Predicting and controlling laser-induced breakup and multidirectional propulsion of liquid droplets

Abstract

Laser-driven control of droplets is important in microfluidics, targeted delivery, and droplet-based laser–matter interactions, yet propulsion direction and breakup remain difficult to predict. Here, we demonstrate that an acoustically levitated droplet’s propulsion polarity and breakup morphology can be selected by controlling axial placement relative to the external (no-droplet) optical focus together with incident pulse energy. We combine time-resolved shadowgraphy with aberration-aware optical simulations that locate the prebreakdown irradiance maxima, and we connect these linear field calculations to laser-induced breakdown using experimentally calibrated thresholds in the liquid and in near-field air. Expressing irradiance in threshold-referenced form yields a first-crossing rule that identifies whether breakdown initiates at the illumination surface, in the interior, at the shadow surface, or in wake-side near-field air. The resulting placement–energy maps anticipate forward, backward, and near-radial droplet responses. A polarity index and a reliability metric quantify directionality and highlight narrow transition corridors in parameter space where competing initiation sites are nearly tied under experimental jitter. The predicted landscapes agree with experiments, and multiphase simulations reproduce the early-time shock-driven deformation. Because the regime structure is set by caustic geometry together with breakdown thresholds, the framework transfers across transparent liquids after threshold calibration and reduces to a surface-trigger condition in the strongly absorbing limit.