Homogeneous ice nucleation observed in single levitated micro droplets

Homogeneous ice nucleation rates in single isolated micro droplets of water are determined under various atmospheric conditions inside an electrodynamic Paul-trap. The droplet size is measured by angle-resolved detection of the scattered light from a Helium-Neon laser. The freezing process is detected by a sudden change of the depolarization ratio which is an indication for the formation of nonspherical particles. The experimentally determined homogeneous ice nucleation rates J(LS) are J(LS) = (2.2 +/- 0.4). 10(8) cm(-3) s(-1) at T = (- 37.2 +/- 0.15)degrees C and J(LS) = (5.6 +/- 1.0). 10(7) cm(-3) s(-1) at T = (- 37.7 +/- 0.15)degrees C. These values are much more precise as determined by measurements inside a cloud expansion chamber. The method of single particle observation eliminates complications arising from these nucleation rate measurements. The results underline the theory that one single ice germ of certain size is sufficient to induce the freezing of the whole droplet. The depolarization ratio of the scattered intensity in the parallel and perpendicular polarization plane I-s/I-p from the frozen particles exhibits a broad distribution with a mean value of I-s/I-p = 0.3.