Effects of acoustic forcing on jet instability and droplet dynamics in micronozzles

Controlled droplet formation is essential in many industrial processes, where flow instabilities govern droplet size and uniformity. In this study, the effects of acoustic forcing on jets and droplets formed from micronozzles were investigated to quantify how external acoustic perturbations interact with the Rayleigh–Plateau instability. Acoustic forcing was applied using a lead zirconate titanate (PZT) ring actuator coupled to a nozzle assembly with interchangeable nozzles with diameters of 16, 32, and 64 μm. High-speed, high-spatial-resolution imaging was employed to characterize breakup length and frequency, as well as the downstream evolution of droplet size, velocity, and aspect ratio up to 200 nozzle diameters from the orifice. An open cylindrical cavity acting as a Helmholtz resonator enabled efficient acoustic coupling between the actuator and the jets. Experiments were conducted at fixed Reynolds (Re) and Weber (We) numbers to decouple viscous-inertial and inertial-capillary effects. Local probability density functions (PDFs) were evaluated to characterize the downstream evolution of droplet diameter, velocity, and aspect ratio. Acoustic forcing was found to shorten the breakup length and increase the breakup frequency while reducing the mean droplet size and aspect ratio across all nozzle geometries. Depending on nozzle diameter and operating condition, the response ranged from off-resonant enhancement of breakup to subharmonic synchronization and near 1: 1 lock-in, with the clearest frequency matching observed for the 64 μm nozzle under the constant We condition. These results indicate strong coupling between the acoustic field and the liquid jet, leading to the formation of more uniform droplets across all nozzle geometries.