Ultrasound assisted nucleation of water during freezing

Nucleation of ice during freezing is an important phenomenon affecting the probability distribution of the ice crystal size and crystal growth. Power ultrasound has been proved to be useful in promoting the nucleation of ice in water based solutions. In the present work, the use of ultrasound waves to induce dynamic nucleation was studied on deionized water, 8% sucrose solution, and agar gel samples; and the results were analyzed in regard to the theories suggested for ultrasound assisted nucleation. The samples were put into tubing vials and were frozen in an ethylene glycol water mixture (-20°C) in an ultrasonic bath system. Ultrasound (25 kHz, 0.25 W cm-1) was applied continuously for 5 s at different sample’s temperatures from just below 0oC to -5oC. For the water, sucrose solution and agar gel samples without ultrasound irradiation, their nucleation temperatures occurred stochastically and followed normal distributions with the values of -7.4 ± 2.4, -10.6 ± 1.7 and -7.5 ± 0.92 oC, respectively. With 5 s ultrasound irradiation, the nucleation temperatures of water and sucrose solution exhibited repeatable results, and a linear relationship between the ultrasound irradiation temperature and the nucleation temperature was obtained. However, 5 s ultrasound irradiation could not obtain the linear relationship in the Tylose samples probably due to the state of water within the gel network or the heat produced as a result of cavitation and therefore, it was differently exposed to the influence of ultrasound.