Hydrodynamics of Liquid Mushrooms

Droplets that impact upon the free surface of a liquid pool may generate a vertically rising jet after the cavity formation and collapse events, provided the droplet has sufficient kinetic energy at impact. Depending on the associated time scales and the effect of the Rayleigh-Plateau instability, the jet may either continue to rise upward as a whole or may form satellite droplets via necking of the jet tip. Collision of these structures (either the jet or the secondary droplets) with a second identical incoming droplet, ejected from the same dispensing source as the first one, may result in the formation of various lamellar or domed fluid structures, and depending on the impact conditions, give rise to liquid mushrooms and/or umbrellas. In this research, we experiment with the hydrodynamics of such liquid mushrooms and study the effects of droplet impact velocity, fluid surface tension, and viscosity on the behavior of such lamellar structures. The role of surface tension and viscosity in the dynamics, evolution, and longevity of the mushrooms is studied. We further explore the role of the orientation of incoming droplet impact, i.e., whether head-on or offset collision with the rising jet/satellite droplet. We discuss the spatiotemporal evolution of the mushroom diameters and its susceptibility to surface tension, viscosity, and droplet impact velocity (release height). We put forward a theoretical model based on energetics to predict the maximum spread diameter of the lamellae, and it yields accurate predictions. Our findings may help to provide important insights into a fluid dynamic phenomenon observed in nature, studied extensively by artists and photography enthusiasts for its aesthetics, and may be important in certain niche utilities as well.