Electrowetting induced transitions on superhydrophobic surfaces

Superhydrophobicity is commonly obtained by artificially adding small-scale roughness to hydro-phobic surfaces. In many applications, ranging from “lab on a chip” devices to microelectronics cooling, it is desirable to adjust the degree of superhydro-phobicity, i.e. by controlling how far the liquid penetrates the microscopic surface features. For a given surface topography a liquid droplet can be in two energetic states: 1) the Cassie Baxter state when air remains trapped inside the microscopic crevasses below the droplet and 2) the Wenzel state when the liquid completely fills those cavities. Previous studies suggest that a transition between the two states can be induced by electrowetting (EW), but the under-lying mechanism is not clear, so far. We investigate, directly on the microscale, the morphology of the electrowetting induced transition between the Cassie- Baxter and Wenzel state for a water droplet on a superhydrophobic surface.

Figure 1 Top: sketch of the water drop on the superhydrophobic surface., Bottom: schematic picture of EW induced transition

In our experiments we use a microstructure with 8, 16 μm wide grooves. We analyze the light reflected from the micromenisci looking through the micro-structure to the solid-liquid interaction area. Modula-tions in the intensity of the light reflected from individual micro menisci, indicates that the entrapped air below the droplet is gradually replaced. When applying an electric field the menisci bend, remaining pinned at the edges of the grooves. When the applied voltage exceeds a critical value, the super-hydrophobic state collapses, resulting in a transition to the Wenzel state. Numerical modeling of the system based on a force balance between the Maxwell stress and the Laplace pressure, shows that above some critical voltage there is no equilibrium possible between these two forces, in accordance with our experimental observation.