Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold

Abstract

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenic microbubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are used for therapeutic and diagnostic applications. This study systematically investigated the effect of excitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds of lipid-coated PFC droplets of three different liquid cores-perfluoropentane (PFP), perfluorohexane (PFH), and perfluorooctyl bromide (PFOB)-and of two different sizes-average diameters smaller than 3 lm and larger than 10 lm-in a tubeless setup. This study found that the ADV threshold increases with frequency for the lowest boiling point liquid, PFP, for both large and small size droplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization for small size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The large PFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFH droplets, ADV threshold decreases with frequency that could possibly be due to the superharmonic focusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresholds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholds indicating that phase transition precedes inertial cavitation. V

Numerical investigation of acoustic vaporization threshold of microdroplets

“…Miles et al [17] extended the classical nucleation theory to the ADV threshold for DDFP microdroplets by combining with the superharmonic effect. Their predicted threshold pressure is about 9.3 MPa for a droplet of 10 μm radius, which is much higher than the experimental data of Aliabouzer et al [16] . As another predictive model for the ADV threshold, one-dimensional numerical models for bubble growth have been applied to determine the conditions for the survival of a small bubble assumed to nucleate.…”

“…For computation of ADV, we use the following acoustic pressure pulse. Here, and vary in this work keeping the number of acoustic cycles as , referring to the experimental condition of Aliabouzer et al [16] . The minus sign is chosen considering that bubble nucleation occurs in the negative pressure period, as experimentally observed in Ref.…”

“…They provided a phase diagram for the three regimes of bubble growth in ADV, which is useful to understand the complex bubble/droplet behaviors in ADV. However, their model needs to be modified in that the bubble has a very high temperature, to the order of a few MKs, while it collapses with oscillations in the intermediate regime III, and the ADV threshold is much higher than the experimental data of Aliabouzer et al [16] .…”

“…The tube setups can cause uncertainties in threshold measurements through the scattering and reflection of acoustic waves. Aliabouzer et al [7] , [16] conducted experiments for the ADV threshold of microdroplets in a tubeless setup to minimize the effects of tube wall. Their results showed that the ADV threshold for dodecafluoropentane (DDFP) droplets increases with the acoustic frequency.…”

“…The van der Waals (VDW) equation is employed to more accurately consider the supercritical state instead of the ideal gas equation, which was used in most of the previous studies referenced above for ADV. The present model is tested on the previous experimental data obtained in a tubeless setup [16] . The effects of fluid properties and droplet radius on the bubble growth pattern and the ADV threshold are investigated.…”