Acoustic Droplet Vaporization Threshold Research Articles

Numerical study of the effect of liquid compressibility on acoustic droplet vaporization

In acoustic droplet vaporization (ADV), a cavitated bubble grows and collapses depending on the pressure amplitude of the acoustic pulse. During the bubble collapse, the surrounding liquid is compressed to high pressure, and liquid compressibility can have a significant impact on bubble behavior and ADV threshold. In this work, a one-dimensional numerical model considering liquid compressibility is presented for ADV of a volatile microdroplet, extending our previous Rayleigh-Plesset based model [Ultrason. Chem. 71 (2021) 105361]. The numerical results for bubble motion and liquid energy change in ADV show that the liquid compressibility highly inhibits bubble growth during bubble collapse and rebound, especially under high acoustic frequency conditions. The liquid compressibility effect on the ADV threshold is quantified with varying acoustic frequencies and amplitudes.

Acoustic Droplet Vaporization of Perfluorocarbon Droplets in 3D-Printable Gelatin Methacrylate Scaffolds

Scientists face a significant challenge in creating effective biomimetic constructs in tissue engineering with sustained and controlled delivery of growth factors. Recently, the addition of phase-shift droplets inside the scaffolds is being explored for temporal and spatial control of biologic delivery through vaporization using external ultrasound stimulation. Here, we explore acoustic droplet vaporization (ADV) in gelatin methacrylate (GelMA), a popular hydrogel used for tissue engineering applications because of its biocompatibility, tunable mechanical properties and rapid reproducibility. We embedded phase-shift perfluorocarbon droplets within the GelMA resin before crosslinking and characterized ADV and inertial cavitation (IC) thresholds of the embedded droplets. We were successful in vaporizing two different perfluorocarbon---perfluoropentane (PFP) and perfluorohexane (PFH)--cores at 2.25- and 5-MHz frequencies and inside hydrogels with varying mechanical properties. The ADV and IC thresholds for PFP droplets in GelMA scaffolds increased with frequency and in stiffer scaffolds. The PFH droplets exhibited ADV and IC activity only at 5 MHz for the range of excitations below 3MPa investigated here and at threshold values higher than those of PFP droplets. The results provide a proof of concept for the possible use of ADV in hydrogel scaffolds for tissue engineering.

Investigation of the Acoustic Vaporization Threshold of Lipid-Coated Perfluorobutane Nanodroplets Using Both High-Speed Optical Imaging and Acoustic Methods

A combination of ultrahigh-speed optical imaging (5 × 106 frames/s), B-mode ultrasound and passive cavitation detection was used to study the vaporization process and determine both the acoustic droplet vaporization (ADV) and inertial cavitation (IC) thresholds of phospholipid-coated perfluorobutane nanodroplets (PFB NDs, diameter = 237 ± 16 nm). PFB NDs have not previously been studied with ultrahigh-speed imaging and were observed to form individual microbubbles (1–10 μm) within two to three cycles and subsequently larger bubble clusters (10–50 μm). The ADV and IC thresholds did not statistically significantly differ and decreased with increasing pulse length (20–20,000 cycles), pulse repetition frequency (1–100 Hz), concentration (108–1010 NDs/mL), temperature (20°C–45°C) and decreasing frequency (1.5–0.5 MHz). Overall, the results indicate that at frequencies of 0.5, 1.0 and 1.5 MHz, PFB NDs can be vaporized at moderate peak negative pressures (<2.0 MPa), pulse lengths and pulse repetition frequencies. This finding is encouraging for the use of PFB NDs as cavitation agents, as these conditions are comparable to those required to achieve therapeutic effects with microbubbles, unlike those reported for higher-boiling-point NDs. The differences between the optically and acoustically determined ADV thresholds, however, suggest that application-specific thresholds should be defined according to the biological/therapeutic effect of interest.

