Charge Sensing Analysis Identifies Group Droplet Emission Dynamics in Microdripping Electrospray Ionization.

Electrospray ionization operates through the formation and evolution of charged droplets, where emission mode critically influences ionization efficiency and stability. In the microdripping mode, droplet emission is commonly assumed to produce a single droplet per cycle. In this research, the study directly investigated emission dynamics using a Charge-Sensing Particle Detector that resolves individual droplet charge and arrival time. The measurements reveal that each apparent emission event consists of multiple, temporally resolved droplets, defining a "group droplet" as the fundamental emission unit. Frequency-domain analysis reveals a principal frequency and higher-order harmonics, indicating periodic, structured emission. As the applied voltage increases, the principal frequency increases while the droplet charge decreases, demonstrating a coupled charge-frequency relationship. Additionally, the number of droplets per group and their charge distribution changes systematically with voltage, indicating a transition toward simpler emission structures at higher voltages. These findings establish microdripping as a time-resolved, multidroplet process ruled by the interaction of electric stress and surface tension, rather than a sequence of independent single droplets. This work provides a new framework for understanding electrospray emission and enables direct characterization of droplet dynamics at the single emission.