Spinning and chaining emulsion droplets in ultrasonic standing waves*

We demonstrate experimentally that emulsion droplets of TPM (3-(trimethoxysilyl)propyl methacrylate) in water spin when levitated in ultrasound, and that the spinning droplets organize themselves into long spinning chains. When the same droplets are solidified into spheres by free-radical polymerization, they no longer spin, and they form crystals rather than chains. We explain these acoustokinetic phenomena through a dipole-order expansion of the incident and scattered waves, by analogy to the theory of optical trapping.