An airbag for drops : high speed interferometry studies of air film lubrication in drop impact

Abstract

The impact of droplets on solid surfaces is of wide-spread relevance in for example pesticide spraying, fluid coating, and ink-jet printing. The impact process includes the formation and spreading of an air film between the droplet and the surface before the droplet actually touches the surface. This so-called ‘air-cushioning effect’ guarantees a relatively soft landing or even prevents the impact altogether – similar to the protection by an airbag in a car accident. Here, the naturally present ambient air acts as the airbag since a lubrication pressure builds up in the air film while it is being squeezed to a thickness of only a few micrometers. In this thesis we describe an improvement of the reflection interference microscopy technique, with which we can measure the spatiotemporal film thickness evolution up to 8 micrometer with a so far unknown resolution of a few tens of nanometers and microseconds. We study the air film for millimeter-sized droplets impacting at Weber numbers We around unity – indicating that both surface tension and inertia play a role on the impact dynamics. The film evolution has important consequences for the actual formation of liquid-solid contact and subsequent spreading dynamics. In particular, the air film thickness develops one or two off-center local minima along the radial direction, from which liquid-solid contact is formed when a critical thickness of 200 nm is reached. While a small air bubble can be entrapped by a ring-shaped contact, for larger interfacial tension effects (We< 4) contact is delayed for at least a few milliseconds. The subsequent single contact spot spreads according to an inertial-capillary scaling. Remarkably, the droplet can even bounce repeatedly on a persisting air film. The observed asymmetry in the spreading and contraction of the air layer during the bounce proves critical for an efficient momentum reversal: the net – purely dissipative – lubrication force remains positive even when the droplet moves away from the substrate. Finally, we show that sharp vertical micro-texture edges (fabricated using lithography) locally change the air flow, and force liquid-solid contact where the air film thickness is smallest. This offers control over the onset of wetting.