Liquid rims collisions and the formation of fines

As an elementary mechanism for the formation of drops from liquid sheets, we investigate the collision of liquid cylinders. This results from the opening of two nearby holes on a liquid film, growing at a constant speed while collecting liquid into two rims, eventually colliding with each other. In this surface tension driven phenomenon, a unique Weber number $We = \rho (2V)^2 2a/\sigma$ controls a variety of behaviors ($\rho, \sigma$ are the liquid density and surface tension, and $2V$ the relative velocity of the impinging rims, each of individual radius $a$). At low $We$, the rims merge through an inelastic, dissipative collision which produces a corrugated ligament, finally breaking into drops of size scaling like $a$, on average. Above a critical $We_c \approx 60$, the collision leads to a splash, with the formation of a thin transverse liquid sheet. We will describe the expansion-retraction dynamics of this secondary sheet and its destabilization, responsible for the production of a mist of finer droplets. These alter sensibly the mean, and overall drops size distribution, thus weighted by a substantial fraction of so-called fines.