Time-dependent, microscale mechanical properties of acoustically responsive scaffolds using atomic force microscopy and confocal imaging

Acoustic droplet vaporization (ADV) has been used to modulate delivery of regenerative molecules from acoustically responsive scaffolds (ARSs), which are composite fibrin hydrogels containing payload-carrying, phase-shift emulsions. Here, we studied the micromechanical properties of ARSs including stiffness as well as elastic and shear moduli over 7 days post-ADV. ARSs, containing dextran-loaded perfluorohexane (PFH) emulsions (diameter: 6 mm) embedded in fluorescently labeled fibrin gels, were exposed to acoustic pressures above the ADV threshold (2.2 ± 0.2 MPa ) at 2.5 MHz. The compressive stiffness as well as elastic modulus of fibrin gels (i.e., without emulsions) and ARSs (pre- and post-ADV) were measured via force-spectroscopy and the Hertz model, respectively. The measured stiffness was lowest (1.9 ± 0.5 mN/m) in regions adjacent to the PFH emulsions and highest (16.0 ± 4.4 mN/m) in regions adjacent to the ADV-generated bubbles. Fibrin compaction at the bubble–fibrin interface was observed using time-lapsed confocal imaging and correlated with bubble growth at the time points studied here. In addition, different ADV-generated bubble responses in ARSs will be discussed using time-lapsed confocal imaging. Elucidating the mechanical microenvironment within the ARS could be used to control mechanically induced, cellular processes and further the understanding of ADV-triggered drug delivery for regeneration.

Preparation of perfluorocarbon emulsions by premix membrane emulsification for Acoustic Droplet Vaporization (ADV) in biomedical applications.

Perfluorocarbon (PFC) droplets are used in acoustic droplet vaporization (ADV), a phenomenon where droplets vaporize into gas microbubbles under exposure to ultrasound. The size and the size distribution of a phase change contrast agent is an important factor in determining the ADV threshold and the biodistribution. Thus, high throughout manufacturing of uniform-sized droplets, required to maintain spatial control of the vaporization process, remains challenging. This work describes a parametric evaluation of a novel process using premix membrane emulsification (PME) to produce homogeneous PFC emulsions at high rate with moderate pressure using Shirasu Porous Glass (SPG) membranes. In this study, we investigated the effect of several process parameters on the resulting pressure and droplet size: membrane pore size, flow rate, and dispersed phase type. The functionality of the manufactured emulsions for ADV was also demonstrated. Vaporization of the PFC emulsions was obtained using an imaging ultrasound transducer at 7.813 MHz, and the ADV thresholds were determined. Here, the pressure threshold for ADV was determined to be 1.49 MPa for uniform-sized perfluorohexane (PFHex) droplets with a mean size of 1.51 μm and a sharp distribution (CV and span respectively of 20% and 0.6). Thus, a uniform-sized droplet showed a more homogeneous vaporization with a uniform response in the focal region of the transducer. Indeed, polydispersed droplets had a more diffuse response outside the focal region due to the presence of large droplets that vaporize at lower energies. The ADV threshold of uniform-sized PFC droplets was found to decrease with the droplet diameter and the bulk fluid temperature, and to increase with the boiling temperature of PFC and the presence of an oil layer surrounding the PFC core.

Manipulation of Nanodroplets via a Nonuniform Focused Acoustic Vortex

Techniques for the manipulation of nanodroplets are central to many miniaturized systems in chemical and biological research and applications. In this study, we demonstrate that the dispersed nanodroplets composed of perfluorocarbon can be manipulated to form a micron-sized elliptical aggregation via a nonuniform focused acoustic vortex, and the aggregation location can be further flexibly regulated. In addition, it is unveiled theoretically and experimentally that the fundamental mechanism for the elliptical aggregation is the nonuniform distribution of the pressure, which makes the acoustic radiation forces acting on the dispersed nanodroplets converge at a certain point. Moreover, the required acoustic droplet vaporization threshold for the nanodroplets is significantly lowered, while the generated microbubbles remain similar in size after aggregation. The findings of this study provide an alternative field of opportunities for targeted drug delivery and imaging using nanodroplet emulsions.

