Fragmentation of Evaporating Droplets in Acoustic Fields: Role of Surface Charge

Evaporating droplets in acoustic fields exhibit interesting fragmentation behaviors under the influence of surface charge. As a charged droplet loses mass through evaporation, it approaches the Rayleigh limit, beyond which it undergoes repeated Coulombic jetting events. While prior studies have investigated this phenomenon using electrodynamic balances, little is known about the fragmentation dynamics of charged droplets levitated in acoustic fields. In this work, we levitate individual droplets using a custom-designed phased-array acoustic levitator and systematically quantify their surface charge using an acoustically transparent Faraday cup. By synchronizing high-speed imaging with charge measurements, we characterize key features of the fragmentation process, including jetting frequency and cone angle. Our observations reveal two distinct regimes: at very low charge, droplet breakup is dominated by acoustic streaming-induced rotation, while at higher charges, classical Rayleigh jetting governs fragmentation. In addition, to validate the applicability of Darcy's law, we also experimentally measure the droplet evaporation rate and compare it to the theoretical predictions. Overall, these results provide new insights into the interplay between charge and acoustic forces, advancing our understanding of droplet stability and fragmentation during acoustic levitation.