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Ultimate fate of a dynamical bubble/droplet system following acoustic vaporization

Abstract : The phase-change of a liquid droplet induced by a supply of acoustic energy is known as “Acoustic Droplet Vaporization,” and it represents a versatile tool for medical applications. In an attempt to understand the complex mechanisms that drive the vaporization threshold, a theoretical concentric three phase model (bubble of vapor dodecafluoropentane + layer of liquid dodecafluoropentane + water) is used to compute numerical simulations of the vapor bubble time evolution. The dynamics are sorted into different regimes depending on their shared characteristic and the system ultimate fate. Those regimes are then organized within a phase diagram that collects all the possible dynamics and that predicts whether the complete vaporization occurs or not. Some innovative medical tools for diagnosis and therapy of malignant diseases used the vaporization of metastable emulsions of biocompatible liquid nanodroplets using ultrasound:1,2 the acoustic droplet vaporization (ADV). This phenomenon is carried out by an adequate supply of acoustic energy to a liquid droplet. Optical observations of the ADV outline a highly nonlinear dynamics with a strong sensitivity to the experimental configuration and noteworthy to the acoustic frequency and pressure.3–5 Numerous experimental studies endeavored to determine the acoustic pressure threshold necessary to reach complete vaporization. However, no clear trend regarding frequency is yet available to predict the optimum parameters (see Table in Ref. 6). Some authors observed a threshold decreasing with frequency,7–9 whereas others found the opposite behavior.5,6,10 These discrepancies are the evidence of the richness and complexity of the vaporization mechanism. The prediction of the ADV threshold is therefore essential to optimize its efficiency. The ADV is usually modeled as a concentric three phase system (see Fig. 1), made of a centered vapor bubble already nucleated, surrounded by a layer of the same species in its liquid state, and immersed in pure water (no dissolved species or gases) at body temperature. Note that this system cannot be at equilibrium even when no acoustic field is supplied. When the nucleus radius is above a critical value, the vapor phase naturally grows by evaporation; otherwise, it shrinks and disappears due to condensation (see Fig. 2 in Ref. 11). The ADV process amounts to counterbalance this natural collapse by applying an acoustic expansion. The liquid/vapor mixture is composed of dodecafluoropentane (C5F12 or DDFP), a promising candidate for its low bulk boiling point12 (29 °C). It nevertheless remains in a metastable liquid phase because of the additional Laplace pressure at the interface. The static pressure p0 is modulated by a continuous harmonic excitation beginning by an expansion p(t) = p0 − pa sin(2πft), for t > 0, where pa is the acoustic amplitude. A more realistic excitation would involve nonlinear distortion, harmonic generation, shock formation, and asymmetrical waveform.13 This would imply to consider many more parameters, would make the ADV landscape even more intricate, and would deserve further studies. However, note that the corresponding superfocusing effect4 has been observed only for droplets larger than 6 μm.
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Submitted on : Wednesday, June 17, 2020 - 9:48:30 AM
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Thomas Lacour, Tony Valier-Brasier, François Coulouvrat. Ultimate fate of a dynamical bubble/droplet system following acoustic vaporization. Physics of Fluids, American Institute of Physics, 2020, 32 (5), pp.051702. ⟨10.1063/5.0004375⟩. ⟨hal-02871138⟩



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