The coalescence of a drop.
When a drop is deposited gently onto the surface of a layer of the same liquid, it sits momentarily before coalescing into the bottom layer. High-speed video imaging reveals that the coalescence process is not instantaneous, but rather takes place in a cascade where each step generates a smaller drop. This cascade is self-similar and we have observed up to six steps. The time associated with each partial coalescence scales with the surface tension time scale. The cascade will, however, not proceed ad infinitum due to viscous effects, as the Reynolds number of the process is proportional to the square root of the drop diameter. Viscous effects will therefore begin to be important for the very smallest drops.
Grazie ad una speciale telecamera in grado di riprendere immagini a 10000 frames per secondo (400 volte più veloce dell’occhio umano) questo gruppo di ricercatori del MIT ci illustra con chiarezza l’effetto della “coalescenza a cascata” di una goccia.