Freezing of micro-droplets driven by power ultrasound

This study is concerned with the freezing of micro-droplets into ice particles driven by power ultrasound. Effects of system key parameters including flowrates of fluids, temperature, and ultrasonic vibration properties (duty cycle, probe position and power output), on droplet freezing and the quality of product ice particles are examined. The onset of ice nucleation in micron-droplets shows strong dependency on their interaction with cavitation bubbles generated by the ultrasound. Partially frozen ice particles are often obtained and their ice fraction increases with increasing sonicator power output, vibration duty cycle, and probe offset distance, or with reducing the cooling temperature. Ice fraction over 90% is achieved but accompanied with a poor ice particle roundness, which, however, can be significantly improved by applying a high vibration intensity. The results provide pivotal data for producing small, spherical ice particles which find applications in many emerging or conventional technologies.