Acoustic droplet vaporization threshold of perfluorocarbon droplets as a function of frequency: Effects of droplet cores and size

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound can be used for many therapeutic and diagnostic applications. The ultrasound activation pressure required for the phase change of these droplets into echogenic microbubbles is termed acoustic droplet vaporization (ADV) threshold. We have studied the ADV and IC (inertial cavitation) thresholds in a tubeless setup using an acoustic means varying frequency of the excitation, the droplet core and their average size. The ADV threshold was found to increase with the increasing frequency for the lowest boiling point liquid, perfluoropentane (PFP), for both large and small average size droplets. For higher boiling point liquids, perfluorohexane (PFH) and perfluorooctyle bromide (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.

Acoustic Droplet Vaporization in Acoustically Responsive Scaffolds: Effects of Frequency of Excitation, Volume Fraction and Threshold Determination Method

Ultrasound-induced vaporization of liquid perfluorocarbon (PFC) droplets into microbubbles, termed acoustic droplet vaporization (ADV), has potential therapeutic and diagnostic applications. Recently, we demonstrated how ADV—a threshold-based phenomenon—can modulate the release of biomolecules from composite hydrogels, thereby stimulating regenerative processes, such as angiogenesis. These composite hydrogels, called acoustically responsive scaffolds (ARSs), consist of monodispersed, micron size PFC emulsions embedded within a fibrin matrix. This study investigated the effects of frequency of excitation (2.25, 5, 7.5 and 10 MHz) and volume fraction (0.05%, 0.2% and 1% [v/v]) of monodispersed, double emulsions in the ARSs on the ADV threshold. We determined and compared the ADV thresholds via acoustic methods, including active detection, passive detection and attenuation, as well as an echogenicity-based method using B-mode imaging. The ADV threshold determined via these four techniques showed an increasing trend with frequency of excitation. Further analysis of the wave propagation showed that the amplitudes of high frequency harmonics were diminished in ARSs with high volume fractions of emulsion. The ADV threshold inversely correlated with the volume fraction of emulsion at the lowest excitation frequency. However, at higher frequencies, possibly due to the high acoustic reflectivity of the PFC emulsions, the ADV threshold correlated directly with the volume fraction of the emulsion. Additionally, the ADV efficiency correlated with the supra-threshold acoustic pressure. Overall, these results elucidate fundamental acoustic properties of the ARSs, which can be used in future applications.

Parametric Study of Acoustic Droplet Vaporization Thresholds and Payload Release From Acoustically-Responsive Scaffolds

Hydrogels are commonly used for the delivery of bioactive molecules, especially growth factors and cytokines capable of stimulating tissue regeneration. Regenerative processes are regulated by the concentrations and spatiotemporal presentations of these molecules. With conventional hydrogels, these critical delivery parameters cannot be actively modulated after implantation. We have developed composite hydrogel scaffolds where payload release is non-invasively modulated, in an on-demand manner, using ultrasound (US). These acoustically-responsive scaffolds (ARSs) consist of a fibrin matrix doped with a payload-carrying, perfluorocarbon (PFC) double emulsion. Previously, acoustic droplet vaporization (ADV) was used to trigger release of a pro-angiogenic growth factor, encapsulated in the ARS, which stimulated blood vessel formation in vivo. In the present study, we assess how characteristics of the monodispersed emulsion, fibrin matrix, and US impact ADV thresholds and the release efficiency of a dextran payload. ADV thresholds increased with the molecular weight of the PFC in the emulsion and inversely with the volume fraction of emulsion in the ARS. Payload release from ARSs with perfluoroheptane (C7) or perfluorooctane (C8) emulsions was dependent on the number of z-planes of US used to generate ADV and inversely dependent on the lateral spacing. Conversely, release from ARSs with perfluoropentane (C5) or perfluorohexane (C6) emulsions was less dependent on these US exposure parameters. After ADV, payload diffusion decreased significantly in ARSs with C5 or C6 emulsions compared with ARSs with C7 or C8 emulsions. The expansion of the ARS after ADV decreased with the molecular weight of the PFC. Non-selective release increased with the molecular weight of the PFC and thrombin concentration. Overall, these findings can be used for optimization of ARS properties and US parameters in future therapeutic applications.

