Optical Trapping of a Growing Water Droplet in Air

A water droplet, which was formed as a nucleation center from an aerosol of ammonium chloride, was trapped by a converged c.w. IR laser (λ = 1064 nm) using a 100× objective lens, and the successive growth of the droplet was observed under supersaturated water vapor. The size of the droplet increased linearly with time and its maximum radius was 5.7 μm at a laser at 5 mW, indicating that the axial trapping efficiency Q was 0.46. This efficiency is much greater than those reported previously; for example, according to Ashkin and Dziedzic [ Science 1975, 187, 1073], Q = 0.08 for a glycerol droplet of radius 6 μm at 40 mW (λ = 514.5 nm).