Dynamics of water spreading on a glass surface

The impact of a water droplet on a glass surface is studied experimentally using a high-speed video camera which can catch up to 60,000 images per second with an exposure time of 10 mus. A wide range of impact velocities are studied by varying the fall height, showing different spreading regimes. Particular attention is given to the dynamics of the contact angle and its relation to the maximum expanding radius and capillary number. A linear relation between the contact line velocity and the impact velocity is found experimentally. Using acoustic analysis, an evaluation of the pressure at the contact line is given. We also confront predicted and experimental jetting times. This work shows that descriptions of drop impact based purely on conservation of energy are inadequate to describe the dynamics of the event. The different shapes taken by the drops between the initial impact and the maximum radius determine the final outcome. (C) 2004 Elsevier Inc. All rights reserved.