Frequency dependence of the vaporization threshold of sonosensitive perfluorocarbon droplets varying their liquid core and size

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound have received increasing attention for therapeutic and diagnostic applications. The ultrasound activation pressure required for the phase change of these droplets into echogenic microbubbles is termed acoustic droplet vaporization (ADV). This study systematically investigates the effect of frequency of excitation¾2.25 MHz, 10 MHz and 15 MHz¾on ADV and inertial cavitation (IC) thresholds of a suspension of PFC droplets by varying the physical properties of the liquid droplets (size and boiling point) in a tubeless setup. We prepared lipid-coated droplets with three different liquid cores, perfluoropentane (PFP), perfluorohexane (PFH) and perfluorooctyle bromide (PFOB), of two different size ranges ¾ one with diameter smaller than 3 I¼m and the other with diameter larger than 10 I¼m. We found that the ADV threshold increases with frequency for the most volatile PFC liquid, PFP, for both large and small size droplets. While for the s...

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

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 μm and larger than 10 μm-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.

Control of Acoustic Cavitation for Efficient Sonoporation with Phase-Shift Nanoemulsions

Acoustic cavitation can be used to temporarily disrupt cell membranes for intracellular delivery of large biomolecules. Termed sonoporation, the ability of this technique for efficient intracellular delivery (i.e., >50% of initial cell population showing uptake) while maintaining cell viability (i.e., >50% of initial cell population viable) has proven to be very difficult. Here, we report that phase-shift nanoemulsions (PSNEs) function as inertial cavitation nuclei for improvement of sonoporation efficiency. The interplay between ultrasound frequency, resultant microbubble dynamics and sonoporation efficiency was investigated experimentally. Acoustic emissions from individual microbubbles nucleated from PSNEs were captured using a broadband passive cavitation detector during and after acoustic droplet vaporization with short pulses of ultrasound at 1, 2.5 and 5 MHz. Time domain features of the passive cavitation detector signals were analyzed to estimate the maximum size (Rmax) of the microbubbles using the Rayleigh collapse model. These results were then applied to sonoporation experiments to test if uptake efficiency is dependent on maximum microbubble size before inertial collapse. Results indicated that at the acoustic droplet vaporization threshold, Rmax was approximately 61.7 ± 5.2, 24.9 ± 2.8, and 12.4 ± 2.1 μm at 1, 2.5 and 5 MHz, respectively. Sonoporation efficiency increased at higher frequencies, with efficiencies of 39.5 ± 13.7%, 46.6 ± 3.28% and 66.8 ± 5.5% at 1, 2.5 and 5 MHz, respectively. Excessive cellular damage was seen at lower frequencies because of the erosive effects of highly energetic inertial cavitation. These results highlight the importance of acoustic cavitation control in determining the outcome of sonoporation experiments. In addition, PSNEs may serve as tailorable inertial cavitation nuclei for other therapeutic ultrasound applications.

Acoustic vaporization threshold of lipid-coated perfluoropentane droplets.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing microbubbles as contrast agents in situ as well as higher stability and the possibility of achieving smaller sizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with a perfluoropentane (PFP) core (diameter 400-3000 nm) is acoustically measured as a function of the excitation frequency in a tubeless setup at room temperature. The changes in scattered responses-fundamental, sub-, and second harmonic-are investigated, a quantitative criterion is used to determine the ADV phenomenon, and findings are discussed. The average threshold obtained using three different scattered components increases with frequency-1.05 ± 0.28 MPa at 2.25 MHz, 1.89 ± 0.57 MPa at 5 MHz, and 2.34 ± 0.014 MPa at 10 MHz. The scattered response from vaporized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV threshold value.

