Nanoliter droplet coalescence in air by directional acoustic ejection

This letter presents a controlled coalescence of nanoliter liquid droplets in air by acoustic directional ejections. An asymmetrical electric field is created within a piezoelectric transducer to produce lopsided acoustic waves, which are focused (through a lens based on the innate impedance mismatch between solid and gas) onto a spot on the liquid surface. The focused acoustic beam is shown to obliquely eject 80-μm -diameter droplets at a traveling speed of 2.3 ms. Up to four such obliquely ejected droplets coalesce in air into a single droplet, which then continue to travel, rotating at 16 000 rads and producing effective micromixing in air.