Using acoustic levitation and an array of carefully-placed speakers, researchers can manipulate droplets without touching them. This lets scientists study the physics of droplet coalescence (top) without interference from solid surfaces, but it also provides opportunities for mixing two different substances in the final droplet. 

On the bottom left, we see a droplet formed from the coalescence of a dyed droplet (visible as gray) and an undyed droplet. The swirling and mixing in the levitating droplet is fairly slow. By contrast, the droplet on the right is vibrated by manipulating the sound waves holding it aloft. This mixes the droplet quite efficiently, allowing it to reach a uniform state more than six times faster than the other droplet. (Image and research credit: A. Watanabe et al., source)