Lowering of acoustic droplet vaporization threshold via aggregation

Acoustically sensitive emulsion nanodroplets composed of perfluorocarbon have shown great potential for advanced medical diagnosis and therapy but are limited by the required high acoustic droplet vaporization (ADV) threshold for clinical applications. This study investigates the use of an ultrasonic standing wave to lower the ADV threshold while maintaining the generated bubble size in the required size range, ensuring the generation of inertial cavitation and corresponding physical effects. The results showed that disperse nanodroplets were manipulated to form micron-sized aggregations, and the required ADV threshold was significantly lowered, while a similar size range of the microbubbles generated by disperse nanodroplets was maintained. The threshold could be further regulated by adjusting the aggregation size via controlling the concentration of the disperse nanodroplets. Furthermore, the internal pressures in the aggregations with different sizes were calculated to determine their ADV thresholds theoretically, which were shown to be in good agreement with the experimental results.

Nanoparticles Formed by Acoustic Destruction of Microbubbles and Their Utilization for Imaging and Effects on Therapy by High Intensity Focused Ultrasound.

This work reports that when PEG-lipid-shelled microbubbles with fluorocarbon interior (C4F10, C5F12, or C6F14) are subjected to ultrasound pulses, they produce metastable, fluid-filled nanoparticles that can be re-imaged upon administration of HIFU. The nanoparticles produced by destruction of the microbubbles (MBNPs) are of 150 nm average diameter and can be re-imaged for up to an hour after creation for C 4F10, and for at least one day for C5F12. The active species were found to be fluid (gas or liquid) filled nanoparticles rather than lipid debris. The acoustic droplet vaporization threshold of the nanoparticles was found to vary with the vapor pressure of the encapsulated fluorocarbon, and integrated image brightness was found to increase dramatically when the temperature was raised above the normal boiling point of the fluorocarbon. Finally, the vaporization threshold decreases in serum as compared to buffer, and administration of HIFU to the nanoparticles caused breast cancer cells to completely detach from their culture substrate. This work demonstrates a new functionality of microbubbles that could serve as a platform technology for ultrasound-based theranostics.

Experimental and theoretical studies on acoustic droplet vaporization

Phase shift nanodroplets are better alternatives to microbubbles due to their enhanced stability and smaller size distribution. These nanodroplets undergo phase transition from liquid to highly echogenic gaseous state under acoustic excitation through a process termed acoustic droplet vaporization (ADV). In this study, we synthesized lipid-coated perfluoropentane (PFP)-filled nanodroplets via sonication and mechanical agitation methods. We investigated the ADV threshold of these nanodroplets as a function of acoustic parameters such as excitation frequency and pulse repetition period (PRP). Experiments were performed at frequencies 2.25, 5, and 10 MHz. The acoustic signature of droplet vaporization was observed to be a broadband signal at all the studied frequencies. The ADV threshold was studied by increasing the excitation pressure in steps of 200 kPa. The ADV threshold at 2.25 and 5 MHz was found to be 1.8 and 2.2 MPa, respectively. The scattered response from droplets were studied at different PRPs of...

Effects of acoustic parameters on nanodroplet vaporization

Phase shift nanodroplets offer a number of advantages over ordinary microbubbles due to their enhanced stability and smaller size distribution. These nanodroplets undergo a phase transition from liquid to highly echogenic gaseous state when activated by sufficient acoustic energy through a process termed acoustic droplet vaporization (ADV). In this study, we synthesized lipid-coated perfluoropentane (PFP) filled nanodroplets via sonication method. We investigated the ADV threshold of these nanodroplets as a function of acoustic parameters such as excitation pressure, frequency, pulse length, and pulse repetition period (PRP). Our results indicate that ADV threshold varies significantly with PRP; while at PRP of 10 ms, the ADV threshold was found to be 3.6 MPa (pk-pk), for PRP of 1 ms, 100 μs, and 500 μs, ADV was not observed even at 10 MPa. At ADV, fundamental and odd harmonics were found to be significantly higher than the background noise. The acoustic response of ordinary perfluorobutane (PFB) filled microbubbles with the same lipid composition is compared to that of PFP nanodroplets when both were excited with the same excitation pressures (450 kPa to 10 MPa). Above ADV threshold both showed a similar response.

