Experimental quantification of contact freezing in an electrodynamic balance

摘要

Heterogeneous nucleation of ice in a supercooled water droplet induced byexternal contact with a dry aerosol particle has long been known to be moreeffective than freezing induced by the same nucleus immersed in the droplet.However, the experimental quantification of contact freezing is challenging.Here we report an experimental method to determine the temperature-dependentice nucleation probability of size-selected aerosol particles. The method isbased on the suspension of supercooled charged water droplets in a laminarflow of air containing aerosol particles as contact freezing nuclei. The rateof droplet–particle collisions is calculated numerically with account forCoulomb attraction, drag force and induced dipole interaction between chargeddroplet and aerosol particles. The calculation is verified by direct countingof aerosol particles collected by a levitated droplet. By repeating theexperiment on individual droplets for a sufficient number of times, we areable to reproduce the statistical freezing behavior of a large ensemble ofsupercooled droplets and measure the average rate of freezing events. Thefreezing rate is equal to the product of the droplet–particle collisionrate and the probability of freezing on a single contact, the latter being afunction of temperature, size and composition of the contact ice nuclei.Based on these observations, we show that for the types of particlesinvestigated so far, contact freezing is the dominating freezing mechanism onthe timescale of our experiment.