Fluid Drops Impacting Solid Surfaces
Drops 'Skate' on a Thin Layer of Air

When a drop of fluid falls onto a solid surface, it has always been assumed that it impacts and wets immediately the surface. Mahadevan and Weitz showed that this is incorrect, and instead, the drop traps a very thin film of air that separates the fluid from the surface. These experimental results are in complete accord with theoretical predictions of Brenner. These measurements require new experimental techniques: The interface is imaged with total reflection microscopy, enabling the thin film to be visualized (left). In addition, a new imaging modality was developed to enhance the temporal resolution of the spreading drop (right). These experiments show that trapped air has an extremely important effect on the impact of a drop on a surface, confirming theoretical calculations.