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A drop hitting a solid surface can deposit, bounce, or splash. Splashing arises from the breakup of a fine liquid sheet that is ejected radially along the substrate. Bouncing and deposition depend crucially on the wetting properties of the substrate. In this review, we focus on recent experimental and theoretical studies, which aim at unraveling the underlying physics, characterized by the delicate interplay of not only liquid inertia, viscosity, and surface tension, but also the surrounding gas. The gas cushions the initial contact; it is entrapped in a central microbubble on the substrate; and it promotes the so-called corona splash, by lifting the lamella away from the solid. Particular attention is paid to the influence of surface roughness, natural or engineered to enhance repellency, relevant in many applications.
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Supplemental Video 1: The impact of a mercury drop onto glass for similar impact conditions as Wortington’s sketch in Figure 1b. Courtesy of Erqiang Li. Supplemental Video 2: Prompt splash during the impact of a mercury drop onto a superhydrophobized glass surface. Courtesy of Erqiang Li. Supplemental Video 3: Corona splash of an ethanol drop impacting onto a dry glass plate. Courtesy of Erqiang Li. Supplemental Video 4: Time-resolved interference imaging of the dimple formation and entrapment of an air disc under a water drop impacting a glass plate, at low impact velocity. The view is through the bottom plate. The debt variation between a dark and bright fringe is 160 nm. Frame rate is 5 Mfps. From Li & Thoroddsen (2015). Supplemental Video 5: Time-resolved interference imaging of the dimple formation and entrapment of an air disc under a water drop impacting a glass plate, at high impact velocity. The compression of the air inside the disc is evident from the rapid expansion following the first contact. Frame rate is 5 Mfps. From Li & Thoroddsen (2015). Supplemental Video 6: Entrapment of an air disc under a water drop impacting a glass plate. The view is through the bottom plate. The air disc contracts producing capillary waves on the free surface. These waves touch the glass at the center, thereby entrapping a small microdrop inside the bubble, inside the drop. Frame rate is 50 kfps. From Thoroddsen et al. (2003). Supplemental Video 7: Prompt splash and entrapment of central air disc, for impacting water drop at high velocity. Azimuthal instability is visible in the base of the ejecta. Frame rate is 500 kfps. From Thoroddsen et al. (2012). Supplemental Video 8: Ejected microdroplets. The smallest and fastest droplets emerge first, with progressively larger and slower droplets detaching from the front of the lamella. Frame rate is 1 Mfps. From Thoroddsen et al. (2012). Supplemental Video 9: Ejected droplets and azimuthal undulations on the lamellar surface. Frame rate is 125 kfps. From Thoroddsen et al. (2012). Supplemental Video 10: Air entrapment under a levitated lamella, resulting from local contacts ahead of the moving contact line. Frame rate is 62 kfps. From Thoroddsen et al. (2010). Supplemental Video 11: The impact of a compound drop, outer drop is a water/glycerin mixture of viscosity 2 cP. It contains 20 inner perfluorohexane droplets, which are more dense and migrate toward the bottom free surface of the large drop. For impact velocity V ≃ 4 m/s, drop D ≃ 4 mm. Frame rate is 12 kfps. Courtesy of Jiaming Zhang and Erqiang Li.