Effect of driving frequency and power on droplet size atomized by a multimodal transducer

Ultrasonic atomization is used in various applications such as medical devices, material synthesis, and humidity control. To meet the needs of different applications, the required droplet size ranges from a few microns to several hundred microns. In this study, we realized wideband multiple-frequency atomization with a multimodal transducer, which could control the size of atomized droplets by switching operating frequencies. A parabolic reflector served to focus the incident ultrasound waves, and a circular plate was employed to excite bending modes. By comparing the results of frequency response experiments with those of finite element simulation, nine different bending modes were acquired in the range of 500-2500 kHz. Then, 734, 949, 1530, and 2063 kHz were selected to examine atomization performance for producing 1-10 μm water droplets. The relationship between the droplet diameter and the operating frequency was clarified. The droplet diameter was found to follow a log-normal distribution. At these four frequencies, the mean droplet diameter was 7.71, 6.57, 4.87, and 4.43 μm, respectively, meaning that the diameter decreased with increasing frequency. The mean droplet diameter decreased with increasing input power at 734 kHz, whereas at 2063 kHz, no significant trend was shown. Furthermore, the atomization rate was found to increase at higher power and exhibited a frequency dependence. These results indicated that the multimodal transducer has promising potential for high-frequency ultrasonic atomization and the capacity to control the droplet size.