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.
