Proceedings of the National Academy of Sciences | 2019
Optical deformation of single aerosol particles
Abstract
Significance Optical tweezers have enabled an array of high-precision spectroscopic investigations into the physicochemical properties of atmospheric aerosol particles. The electromagnetic stress from the optical tweezers should deform the trapped particle, but this has previously been disregarded as a negligible effect. We demonstrate that nanometer-scale optical deformations on micron-sized aerosol particles are not always insignificant and can actually be measured using whispering gallery mode splitting. Interpretation of the optical spectrum of the deformed droplet is in itself fascinating as the proper explanation answers fundamental questions concerning optical trapping and how light interacts with matter, which we describe in detail. Furthermore, we present a methodology for contactless tensiometry measurements of single subpicoliter volume aqueous aerosol particles under both stable and metastable conditions. Advancements in designing complex models for atmospheric aerosol science and aerosol–cloud interactions rely vitally on accurately measuring the physicochemical properties of microscopic particles. Optical tweezers are a laboratory-based platform that can provide access to such measurements as they are able to isolate individual particles from an ensemble. The surprising ability of a focused beam of light to trap and hold a single particle can be conceptually understood in the ray optics regime using momentum transfer and Newton’s second law. The same radiation pressure that results in stable trapping will also exert a deforming optical stress on the surface of the particle. For micron-sized aqueous droplets held in the air, the deformation will be on the order of a few nanometers or less, clearly not observable through optical microscopy. In this study, we utilize cavity-enhanced Raman scattering and a phenomenon known as thermal locking to measure small deformations in optically trapped droplets. With the aid of light-scattering calculations and a model that balances the hydrostatic pressure, surface tension, and optical pressure across the air–droplet interface, we can accurately determine surface tension from our measurements. Our approach is applied to 2 systems of atmospheric interest: aqueous organic and inorganic aerosol.