Observation of bistability of trapping position in aerosol optical tweezers

Bistability in the axial trapping position of aqueous aerosol droplets has been observed for the first time, to our knowledge, in optical tweezers. The behavior has been observed for two distinct trapping configurations, with the trapping beam oriented either along the vertical or horizontal axis. This represents the first report of the optical tweezing of aerosol droplets with a horizontally propagating laser beam. Side imaging was used in conjunction with imaging in the plane of the optical trap. Droplet sizing was performed using cavity-enhanced Raman spectroscopy or by applying a circular regression routine to the acquired images. Predictions from a theoretical model of optical forces are shown to be in good agreement with the experimental observations of bistability in the trapping position. These studies have significance both for the rigorous interpretation of data obtained using aerosol optical tweezers and for the modeling of aerosol optical traps.