Systematic characterization and mechanistic insights into ultrasonically actuated sharp-tip capillary droplet generation.

Ultrasonically actuated sharp-tip capillary droplet generation offers a chip-free approach to produce microdroplets for applications such as micro-/nanoparticle synthesis and biochemical analysis, eliminating the need for complex microfluidic fabrication and bulky pumping systems. This method exploits the synergy between acoustically driven centrifugal pumping and acoustic streaming for on-demand droplet formation. Despite its promise, a thorough understanding of how key parameters influence droplet dynamics has remained elusive, hindering further optimization and broader adoption. Here, we present the first systematic characterization of droplet generation dynamics in an ultrasonically actuated sharp-tip capillary system. We investigate the effects of excitation voltage, amplitude modulation (AM) waveform, capillary tip diameter, and liquid viscosity (both dispersed and continuous phases) on droplet size, monodispersity, and generation stability. A simplified theoretical model is developed to elucidate the three-stage droplet formation mechanism: centrifugal pumping, acoustic streaming-induced neck elongation, and Laplace pressure-driven pinch-off upon vibration cessation. Crucially, leveraging the precise control enabled by AM modulation, we demonstrate the novel programmable generation of multi-volume droplet sequences within a single stream. We further demonstrate the platform's versatility through the synthesis of highly monodisperse calcium alginate (CV ∼3.38%) and poly(ethylene glycol) diacrylate (PEGDA) hydrogel microspheres (CV ∼2.94%). This study offers fundamental mechanistic insights and practical guidelines for optimizing vibrating sharp-tip capillary droplet generators, facilitating their potential use in point-of-care diagnostics, combinatorial screening, and advanced material synthesis.