Ultrafast dynamics of the acoustic vaporization of phase-change microdroplets

Abstract

Acoustically sensitive emulsions are a promising tool for medical applications such as localized drug delivery. The physical mechanisms underlying the ultrasound-triggered nucleation and subsequent vaporization of these phase-change emulsions are largely unexplored. Here, the acoustic vaporization of individual micron-sized perfluoropentane (PFP) droplets is studied at a nanoseconds timescale. Highly diluted emulsions of PFP-in-water and oil-in-PFP-in-water droplets, ranging from 3.5 to 11 μm in radius, were prepared and the nucleation and growth of the vapor bubbles was imaged at frame rates of up to 20 Mfps. The droplet vaporization dynamics was observed to have three distinct regimes: (1) prior to nucleation, a regime of droplet deformation and oscillatory translations within the surrounding fluid along the propagation direction of the applied ultrasound; (2) a regime characterized by the rapid growth of a vapor bubble enhanced by ultrasound-driven rectified heat transfer; and (3) a final phase characterized by a relatively slow expansion, after ultrasound stops, that is fully dominated by heat transfer. A method to measure the moment of inception of the nucleation event with respect to the phase of the ultrasound wave is proposed. A simple physical model captures quantitatively all of the features of the subsequent vapor bubble growth.

Acoustic droplet vaporization is initiated by superharmonic focusing

“…There is also an unexplained decrease of the threshold pressure with increasing size of the droplets (20,21). Finally, ultrahigh-speed imaging has allowed for the construction of spatial and temporal nucleation maps (22). This showed that the nucleation spots inside the droplets were highly localized for some bubbles, whereas other bubbles had nucleation spots at random positions throughout the droplet.…”

“…This showed that the nucleation spots inside the droplets were highly localized for some bubbles, whereas other bubbles had nucleation spots at random positions throughout the droplet. The authors suggested that the location of such spots may be a function of the droplet size (22). The authors also pointed out that temporally the initiation of the nucleation is shifted toward the end of the rarefactional half cycle of the ultrasound pulse.…”

Monodisperse bubbles and droplets for medical applications

“…7.5 it can be observed that the nucleation process initiated by the ultrasound exposure is not instantaneous in time, as was also shown before 183 . Fig.…”

“…Nucleated droplets are marked by the red-dotted circles. It is observed that the nucleation bubble grows rapidly under acoustic forcing 183 . When the ultrasound is turned of, the bubble rapidly decreases in size.…”

Individual lipid encapsulated microbubble radial oscillations: Effects of fluid viscosity

“…Experiments in either fluid were conducted separately, and small differences in focal alignment between experiments (less than a microsecond), while not affecting the peak pressure, render the exact phase of ultrasound in each frame difficult to ascertain. Indeed, a novel cross-correlation technique to do so by employing the bubble as a pressure sensor was recently demonstrated (Shpak et al, 2013), however, this was not attempted in the present study. Figure 6 summarizes the size dependent results for all the microbubbles insonicated at either (a) 100 or (b) 250 kPa in both 1 (circles) and 4 cP (triangles) fluid.…”