Optical fluorescence studies of perfluorocarbon droplet vaporization

Droplets of liquid perfluorocarbon (PFC) can be vaporized into gas bubbles in-situ by externally applied ultrasound, with applications to diagnosis and therapy. However, little is known about the mechanisms of droplet vaporization and the composition of the bubbles so produced. Here we optically observe vaporization of PFC droplets with either fluorescently labelled PFC liquid or fluorescently labelled encapsulating shells. DiIC-18 or Rhodamine fluorescent markers were used to selectively label either the liquid PFC core or the shell of the droplets. Following vaporization, the newly created bubbles exhibit a rapid decrease in overall fluorescence signal. However, droplets with fluorescent shells were shown to retain the signal around the bubble edge. The results show that it is possible to create emulsions of liquid PFC droplets with fluorescently labelled liquid PFC bulk or shell material, which can be vaporized with applied ultrasound. Loss of fluorescence suggests dissolution of fluorescent marker from the vaporized droplet. Nevertheless, retention of fluorescent signal at the edge of the bubble suggests the possibility of the presence of coating on the vaporized droplet. This model may help in facilitating the characterization of stability of the bubbles after vaporization, and will be a valuable tool in the study of droplet vaporization mechanisms on microsecond timescales.