Optical observations of single droplets during acoustic vaporization

Abstract

It has been shown previously that Acoustic Droplet Vaporization (ADV) causes bubbles to form from small droplets. This paper shows that prior to vaporization the droplets undergo translatory (dipole) oscillations when exposed to tone bursts. A high speed video system was used to monitor droplets in a flow tube when single-element focused transducers (3.5 and 10 MHz) were used for ADV on individual droplets. Single sinusoidal tone burst (<3.3 μs) were sufficient for ADV of droplets with 5 to 25 micrometer diameter. Dipole oscillations of 1.3 μm independent of diameter were found. Variations in the droplet diameter of up to 15% were observed during the onset of acoustic irradiation. The pressure threshold for ADV decreased with increasing droplet diameter. The onset of vaporization was seen either as a localized event within or homogeneously throughout the imaged droplet (possibly due to the temporal resolution of the imaging system). Localized nucleation was solely observed along the direction of dipole motion of the droplet, which is the same as the direction of the propagating acoustic wave. Typically, sites on ‘‘north and/or south poles’’ were observed. [Research supported by PHS Grant No. R01HL54201 and US Army Grant No. DAMD17-00-1-0344.]