Periodic Droplet Impact Flows: Wave and Non-Wave Components

Video frames are provided that illustrate the rapid evolution of the fine structure of the flow during the merging of a droplet of water freely falling into a laboratory pool of tap water. At the initial stage of merging in the impact mode, a packet of running periodic annular perturbations around the growing crown was visualized for the first time. The movement of a packet consisting of five to seven rings was tracked by the movement of a group of glares reflected from inclined areas of the liquid surface. The radial distances between the crests of perturbations (analogs of wavelengths) in the packet increases monotonically with distance from the source, which is interpreted as dispersion spreading of the wave packet. Dispersion relations for wave-like components and “classical” capillary waves are derived and their correspondence to the theoretical model is assessed.