droplet ejection

The microgravity environment of space is an excellent place to investigate fluid properties. In particular, surface tension and capillary action appear more dramatic in space because gravitational effects are not around to overwhelm them. In this animation, astronaut Don Petit injects a jet of air into a large sphere of water. Some of the water’s reaction is similar to what occurs on Earth when a drop falls into a pool; the jet of air creates a cavity in the water, which quickly inverts into an outward-moving jet of water. In this case, the jet is energetic enough to eject a large droplet. Meanwhile, the momentum, or inertia, from the air jet and subsequent ejection causes a series of waves to jostle the water sphere back and forth. Surface tension is strong enough to keep the water sphere intact, and eventually surface tension and viscosity inside the sphere will damp out the oscillations. You can see the video in full here. (Image credit: Don Petit/Science off the Sphere)

Rub your hands on the handles of a Chinese resonance bowl and you can generate a spray of tiny droplets. The key to this, as the name suggests, is vibration. Rubbing the handles vibrates the bowl, causing small oscillations in the bowl’s shape that are too small for us to see. But those vibrations do produce noticeable ripples on the water in the bowl. When you hit the right frequency and amplitude, those vibrations disturb the water enough that the up-and-down vibration at the surface actually ejects water droplets. The vibration of the bowl affects water near the wall most strongly, which is why that part of the bowl has the strongest reaction. It takes even larger amplitude vibrations to get droplets jumping in the middle of the bowl, but you can see that happening in this video of a Tibetan singing bowl. (Image/video credit: Crazy Russian Hacker, source)


Milk and juice vibrating on a speaker can put on a veritable fireworks display of fluid dynamics. Vibrating a fluid can cause small standing waves, called Faraday waves, on the surface of the fluid. Add more energy and the instabilities grow nonlinearly, quickly leading to tiny ligaments and jets of liquid shooting upward. With sufficiently high energy, the jets shoot beyond the point where surface tension can hold the liquid together, resulting in a spray of droplets. (Image credit: vurt runner, source video; h/t to @jchawner)


I love science with a sense of humor. This video features a series of clips showing the behavior of droplets on what appears to be a superhydrophobic surface. In particular, there are some excellent examples of drops bouncing on an incline and droplets rebounding after impact. For droplets with enough momentum, impact flattens them like a pancake, with the rim sometimes forming a halo of droplets. If the momentum is high enough, these droplets can escape as satellite drops, but other times the rebound of the drop off the superhydrophobic surface is forceful enough to overcome the instability and draw the entire drop back off the surface.  (Video credit: C. Antonini et al.)


Large droplets ejected from a liquid pool do not coalesce immediately back into the whole.  Instead, a thin layer of air gets trapped beneath them, much like the oil lubricating bearings.  The weight of the droplet causes the air to drain away, and eventually the droplet comes in contact with the pool. Some of the droplet gets drained away before surface tension snaps the interface back into a low energy state. A new smaller droplet then bounces upward before repeating the process over again. Eventually the droplet becomes small enough that its entire mass gets sucked away by the pool. Researchers call this process the coalescence cascade.