Experimental study of the impact of cooling rate on the freezing process of three kinds of water drops

Large number of the previous experimental researches have indicated that heterogeneous freezing is caused by nucleus contained in drops and stochastic process both. In this research, experiments are designed to verify the previous experiment results. On the other hand, a quantitative relationship of the freezing phenomenon of water drops with temperature and time is also studied. Besides, the interaction of the stochastic hypothesis with the singular hypothesis is also examined. The constant cooling rate experiments for rainwater, pure water and bottled water, and the constant temperature experiment for rainwater only, were conducted using a freezing experimental device of uniform drops designed by Chinese Academy of Meteorological Sciences. According to the results, it is seen that while the temperature decreasing, the accumulative freezing fraction in the slow cooling rate experiment is larger than that in the fast cooling rate experiment in general. This phenomenon is more obvious for pure water and bottled water. The faster cooling rate is corresponding to the lower average freezing temperature and median freezing temperature. However, the relationship between freezing temperature and cooling rate in rainwater is logarithmic while their relationship is linear in bottled water (there are only two rates of cooling in pure water experiment, so it is not certain to belong to which relationship). The probability of freezing increases exponentially with the decreasing temperature in each water sample, and in the probability of freezing function, the exponential coefficient decreases exponentially with the increasing cooling rate in bottled water; during the constant temperature period, the probability of freezing decays exponentially with time in rainwater. The decay is faster in first two minutes, and becomes slower later. No freezing event occurs after 15 minutes. This phenomenon can be explained by combining the stochastic hypothesis and singular hypothesis as follows: when the temperature of drops reaches or is lower than hte characteristic temperature, water molecules need a period of time to aggregate on freezing nucleus, which lead the freezing of drops to occurring in constant temperature period. Freezing of drops is caused by a combined effect of the two hypotheses.