Experiments on the breakup of drop-impact crowns by Marangoni holes

Abstract

We investigate experimentally the breakup of the Edgerton crown due to Marangoni instability when a highly viscous drop impacts on a thin film of lower-viscosity liquid, which also has different surface tension than the drop liquid. The presence of this low-viscosity film modifies the boundary condition, giving effective slip to the drop along the solid substrate. This allows the high-viscosity drop to form a regular bowl-shaped crown, which rises vertically away from the solid and subsequently breaks up through the formation of a multitude of Marangoni holes. Previous experiments have proposed that the breakup of the crown results from a spray of fine droplets ejected from the thin low-viscosity film on the solid, e.g. Thoroddsen et al. (J. Fluid Mech., vol. 557, 2006, pp. 63–72). These droplets can hit the inner side of the crown forming spots with lower surface tension, which drives a thinning patch leading to the hole formation. We test the validity of this assumption with close-up imaging to identify individual spray droplets, to show how they hit the crown and their lower surface tension drive the hole formation. The experiments indicate that every Marangoni-driven patch/hole is promoted by the impact of such a microdroplet. Surprisingly, in experiments with pools of higher surface tension, we also see hole formation. Here the Marangoni stress changes direction and the hole formation looks qualitatively different, with holes and ruptures forming in a repeatable fashion at the centre of each spray droplet impact. Impacts onto films of the same liquid, or onto an immiscible liquid, do not in general form holes. We furthermore characterize the effects of drop viscosity and substrate-film thickness on the overall evolution of the crown. We also measure the three characteristic velocities associated with the hole formation: i.e. the Marangoni-driven growth of the thinning patches, the rupture speed of the resulting thin films inside these patches and finally the growth rate of the fully formed holes in the crown wall.

Miscibility and wettability: how interfacial tension influences droplet impact onto thin wall films

“…Aljedaani et al. (2018) focused on the occurrence of holes in the crown, similar to some extent to those observed in the right-hand column of figure 1. The holes formed on the crown wall grow until they join and form a net-like structure which finally disintegrates into secondary droplets (see the movie in the supplementary material available at of the impact process corresponding to the experiments in figure 1).…”

“…Although Aljedaani et al. (2018) noticed only a weak reduction in the growth rate of the holes for increasing droplet viscosity, a large viscosity ratio between droplet and wall film influences the liquid repartition in the crown. For droplets with a much higher viscosity than the wall film, a two-stage crown was observed with the wall-film liquid at the upper part of the crown.…”

“…This particular type of splashing event was first observed for a water/glycerol droplet impacting onto very thin wall films of ethanol (Thoroddsen et al 2006) and is a unique feature of binary droplet/wall-film systems. The hole formation was attributed to Marangoni-driven flows initiated by small secondary droplets impacting onto the inside part of the crown wall (Thoroddsen et al 2006;Aljedaani et al 2018;Kittel 2019). This phenomenon was also observed with very similar surface tensions for droplet and wall film (hyspin and hexadecane liquids) and was thus attributed more generally to inhomogeneities in viscosities and/or surface tensions in the crown wall (Geppert et al 2016).…”

“…Hence, only one-stage crowns are observed, where the droplet is expected to cover uniformly the wall-film liquid in the crown wall. In addition to the miscible configurations leading to the formation of holes, Aljedaani et al (2018) studied the impact of immiscible liquid pairs. They did not report any major differences in the crown dynamics (looking especially at the crown angle), but the hole formation vanishes despite differences in surface tension between droplet and wall film.…”

“…2006; Aljedaani et al. 2018; Kittel 2019). This phenomenon was also observed with very similar surface tensions for droplet and wall film (hyspin and hexadecane liquids) and was thus attributed more generally to inhomogeneities in viscosities and/or surface tensions in the crown wall (Geppert et al.…”