Kinematics and passing modes of a droplet impacting on a soap film

Abstract

<sec> Droplet passing through a film is a ubiquitous phenomenon in nature, such as a rain drop impacting on a rain bubble in paddle and pouring beer onto the beer foam, etc. This phenomenon has not been sufficiently investigated and many interfacial interaction mechanisms are still unknown. In this paper, the passing modes and the kinematics of a droplet impacting on a soap film are studied with the help of a high-speed cameral. The impacting Weber number of the droplet varies from 10 to more than 350. The droplet position and velocity are extracted from the video by a self-designed Matlab codes. </sec><sec> Experimental results show that the droplet may pass through the soap film in five modes, i.e., bouncing, bagless packaging, package peeling, bag packaging, and instaneous coalescence. A " drop-cushion-shell”-type compound droplet can be formatted in bag-[<i>We</i> ∈ (10.8, 60)] and bagless [<i>We</i> ∈ (120, 240)] packaging mode, while in the package peeling [<i>We</i> ∈ (60, 120)] and coalescence [<i>We</i> ∈ (240, 350)] mode it will form single phase droplets, however, with the surface coated with a soap solution layer (original soap film). Although compound droplets have three surfaces, i.e., the droplet’s original surface and the inner and outer surface of the soap film, the apparent surface tension for the bagless-packed droplet is just that of the soap solution, while for the bag-packed droplet it is the sum of the three surface tensions. The outer shell of the compound droplet may peel off and eject a bubble when the Weber number is in the certain range (<i>We</i> ∈ (60, 120) for droplet with <i>D</i>₀ = 3.0 mm), the lower limit decreases and the upper limit increases with the increase of the initial diameter of the droplets and thus expands the bubble-shooting range. The droplet performs a free fall motion, however, it is interfered by the soap film. The droplet can be stopped and rebounded when <i>We</i> < 10.8, and penetrate the film and start another free fall when <i>We</i> > 10.8. The velocity loss before and after the penetration decrease with impact velocity increasing, hence the motion of the higher We droplet is less retarded by the soap film, the motion curve approaches to the free fall curve. The approaching is not a linear but an accelerating behavior. </sec>