Quantifying surface tension of metastable aerosols via electrodeformation

Accurate surface tension measurements are key to understanding and predicting the behavior of atmospheric aerosols, particularly their formation, growth, and phase transitions. In Earth's atmosphere, aerosols often exist in metastable states, such as being supercooled or supersaturated. Standard tensiometry instruments face challenges in accessing these states due to the large sample volumes they require and rapid phase changes near surfaces. We present an instrument that uses a strong electric field, nearing the dielectric strength of air, to deform aerosol microdroplets and measure surface tension in a contact-free, humidity-controlled environment. A dual-beam optical trap holds single microdroplets between two electrodes and excites Raman scattering. When a high voltage is applied, droplet deformations reach tens of nanometers. These small shape changes are precisely measured through the splitting of morphology-dependent resonances, seen as sharp peaks in Raman spectra. Our measurements cover water activities where droplets are supersaturated, a region with limited previous data, and show good agreement with existing data where comparisons are possible. Unlike prior levitation-based methods, this approach measures surface tension in systems with viscosities over 10² Pa s without relying on dynamic processes.