arXiv subjects

Anunay Prasanna

Publications and source records attributed to Anunay Prasanna.

6 recordsLinked to original sources

arXiv2026

Bubble jetting in acoustic microdroplet vaporization

Acoustic droplet vaporization denotes the phase-change of micron- and sub-micron-sized droplets upon the application of high-amplitude ultrasound. The asymmetric collapse of the incepted vapor bubbles within the droplets can give rise to high-speed liquid microjets. Here, we describe acoustically-driven and bubble-pair jetting arising within the vaporizing droplet, observed experimentally with ultra-high-speed imaging at the microscale. The existence of complex pressure fields due to the continued acoustic wave-droplet interaction and the nucleation of multiple bubbles within the droplet leads to rich dynamics, with the jets presenting behavioral self-similarity to millimetric bubbles under comparable conditions. Evaporative instabilities that develop during bubble growth impede jet formation during bubble collapse. Furthermore, the ability of the jets to pierce the droplet interface and penetrate into the surrounding fluid is discussed. These powerful microjets could be harnessed to induce cell permeabilization for targeted drug delivery and treatment of cancerous tissue.

physics.flu-dyn↗

arXiv2026

Cavitation by phase shift of focused shock waves inside a droplet

Localized cavitation in liquids and soft tissues, typically initiated by the rarefaction phase of high-amplitude ultrasound waves, is leveraged in several biomedical applications such as ablation techniques and drug delivery with vaporizing agents. However, safety considerations aimed at avoiding unwanted bubble activity outside the targeted region pose a limit to the maximum allowed peak rarefaction pressure, which on the other hand can hinder the therapeutic efficacy of these techniques. This study shows that a purely compressive shock wave can generate localized, negative pressure and initiate cavitation inside a sub-millimetric perfluorohexane droplet, without requiring any externally applied rarefaction wave. The Gouy phase shift is identified as the physical mechanism responsible for the conversion of positive pressure into tension during shock focusing, and its occurrence is demonstrated through numerical simulations and direct experimental measurements. Comparison of the regions affected by cavitation, visualized \emph{in-situ} by means of high-speed x-ray phase-contrast imaging, with prediction from Classical Nucleation Theory suggests homogeneous nucleation as the underlying mechanism behind bubble formation. The presented findings offer valuable insights into the physics of shock wave propagation which can inspire the development of novel acoustic driving strategies for cavitation generation, facilitating the reduction of negative pressures outside the target region and improving the safety and precision of biomedical treatments.

physics.flu-dyn↗

arXiv2025

Shape modes and jet formation on ultrasound-driven wall-attached bubbles

Understanding how substrate-attached bubbles respond to ultrasound is important for applications from industrial cleaning to biomedical therapy. Under ultrasonic excitation, bubbles can deform through Faraday instability and periodically emit high-speed jets. Although this behavior is increasingly well understood for free bubbles, the dynamics of wall-attached bubbles remain largely unexplored. In particular, the three-dimensional selection and evolution of non-spherical modes and their relation to jetting have not been resolved. We investigate micrometric air bubbles in contact with a rigid substrate and driven by ultrasound, using a dual-view imaging setup combining top-view bright-field microscopy with side-view phase-contrast X-ray imaging. This approach reveals a stepwise evolution of bubble shape through four regimes: spherical oscillations, harmonic axisymmetric meniscus waves, half-harmonic axisymmetric Faraday waves, and the superposition of half-harmonic sectoral Faraday waves. This contrasts with free bubbles, which jump directly to their final Faraday pattern at instability onset. For the chosen substrate, the observed shape-mode spectrum is degenerate and spans a continuous range of mode degrees, consistent with theoretical predictions based on kinematic arguments. Free bubbles, although also degenerate, remain limited to discrete spherical harmonics. Measured ultrasound pressure thresholds for Faraday instability agree with classical interface-stability theory modified for a rigid boundary. Complementary 3D boundary-element simulations reproduce the observed shape evolution. Finally, we identify the acceleration threshold for cyclic jetting: unlike free bubbles, wall-attached bubbles always jet from the side not constrained by the substrate.

physics.flu-dyn↗

arXiv2025

Shock compression-based equation of state for perfluorohexane

Perfluorohexane is a biocompatible material that serves as a liquid core for acoustically-responsive agents in biomedical applications. Despite its relatively widespread usage, there is a lack of experimental data determining its thermodynamic properties. This challenges numerical simulations to predict the acoustic response of agents developed using this material. In this study, we employ the well-established method of shock compression of materials at relatively high pressures (100--400 MPa) to estimate a kinematic equation of state for perfluorohexane. We use multi-objective optimization to obtain the Noble-Abel Stiffened-Gas equation of state, which is suitable for hydrodynamic numerical simulations. We then apply the extrapolated equation of state to simulate shock-wave propagation within a perfluorohexane droplet showing excellent agreement with equivalent experiments. This validates the equation of state and promotes the use of numerical simulations as a valuable tool for understanding the complex acoustic interactions involved in these biomedical agents, ultimately facilitating their translation for clinical purposes.

physics.flu-dyn↗

arXiv2024

Positive pressure matters in acoustic droplet vaporization

Acoustically vaporizable droplets are phase-change agents that can improve the effectiveness of ultrasound-based therapies. In this study, we demonstrate that the compression part of an acoustic wave can generate tension that initiates the vaporization. This counter-intuitive process is explained by the occurrence of Gouy phase shift due to the focusing of the acoustic wave inside the droplet. Our analysis unifies the existing theories for acoustic droplet vaporization under a single framework and is supported by experiments and simulations. We use our theory to identify governing parameters that allow to vaporize droplets using predominantly compression waves, which are safer in medical use.

physics.flu-dyn↗

arXiv2023

Synchrotron X-ray phase-contrast imaging of ultrasonic drop atomization

Ultrasonic atomization is employed to generate size-controllable droplets for a variety of applications. Here, we minimize the number of parameters dictating the process by studying the atomization of a single drop pending from an ultrasonic horn. Spatiotemporally resolved X-ray phase-contrast imaging measurements show that the number-median sizes of the ejected droplets can be predicted by the linear Navier-Stokes equations, signifying that the size distribution is controlled by the fluid properties and the driving frequency. Experiments with larger pendant water drops indicate that the fluid-structure interaction plays a pivotal role in determining the ejection onset of the pendant drop. The atomization of viscoelastic drops is dictated by extended ligament formation, entrainment of air, and ejection of drop-encapsulated bubbles. Existing scaling laws are used to explain the required higher input amplitudes for the complete atomization of viscoelastic drops as compared to inviscid drops. Finally, we elucidate the differences between capillary wave-based and cavitation-based atomization and show that inducing cavitation and strong bubble oscillations quickens the onset of daughter drop ejection but impedes their size control.

physics.flu-dyn↗