Measurement of a microcapsule vibration having a hard plastic shell in an acoustic standing wave

Observation techniques for measuring the vibration of a single microcapsule with a hard plastic shell in an acoustic standing wave field are discussed. First, optical observation of a microcapsule vibration by using a laser Doppler vibrometer (LDV), which can observe small capsule vibration amplitudes at high-frequency, was investigated. Capsules of tens of micrometer size were trapped at the antinode of an acoustic standing wave generated in an observational cell, and the capsule vibration with displacement amplitude of tens of nano-meters could be measured. The acoustic radiation force acting on microcapsules in the acoustic standing wave was measured from the trapped position of the standing wave and the vibrational displacement amplitude of the capsules was estimated from the theoretical equation of the acoustic radiation force, giving results in good agreement with the LDV measurements. The vibrational amplitude of a capsule was found to be proportional to the amplitude of the driving sound pressure. A larger expansion ratio was observed for capsules closer to the resonance condition under the same driving sound pressure and frequency.