Simultaneous in–situ measurement of electric charge and mass of single optically trapped droplets

Electrically charged droplets play a crucial, yet often unquantified, role in numerous natural and laboratory-based processes. Bridging this critical knowledge gap is challenging, primarily due to the current lack of experimental methods capable of quantifying the charge state of individual droplets with high precision under controlled conditions. Here, we present a novel optical electrometer that enables simultaneous, in-situ determination of electric charge and mass of single optically trapped droplets within seconds. The charged droplet is immobilized in air between two ring capacitor plates using counter-propagating optical tweezers. An oscillating electric field between the two ring plates sets the droplet into oscillatory motion, which is precisely tracked. A lock-in amplifier controls the applied voltage and records the frequency-dependent droplet response from which charge and mass are deduced. This versatile method extends our established single-droplet mass measurement technique to include simultaneous charge retrieval, providing a powerful platform for probing charge-dependent phenomena of individual droplets by readily combining it with complementary spectroscopic techniques.