#buckling instability

When your predators use echolocation to locate you, it pays to have an ultrasonic deterrence. So, many species of ermine moths have structures on their wings known as tymbals.  (Image credit: Wikimedia/entomart; research credit: H. Mendoza Nava et al.; via APS Physics) Read the full article

When a viscous drop falls into a pool of a less viscous liquid, the drop can deform into some beautiful and complex shapes. Typically, shear forces between the drop and its surroundings cause a vortex ring to roll up and advect downward, thereby stretching the remainder of the drop into thin sheets that can buckle and wrinkle. Here the drop is about 150 times more viscous than the pool and impacts at 1.45 m/s, making a rather energetic entry. The vortex ring (not visible) has stretched the drop’s remains downward while a buoyant bubble caught by the impact pulls some of the drop back toward the surface. As a result, the thin sheets of the drop’s fluid are buckling and folding back on themselves like an elaborate and delicate glass sculpture. This entire paper is full of gorgeous images and videos. Be sure to check them out! (Image and research credit: E. Q. Li et al.; see supplemental info zip for videos)

A sphere falling into water generates a spectacular crown splash at the surface. The object’s impact ejects a thin sheet of fluid that rises vertically. The air pulled down into the cavity by the sphere’s passage makes the air pressure inside the sheet lower than the ambient air pressure on the exterior of the sheet. This pressure difference is part of what draws the crown inward to seal the cavity. As the splash collapses inward and seals, the liquid sheet starts to buckle and wrinkle, leaving periodic stripes around the closing neck. This so-called buckling instability occurs when the radius of the neck collapses faster than the vertical speed of the splash. For more, see the research paper or this award-winning video. (Image credit: J. Marston et al., source)