Resonantly enhanced evaporation sensing in liquid droplet whispering-gallery cavities

We experimentally demonstrate resonantly enhanced measurements of droplet evaporation by tracking whispering-gallery-mode resonances. Droplet optical Q-factors exceeding 2 × 10⁸ , coupled to a standard optical fiber, enable operation for several days and evaporation measurement as fine as the release of a single-molecular layer every 50 s! We observe that lower viscosities are associated with faster evaporation, since its molecules are shorter and, hence, have weaker cohesive forces per molecule and higher Brownian velocities. To our surprise, long-term measurements resulted in an exponentially decaying evaporation rate, explained by the early evaporation of shorter molecules, leaving a majority of longer molecules possessing lower vapor pressure.