Non-contact mass measurement of droplet based on free oscillation under ultrasonic levitation

The position of solid object or droplet will vibrate when it is levitated in a standing ultrasonic field. The frequency of the free oscillation is determined by the mass of the object and the gradient of the ultrasonic radiation force. Using this phenomenon, in this report, we tried to measure the mass of the levitated droplet. First, we investigated the relationship between the effective spring constant and the sound pressure on the levitated object by using plastic spheres with known mass. Using the relationship, we executed non-contact mass measurement of ethanol and water droplets. As a result, mass can be measured with error rate of 5-40%.