How microstructures affect air film dynamics prior to drop impact.

When a drop impacts a surface, a dimple can be formed due to the increased air pressure beneath the drop before it wets the surface. We employ a high-speed color interferometry technique to measure the evolution of the air layer profiles under millimeter-sized drops impacting hydrophobic micropatterned surfaces for impact velocities of typically 0.4 m s(-1). We account for the impact phenomena and show the influence of the micropillar spacing and height on the air layer profiles. A decrease in pillar spacing increases the height of the air dimple below the impacting drop. Before complete wetting, when the impacting drop only wets the top of the pillars, the air-droplet interface deforms in between the pillars. For large pillar heights the deformation is larger, but the dimple height is hardly influenced.

Spatiotemporal evolution of thin liquid films during impact of water bubbles on glass on a micrometer to nanometer scale.

Collisions between millimeter-size bubbles in water against a glass plate are studied using high-speed video. Bubble trajectory and shape are tracked simultaneously with laser interferometry between the glass and bubble surfaces that monitors spatial-temporal evolution of the trapped water film. Initial bubble bounces and the final attachment of the bubble to the surface have been quantified. While the global Reynolds number is large (∼10(2)), the film Reynolds number remains small and permits analysis with lubrication theory with tangentially immobile boundary condition at the air-water interface. Accurate predictions of dimple formation and subsequent film drainage are obtained.