Marangoni effect on the impact of droplets onto a liquid-gas interface

The impact dynamics and internal mixing of a droplet onto a liquid-gas interface of lower surface tension was studied both experimentally and numerically, with both the Ohnesorge number (Oh) and the Bond number (Bo) being fixed. Compared to the droplet impact onto a pool of identical liquid, the interfacial Marangoni flow entrains abundant fluid upward and hence induces an additional jet breakup during crater formation (the first breakup), and it facilitates the emergence of the Rayleigh jet breakup (the second breakup) during crater restoration and enhances the vortical mixing beneath the liquid surface. Specifically, with the increase of the impact inertia, the first breakup manifests a nonmonotonic trend of "absence-presence-absence."The former transition of "absence-presence"at a low droplet-based Weber number (Wed) is caused by the shortened path of the Marangoni flow on the faster-growing liquid bridge, and the later transition of "presence-absence"at a high Wed is resulted from the reduced displacement velocity of the pool fluid on the expanding crater surface. The second breakup corresponds to the Rayleigh jet breakup without surface tension difference and occurs monotonically beyond a certain Wed. Due to the relatively short displacement time of the Marangoni flow on the crater surface compared to the time for crater formation, the critical condition for the emergence of the second jet breakup could be described by the critical reservoir-fluid-based We number (Wer). The critical Wer contains two parts: The Bo-dependent critical Wer0 without surface tension difference, and the increased viscous dissipation from the wrap-up motion of the Marangoni flow. Furthermore, capillary waves are also induced by the Marangoni flow during crater restoration, and the accompanied vorticity generation causes the mixing pattern to exhibit multiple vortex rings and even a clawlike structure, which is substantially enhanced compared to the vortical mixing without surface tension difference.