FYFD

Cold rain splashing on airplane wings can freeze in instants. To prevent that, researchers look for ways to minimize the time and area of contact a drop has. Hydrophobic coatings and textures can do some of the work, but they are easily damaged and don’t always work well when it comes to freezing.

The new technique shown here uses ring-shaped “waterbowls” to help deflect drops. As the drop impacts and spreads, the walls of the ring texture force the lamella up and off the surface. This reduces both the time and area of contact and, under the right circumstances, cuts the heat transfer between the fluid and surface in half. The technique is useful for more than just preventing freezing, though; it would also be helpful for waterproofing breathable fabrics, where shedding moisture quickly without clogging pores is key to keeping the wearer dry. (Image and research credit: H. Girard et al.; via MIT News and Gizmodo)

Diving can generate some remarkable splashes. Here researchers explore the splashes from a wedge-shaped impactor. At high speeds, they found that the splash sheet pushed out by the wedge curls back on itself and accelerates sharply downward to “slap” the water surface (top). Studying the air flow around the splash sheet reveals some of the dynamics driving the slap (bottom). The splash sheet quickly develops a kink that grows as the sheet expands. This creates a constriction that accelerates flow on the underside of the sheet. That higher velocity flow means a low pressure inside the constriction, which pulls the thin sheet down rapidly, making it slap the surface. For more, check out the full video. (Image and research credit: T. Xiao et al., source)

Shoot a sphere through a drop with sufficient speed, and you’ll see something like the composite photo above. Going from right to left, the projectile is initially coated in liquid and stretches the fluid behind it as it continues flying. This forms a thin sheet of fluid called a lamella with a thicker, uneven rim at its far end. The lamella continues stretching until the projectile breaks through and detaches. Now the lamella starts rebounding back on itself as surface tension struggles to keep the fluid together. A new rim forms on the front, and both the front and back rims thicken as the lamella collapses. Along the rims thicker portions start forming droplets -- like spikes on a crown -- as the surface-tension-driven Plateau-Rayleigh instability starts breaking the structure down. The untenable sheet of fluid will break up into a cloud of smaller, satellite droplets when it can hold together no longer. (Image credit: V. Sechenyh et al., video)

Viscosity can have a notable effect on droplet impacts. This poster demonstrates with snapshots from three droplet impacts. The blue drops are dyed water, and the red ones are a more viscous water-glycerol mixture. When the two water droplets impact, a skirt forms between them, then spreads outward into a sheet with a thicker, uneven rim before retracting. The second row shows a water droplet impacting a water-glycerol droplet. The less viscous water droplet deforms faster, wrapping around and mixing into the other drop before rebounding in a jet. The last row switches the impacts, with the more viscous drop falling onto the water. As in the previous case, the water deforms faster than the water-glycerol. The two mix during spreading and rebound slower. In the last timestep shown, the droplet is still contracting, but it does rebound as a jet thereafter. (Image credit: T. Fanning et al.)