液滴撞击液膜的穿越模式及运动特性

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

液滴撞击液膜是自然界中广泛存在的一种有趣的物理现象,如雨滴撞击水坑中的雨泡,啤酒穿过而不破坏杯中泡沫等,科学家对此现象进行了一些研究,但对其相互作用机理仍然认识不足.本文利用高速摄像机记录液滴撞击肥皂膜的瞬态过程,研究了不同韦伯数(We∈(10.8, 350))下液滴与液膜的相互作用过程、穿越模式及运动特性.研究结果表明,随着We的增加,液滴从反弹过渡到穿越(临界We=10.8),穿越后,根据液膜与液滴的作用形式,又可细分为无袋型包裹、射泡、有袋型包裹和融合等四种不同穿越模式.穿越后的液滴可形成"液滴-气垫-液膜"型复合液滴(We∈(120, 240))和表皮为微米厚溶液层的单相液滴(We∈(240, 350)).根据气垫层厚度不同,无袋型复合型液滴的表观表面张力为最外层液膜之表面张力,而有袋型复合液滴则为3个气液界面的表面张力之和.复合型液滴外层包膜可能破裂、剥落并产生射泡现象,该现象发生在一定的We数区间内(We∈(60, 120)),且该区间随着液滴直径的增加而增大. We数越大,液滴穿越液膜过程中的速度损失越小,液滴位移曲线越靠近理论曲线.

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.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-[We∈(10.8,60)]and bagless[We∈(120,240)]packaging mode,while in the package peeling[We∈(60,120)]and coalescence[We∈(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 bagpacked 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(We∈(60,120)for droplet with D0=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 We<10.8,and penetrate the film and start another free fall when We>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.