Ultrasonic chaining of emulsion droplets

Emulsion droplets trapped in an ultrasonic levitator organize themselves in a way that solid spheres do not. Rather than coalescing into planar colloidal crystals, monodisperse emulsion droplets instead form single-file chains. These chains' collective behavior and their influence on nearby droplets suggest that their constituent droplets are spinning rapidly around their common axis. Such acoustically induced spinning also distinguishes fluid droplets from solid spheres and naturally accounts for the droplets' propensity to form chains. In this interpretation, solid spheres do not form chains because they do not spin. We demonstrate the chain-to-crystal transition with a model system in which fluid emulsion droplets can be photopolymerized into solid spheres without significantly changing other material properties. The behavior of this experimental system is quantitatively consistent with an acoustohydrodynamic model for spinning spheres in an acoustic levitator. This study therefore introduces acoustically driven spinning as a mechanism for guiding self-organization of acoustically levitated matter.