Drop Impacts

When a drop strikes a water surface, several phenomena can occur including a splash, the formation of drops, the formation of bubbles, and in rare cases, the bouncing of the drop off of the water surface. We study these phenomena with particular focus on how surfactant monolayers affect all of these processes.

At drop impact velocities that are very low, the drop can actually float or bounce on the water surface. Here the air layer between the drop and the water surface supports the weight of the drop, causing it to bounce away (several times!). Movie by Aynsley Zollinger

At intermediate drop impact velocities, a phenomenon called Mesler entrainment can occur. Here, the sheet of air between the drop and the water bulk collapses in such a way that a very large number of very small bubbles are created. This is shown in the following movie. Note that we are below the water surface in this movie and cannot actually see the drop, which forms the black impact crater at the top of the frame (movie by Brantley Mills).

We have found that viscosity can play an important role in the type of Mesler entrainment that is observed. The following two movies (both by Garrett D. Bounds) were obtained using two silicone oils, each differing in viscosity. The following movie is for a silicone oil having a viscosity of 0.65 centistokes (water has a viscosity of 1.0 centistoke).

And this next movie is for silicone oil having a viscosity of 10.0 centistokes:

Whether or not Mesler entrainment occurs is partially due to the impact velocity. It is also affected by the shape of the drop at impact. Drops oscillate as they fall, as shown in the movie below. The closer the drop is to spherical, the more likely it is that Mesler entrainment will occur.

At high drop velocities, a splash usually occurs. This is shown in the sequence of images presented below. These images were obtained using a high speed video system, framing at 4500 frames/sec. The first image was obtained just before the drop struck the water surface. The second two images were obtained 6,889 and 40,889 microseconds after the first, respectively.

The second image illustrates the presence of what is commonly referred to as the `crown,' which subsequently collapses. In the third image, the crown has collapsed, and a `jet' or `stalk' has formed, rising above the water surface. Depending on the drop impact velocity and diameter, the jet may or may not form satellite droplets which separate from the jet and eventually strike the water surface, reinitiating the entire sequence of events. In this particular set of images, the water surface is highly contaminated with the surfactant Triton X-100.

Movies of the droplet splash process are presented below (Movies by J. R. Saylor).

Low Resolution - 6.3 Mb (QuickTime)

High Resolution - 37.3 Mb (QuickTime)