One method of breaking a liquid into droplets, or atomizing it, uses a slow liquid jet surrounded by an annulus of fast-moving gas. The gas along the outside of the liquid shears it, creating waves that the wind blowing past can amplify. This draws the liquid into thin ligaments that then break into droplets. This is a popular technique in rocket engines, where cryogenic liquid fuels often need to be atomized for efficient combustion. When things aren’t working exactly right, however, the liquid jet may start flapping instead of breaking up. In this case, the jet will swing back and forth, but only part of it will atomize. For a rocket engine, this would mean slower and less efficient combustion – never desirable outcomes! (Image credit: A. Delon et al.)