Air cushioning in droplet impact. II. Experimental characterization of the air film evolution

A liquid drop approaching a solid surface deforms substantially under the influence of the ambient air which needs to be squeezed out before the liquid can actually touch the solid. We use nanometer- and microsecond-resolved dual wavelength interferometry described in Part I (also published in this issue) to reveal the complex spatial and temporal evolution of the squeezed air layer. In low-velocity droplet impact, i.e., We numbers of order unity, the confined air layer below the droplet develops two local minima in thickness. We quantitatively measure the evolution of the droplet bottom interface and find that surface tension determines the air film thickness below the first kink, after which fluid is diverted outward to form a second even sharper kink. Depending on We, one of the two kinks approaches the surface more closely forming liquid-solid contact. The early time spreading of liquid-solid contact is controlled by the capillary driving force and the inertia of the liquid. The cushioned air film geometry, i.e., a flat micrometer-thin gap, induces an increase of the spreading velocity; the contact area first spreads over the cushioned region, only then followed by radial spreading. This spreading mechanism can lead to the entrapment of one or more air bubbles.