New technique for investigating phase transition processes of optically levitated droplets consisting of water and sulfuric acid

Experimental techniques for studying supercooled or frozen droplets will be presented. Single particles consisting of water or of water contaminated with sulfuric acid have been levitated optically and stabilized in a vertical laser beam by radiation pressure forces. In the experiments the levitated droplet has been observed in a chamber, in which temperatures down to -60 degrees C can be reached, in order to simulate conditions in contrails. After introduction into the chamber the droplet is supercooled rapidly. Depending on temperature and vapor pressure in the chamber, the droplet will grow or evaporate until it disappears. During this process the droplet is observed by a video camera. Furthermore the scattered laser light is evaluated at scattering angles of approximately 45 degrees to characterize the droplet and to determine the droplet size. Simultaneously with the size, the position of the droplet along the axis of the laser beam is determined. The droplet oscillates for a fixed laser power along the axis of the laser beam, when the droplet radius changes with time. These oscillations are due to oscillations of the radiation pressure forces during monotonic decrease or increase of the radius, i.e., for evaporation or condensation. With this new technique it is possible to associate directly the frequency of these oscillations with the radius change rate. Typical experimental results for size and for the radius change rate are shown.