Making a supersonic jet in your kitchen

When scientists speak of a “jet” they are usually referring to a fast flowing column of material, typically air or water. These jets range from the mundane, like water rushing out of a hose, to the exotic, such as the relativistic plasma jets that beam out from quasars or monster black holes. A turbo-jet, for example, pushes an aircraft forward using the supersonic thrust of air that streams out of the back of the engine. Writing in Physical Review Letters, Stephan Gekle and colleagues at the University of Twente, the Netherlands, in collaboration with the Universidad de Sevilla, Spain, have found a supersonic jet in a surprising place: the collapsing splash from an object falling into water [1]. Their general setup is easy to reproduce by dropping a marble into a deep bowl of water (a billiard ball into a full bucket works even better). This effort is rewarded with not one, but three jets (see Fig.1): First, one of upward streaming supersonic air, followed by an obvious upward jet of water, along with a less evident downward jet of water toward the marble. Although Gekle et al. perform a more controlled experiment—they pull a disk downward through the liquid surface at a controlled speed—the general features of what they find are the same. Moreover, the disk enables Gekle et al. to have good control of the experimental conditions. In the kitchen version of the experiment, the marble creates a crown-shaped splash and crater as it falls into the liquid. The crater deepens to the point at which the walls start to contract. This is due to both the weight of the water outside and possibly surface tension, both of which create pressure gradients that force the collapse. Air inside this collapsing neck must escape upward or downward as the neck approaches pinch-off. It is in this escaping air that Gekle et al. found supersonic velocities—the first jet in this simple experiment (see Video 1). The shape of the neck plays an interesting role. As the air escapes through the neck, right before the neck FIG. 1: The impact of a heavy object on a fluid surface causes a sequence of events leading to the creation of three jets. From left to right in time: The crater first deepens and forms a crown as the object descends. When sufficiently deep, the crater begins to pinch radially due to the pressure of the surrounding fluid. A supersonic jet of air escapes through this collapsing neck. Following the impact of the fluid surface on itself at the neck pinch-off, two fluid jets emerge upward and downward from the pinch. This process illustrates the self-focusing and near singularities that occur when the liquid adjusts to the changing topology of its surface. (Alan Stonebraker)