Size and polarization behaviour of optically levitated frozen water droplets

Experiments with single optically levitated droplets were performed in order to simulate phase transition processes during contrail formation. Freezing, sublimation, and crystal growth of levitated droplets were studied. Shadow pictures of the levitated droplets showed, that the droplets remain spherical immediately after freezing and during the sublimation process with dry ambient conditions. Under supersaturated conditions growing of ice crystals occurred on the frozen droplets. Size measurements were performed by evaluating the shadow pictures. The experiments showed, that the polarization ratio becomes non-zero at the moment of freezing. Immediately after freezing and for sublimating frozen droplets mean values of the polarization ratio ranging between 0.1 and 0.25 were measured. Measurements on frozen droplets with growing ice crystals showed increasing polarization ratios with a level off between 0.4 and 0.5. (C) 1998 Elsevier Science Ltd. All rights reserved.