Scavenging dissolved oxygen via acoustic droplet vaporization

Acoustic droplet vaporization (ADV) of perfluorocarbon emulsions has been explored for diagnostic and therapeutic applications. Previous studies have demonstrated that vaporization of a liquid droplet results in a gas microbubble with a diameter 5–6 times larger than the initial droplet diameter. The expansion factor can increase to a factor of 10 in gassy fluids as a result of air diffusing from the surrounding fluid into the microbubble. This study investigates the potential of this process to serve as an ultrasound-mediated gas scavenging technology. Perfluoropentane droplets diluted in phosphate-buffered saline (PBS) were insonified by a 2MHz transducer at peak rarefactional pressures lower than and greater than the ADV pressure amplitude threshold in an in vitro flow phantom. The change in dissolved oxygen (DO) of the PBS before and after ADV was measured. A numerical model of gas scavenging, based on conservation of mass and equal partial pressures of gases at equilibrium, was developed. At insonation pressures exceeding the ADV threshold, the DO of air-saturated PBS decreased with increasing insonation pressures, dropping as low as 25% of air saturation within 20s. The decrease in DO of the PBS during ADV was dependent on the volumetric size distribution of the droplets and the fraction of droplets transitioned during ultrasound exposure. Numerically predicted changes in DO from the model agreed with the experimentally measured DO, indicating that concentration gradients can explain this phenomenon. Using computationally modified droplet size distributions that would be suitable for in vivo applications, the DO of the PBS was found to decrease with increasing concentrations. This study demonstrates that ADV can significantly decrease the DO in an aqueous fluid, which may have direct therapeutic applications and should be considered for ADV-based diagnostic or therapeutic applications.

A model for acoustic vaporization of encapsulated droplets

The use of encapsulated liquid nanoparticles is currently largely investigated for medical applications, mainly because their reduced size allows them to enter targeted areas which cannot be reached by large microbubbles (contrast agents). Low-boiling point perfluorocarbon droplets can be vaporized on-site under the action of the ultrasonic field, in order to turn them into echogeneous-eventually cavitating-microbubbles. This paper presents a theoretical model describing this phenomenon, paying particular attention to the finite size of the droplet and its encapsulation by a thin viscoelastic layer. Numerical simulations are done for droplets of radii 1 and 10 μm and for frequencies of 1-5 MHz. Results reveal that droplet surface tension and shell rigidity are responsible for an increase of the acoustic droplet vaporization threshold. Furthermore, this threshold does not vary monotonically with frequency, and an optimal frequency can be found to minimize it. Finally, the role of some physical properties on the dynamics of the particle is analyzed, such as the contrast of inner and outer liquids densities and the mechanical properties of the shell.

Scavenging dissolved oxygen via acoustic droplet vaporization

Acoustic droplet vaporization (ADV) has been investigated for capillary hemostasis, thermal ablation, and ultrasound imaging. The maximum diameter of a microbubble produced by ADV depends on the gas saturation of the surrounding fluid. This dependence is due to diffusion of dissolved gases from the fluid into the perfluoropentane (PFP) microbubble. This study investigated the change in oxygen concentration in the surrounding fluid after ADV. Albumin-shelled PFP droplets in air-saturated saline (1:30, v/v) were continuously pumped through a flow system and insonified by a focused 2-MHz single-element transducer to induce ADV. B-mode image echogenicity was used to determine the ADV threshold pressure amplitude. The dissolved oxygen concentration in the fluid upstream and downstream of the insonation region was measured using inline sensors. Droplet size distributions were measured before and after ultrasound exposure to determine the ADV transition efficiency. The ADV pressure threshold at 2 MHz was 1.7 MPa (peak negative). Exposure of PFP droplets to ultrasound at 5 MPa peak negative pressure caused the dissolved oxygen content in the surrounding fluid to decrease from 88 ± 3% to 20 ± 4%. The implications of oxygen scavenging during ADV will be discussed.