AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Homogeneous Ice Nucleation From Supercooled Nanodroplets ~230 K
TONG SUN, Barbara Wyslouzil, The Ohio State University
Abstract Number: 197 Working Group: Aerosol Physics
Abstract Homogeneous ice nucleation within highly supercooled water droplets (160 K<T< 235 K) is a nonequilibrium phenomenon of ongoing interest in part because of the insight it provides into the phase behavior of water under extreme conditions. The reported ice nucleation rates ~230 K remain controversial: rates reported in droplets with radii ~10nm at temperatures of ~225K are ~9 orders of magnitude higher than those reported in 10 μm droplets at ~227K. Here, we seek to make droplets with radii around 20 nm and freeze them at higher temperature in order to fill the gap between the previous studies. To increase the size of the droplets, we use a shaped supersonic nozzle to condense water vapor and then freeze the supercooled droplets. The special design of the nozzle allows us to produce droplets with radii around 20 nm and the extended expansion region length makes it possible for the droplets to freeze at ~230K. A movable probe is used to measure the position-resolved pressures and by integrating the adiabatic flow equations, the temperature, density, velocity of the gas mixture, effective area ratio of the nozzle and the mass fraction of the condensate are obtained. By recording the peak position and intensity changes, Fourier Transform Infrared spectroscopy provides the onset conditions for ice nucleation and fractions of the aerosol that has frozen. Small Angle X-ray Scattering characterizes the aerosol size and number density. The behavior of ice nucleation ~230 K is elucidated based on these thorough characterizations. The comparison of the experimental data with the rates given by classic nucleation theory further illustrates the effect of the Laplace pressure on ice nucleation rates of nano-sized droplets.