Ultrasonic chaining of emulsion droplets

TL;DR

This paper explores how ultrasonic levitation induces unique spinning and chaining behaviors in emulsion droplets, revealing a new mechanism for self-organization driven by acoustically-induced spinning and hydrodynamic interactions.

Contribution

It introduces the concept of acoustically-driven spinning as a novel mechanism for self-organization of levitated matter, supported by experimental and modeling evidence.

Findings

Droplets spin rapidly about an axis parallel to the trapping plane.

Spinning droplets form long chains aligned with their rotation axis.

Chains can merge or form 3D bundles through hydrodynamic interactions.

Abstract

Emulsion droplets trapped in an ultrasonic levitator behave in two ways that solid spheres do not: (1) Individual droplets spin rapidly about an axis parallel to the trapping plane, and (2) coaxially spinning droplets form long chains aligned with their common axis of rotation. Acoustically-organized chains interact hydrodynamically, either to merge into longer chains or to form three-dimensional bundles of chains. Solid spheres, by contrast, form close-packed planar crystals drawn together by the sound-mediated secondary Bjerknes interaction. 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.