Interesting Physics - Photographs

Drop falls into a cup of coffee

The fall of a drop into a cup of coffee and the consequent process is a remarkable event. After the drop touches the surface of the liquid, a fight of gravity, surface tension, inertia and viscosity starts and results in what we see in the pictures. The drops were falling from about 25 centimeters height and triggered a photo gate connected to a photographic flash. The flash fired with a tunable delay which enabled us to capture different phases of the process. The tearing of the liquid column into individual drops is an especially interesting moment. Note also that the shape of the drops is spherical due to the surface tension and not the one usually shown in children books. The last picture shows collision of a secondary drop with a fast primary drop that is just coming down. We used an ordinary camera Nikon D50 to take all the photos.

You can also see a short video (formate Quick Time 6, 56kbps) composed of the photos.