Impact on Liquids: Void Collapse and Jet Formation

A spectacular example of free surface flow is the impact of a solid object on a liquid: At impact a “crown” splash is created and a surface cavity (void) emerges which immediately starts to collapse due to the hydrostatic pressure of the surrounding liquid. Eventually the cavity closes in a single point about halfway down its length and shoots out a fast and extremely slender water jet. In the present thesis we study the impact of thin circular discs a few centimeters in radius with impact velocities of a few meters per second. Combining high-speed imaging with sophisticated boundaryintegral computer simulations we elucidate various aspects of this fascinating process.

Next to their undisputable interest for fundamental science such impacts can also be of practical relevance in other disciplines. For oceanographic research it is important that raindrops falling on the ocean entrain small air bubbles after the pinch-off of the impact cavity. This behavior constitutes the main mechanism for carbon dioxide exchange between the sea and the atmosphere and is furthermore a major source of underwater noise. As a medical application, the thin liquid jets which are generated during the collapse of a liquid cavity such as the cavities produced during liquid impact represent a promising possibility for very localized drug delivery into cells or through a patient’s skin.