432 水の蒸発凍結現象に及ぼす極短パルスレーザー照射の影響

Abstract

In this study, the effect of femtosecond very-short laser irradiation on the evaporation-freezing behavior of water droplet was experimentally examined under abrupt evacuation condition. In order to improve the performance of a novel transport-storage system for cold, which utilizes the evaporation-freezing phenomenon of water, it is desirable to reduce the supercooling of water by adding freezing promoters into the water. However, the evaporation characteristics of water might be affected by the freezing promoters, since some kinds of the promoters may strengthen the hydrogen bondings among water molecules. In this study, the effect of the "strength" of hydrogen bondings on the evaporation-freezing characteristics of water was therefore experimentally examined by using the femtosecond very-short pulse laser irradiation. Wavelength of the irradiated laser was about 780 nm, which corresponds to the third-overtone of an O-H stretching vibration of water, and thus the irradiation must weaken the hydrogen bondings among the water molecules. From the results, it was confirmed that the femtosecond laser irradiation results in increase in cooling rate, reduction of freezing time, and decreasing the freezing temperature. These results suggest that the freezing promoters added into water results in reduction of cooling rate of water due to evaporation, and that the performance of the cold transport-storage system is not always improved by adding such kind of freezing promoters.