The influence of resonant absorption and heating on the equilibrium size of aqueous-solute aerosol droplets

The time-dependent evolution in the equilibrium size of an optically trapped aqueous sodium chloride droplet (>2 μm radius) within an environment of varying relative humidity (RH) is shown to depend on both the depression in vapour pressure due to the presence of the solute and the elevation in temperature due to optical absorption. In particular, the level of optical absorption is highly dependent on the size of the droplet relative to the wavelength of the absorbed light. Thus, as the droplet size tunes into a Mie resonance at the trapping laser wavelength, the increased level of optical absorption leads to an elevation in droplet temperature. This increase in resonant heating can balance a continual increase in RH, leading to only marginal growth in droplet size and change in solute concentration. Once the RH is sufficiently high that the resonance condition can be surpassed, the droplet cools instantaneously and the solute concentration again dominates in determining the vapour pressure, with a rapid increase in size and a decrease in solute concentration returning the droplet to equilibrium with the gas phase RH. Thus, a growing droplet is observed to pass through periods of apparent size stability followed by instantaneous growth, consistent with the variation in absorption efficiency with droplet size. This provides a clear example of the coupling between the optical and physical properties of an aerosol and their influence on the equilibrium state.