water drop on a smooth hydrophobic substrate in an ambient oil bath driven by electrowetting. We find that a thin film of oil is entrapped under the drop. Its thickness is described by an extension of the Landau-Levich law of dip coating that includes the electrostatic pressure contribution. Once trapped, the thin film becomes unstable under the competing effects of the electrostatic pressure and surface tension and dewets into microscopic droplets, in agreement with a linear stability analysis. Our results recommend electrowetting as an efficient experimental approach to the fundamental problem of dynamic wetting in the presence of a tunable substrate-liquid interaction. Apart from technological applications, EW has also proven to be a very useful tool for studying fundamental problems in wetting and thin film hydrodynamics, where the contact angle is often a crucial parameter that is difficult if not impossible to vary experimentally without changing other important aspects of the system. Examples include wetting of complex surfaces [3,4], capillary pinch-off and microdroplet generation [5][6][7], and deposition [8]. Frequently, electrowetting experiments are performed in an ambient oil bath in order to minimize both the evaporation of liquid and contact angle hysteresis. It has been indicated by several authors [9][10][11][12][13] that thin layers of the ambient oil might form between the drop and substrate in such a twophase configuration. Quilliet and Berge [9] found theoretically that the balance between electrical forces and the disjoining pressure should give rise to an equilibrium thickness of the films of approximately 10 -20 nm for typical values of the applied voltage. However, despite the importance of these layers-for instance for the reduction of contact angle hysteresis, but also for the protection of the surfaces from adsorption of biomolecules [10,14]their properties and formation mechanism remained elusive in previous experimental studies [11].In the present Letter we study the dynamics of moving contact lines in EW systems with a two-phase configuration, as just described. We show that a layer of oil is indeed entrapped under the drop with an initial thickness that is determined by the hydrodynamics of the moving contact line rather than by equilibrium properties. To describe the entrapment process we extend the Landau-Levich [15] treatment of dynamic wetting by an additional electrostatic pressure contribution, a topic that attracted considerable attention in the recent wetting literature [16 -18]. Following the entrapment, the oil film turns out to be unstable and breaks up into a number of smaller oil droplets. The size distribution of these droplets is described by a linear stability analysis of the thin film in the lubrication approximation, taking into account the balance between surface tension and electrostatic pressure. The problem thus combines two aspects: the entrapment process itself and the subsequent time evolution of the entra...

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