On the splashing of water-in-oil compound droplets impinging on lyophilic substrates

Droplet splashing is a common and important phenomenon that occurs when droplets impact solid surfaces. This study experimentally investigates the splashing behaviors of compound droplets—composed of a water core encapsulated by a silicone oil shell—on lyophilic glass substrates, with a focus on the effects of silicone oil viscosity (μo) and core-to-shell volume ratio (α). Interestingly, increasing μo from 0.8 to 9.3 mPa s promotes both receding splash and spreading splash. However, a further increase in μo first suppresses spreading splash at 18.9 mPa s and eventually inhibits both kinds of splashing at 96.2 mPa s. We also find that adding a water core into the silicone oil droplets up to α=0.6 slightly enhances receding splash, while exerting a non-monotonic influence on the spreading splash. Detailed analysis of the ejected lamella after the initial stage of its ejection (0.3–0.8 ms) reveals that both types of splashing require the radial velocity at the lamella tip to exceed that at its root, with a critical velocity ratio of approximately 1.5. Comparisons between the experimental results and a theoretical model developed for single-phase droplets show agreement only for the receding splash under low-viscosity oil conditions, while for high-viscosity liquids, the viscous dissipation must be taken into account. Significant discrepancies are observed for spreading splash due to multiple reasons, highlighting the need for further theoretical development.