FORMATION AND GROWTH OF ICE PARTICLES IN STATIONARY ULTRASONIC FIELDS

The formation of ice particles in stationary ultrasonic fields (SUSFs) from ice aerosol was observed. Under suitable experimental conditions aerosol particles gather in stable ellipsoidal systems at temperature s down to <50 K below ambient. We call these systems cold gas traps. T he aerosol concentrates in the pressure node areas of the SUSF and con glomerates to form larger particles and snowflakes with diameters up t o 10 mm. The resulting shapes are shown. Most of them are similar to t hose occurring in nature. An attempt is made to explain the effect. Th ree different processes of ice particle formation are visualized and d iscussed. Here the existence of a quasiliquid layer (QLL) on the parti cle surfaces seems to be essential because both the formation of ice p articles and the QLL occur only at temperatures higher than about -25 degrees C. The observed trapping and generation process can be used, f or example, in atmospheric physics for the study of particle aggregati on, precipitation formation, and water and radiation transport phenome na. (C) 1998 American Institute of Physics. [S0021-9606(98)70633-6].