Air bubble entrapment under an impacting droplet

We simulated impact of water, n-heptane, and molten nickel droplets on a solid surface. A numerical code was developed to model the motion of both the liquid in the droplet and the surrounding air. The model used a volume-of-fluid method to track the droplet surface and assumed that only one flow field governed the motion of all the fluids present. Predicted droplet shapes during impact agreed well with photographs. When a droplet approached another surface, air in the gap between them was forced out. Increased air pressure below a droplet created a depression in its surface in which air was trapped. The magnitude of pressure rise could be predicted using a simple analysis of fluid between two solid planes moving closer together. The air bubble formed at the solid-liquid interface remained attached to the solid surface in a water droplet. In an n-heptane droplet the bubble moved away from the surface and broke into two or three smaller bubbles before escaping through the droplet surface. This difference in behavior was attributed to the contact angle of water being much larger than that of n-heptane. (C) 2003 American Institute of Physics.