Viscous trees and Voronoi-structure formation in expanding systems

A viscous tree arises when two plates sandwiching a fluid are being separated. By experimental investigations of such self-organized trees it is revealed that they originate from the competition for space between viscous fingers. Scaling properties are addressed and a computer simulation based on a phenomenological model is presented. If the two plates are separated rapidly (or in vacuum) cavitation appears leading to the formation of a foam. Cavitation is suggested as a possible mechanism for the formation of large-scale structure in the Universe.