Optical and acoustic study on phase transition of nanodroplets: Acoustic droplet vaporization versus photoacoustic cavitation

The phase transition of nanodroplet has great potential in localized therapy, e.g. gas embolotherapy, drug delivery and the extravascular tumor-targeted theranostics. To improve the temporal and spatial resolution and reduce the energy threshold of nanodroplet phase transition, photoacoustic cavitation (PAC) using synergistic laser and ultrasound was introduced and compared with acoustic droplet vaporization (ADV) in the study. Nanodroplet phase transition was performed in a confocal microscopic system where the ultrasound and focused laser were co-focused in a 200 μm inner diameter cellulose tube in vitro. The perfluoropentane (PFP) droplets were perfused by a syringe. The phase transition process was monitored by both high speed microphotography and acoustic detection. The acoustic signals were converted to the frequency domain and compared with ADV. Microscopic images revealed the distinct patterns of phase transition by ADV and PAC in the cellulose tube. Acoustic signals from PAC showed distinct photoacoustic characteristics with wide-band low frequency components and harmonic components. Compared with ADV, both patterns showed the distinct advantage for predetermining the exact spatial and temporal occurrence of phase transition by the additional focused laser. The energy threshold, although relevant with droplet diameter, was also significantly reduced.