Controlled interference of laser-induced shock waves in microdroplet jetting

We report the formation of high-velocity microjets driven by the interference of two laser-induced shock waves in a tin microdroplet. A first 0.4-ns laser pulse launches a shock wave into the droplet (diameter D₀=40, 50, and 60µm). Due to the droplet's spherical geometry, the shock converges and diverges as it traverses the droplet center and reaches the back surface. Upon reflection from the free back surface, the shock reverses polarity, producing an intense, focused tensile wave. This wave nucleates an ∼1µm cavitation bubble, trapping itself on the front side of the droplet, between the bubble and the droplet's free laser-facing surface. Impact of a second, delayed, identical laser pulse introduces a second shock wave and leads to the formation of a microjet. Scanning the delay of this second pulse reveals a complex pattern of constructive and destructive interferences of the two shocks through the observed change in jet velocity, with velocities reaching 110 m/s. This laser-droplet system presents a unique case for the study of acoustics and cavitation at the microscale.