Optical droplet vaporization (ODV): Photoacoustic characterization of perfluorocarbon droplets

Optical droplet vaporization (ODV) of nanoscale and micron-sized liquid perfluorocarbon (PFC) droplets via a 1064 nm laser is presented. The stability and laser fluence threshold were investigated for PFC compounds with varying boiling points. Using an external optical absorber to facilitate droplet vaporization, it was found that droplets with boiling points at 29°C and 56°C were consistently vaporized upon laser irradiation using a fluence of 0.7 J/cm² or greater, while those with higher boiling points did not, up to a maximum laser fluence of 3.8 J/cm². Upon vaporization, the droplet rapidly expanded to approximately 10-20x the original diameter, then slowly and continuously expanded at a rate of up to 1 μm/s. Lead sulphide (PbS) nanoparticles were incorporated into perfluoropentane (PFP) droplets to facilitate vaporization. The fluence threshold to induce vaporization ranged from 0.8 to 1.6 J/cm², the wide range likely due to variances of the PbS concentration within the droplets. Prior to vaporization, the photoacoustic spectral features of individual droplets 2-8 μm in diameter measured at 375 MHz agreed very well with the theoretical prediction using a liquid sphere model. In summary, the use of liquid droplets for photoacoustic imaging and cancer therapy has been demonstrated.