Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session G21: Bubbles: Cavitation, Acoustics and Biomedical |
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Chair: Marc K. Smith, Georgia Institute of technology Room: D139-140 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G21.00001: Interfacial Dynamics of Condensing Vapor Bubbles in an Ultrasonic Acoustic Field Thomas Boziuk, Marc Smith, Ari Glezer Enhancement of vapor condensation in quiescent subcooled liquid using ultrasonic actuation is investigated experimentally. The vapor bubbles are formed by direct injection from a pressurized steam reservoir through nozzles of varying characteristic diameters, and are advected within an acoustic field of programmable intensity. While kHz-range acoustic actuation typically couples to capillary instability of the vapor-liquid interface, ultrasonic (MHz-range) actuation leads to the formation of a liquid spout that penetrates into the vapor bubble and significantly increases its surface area and therefore condensation rate. Focusing of the ultrasonic beam along the spout leads to ejection of small-scale droplets from that are propelled towards the vapor liquid interface and result in localized acceleration of the condensation. High-speed video of Schlieren images is used to investigate the effects of the ultrasonic actuation on the thermal boundary layer on the liquid side of the vapor-liquid interface and its effect on the condensation rate, and the liquid motion during condensation is investigated using high-magnification PIV measurements. High-speed image processing is used to assess the effect of the actuation on the dynamics and temporal variation in characteristic scale (and condensation rate) of the vapor bubbles. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G21.00002: Reduced Order Modeling of Bubble Cloud Dynamics in a Focused Ultrasound Field Kazuki Maeda, Tim Colonius In order to characterize the cloud cavitation in burst wave lithotripsy, reduced order modeling of the dynamics of a spherical bubble cloud of a radius $O(1)$ mm interacting with traveling ultrasound waves of an amplitude O(1) MPa in water is presented. Bubbles are treated as spherical, radially oscillating cavities dispersed in continuous liquid phase. The volume of Lagrangian point bubbles is mapped with a regularization kernel as void fraction onto three-dimensional Cartesian grids that define the Eulerian liquid phase. The flow field is solved using a WENO-based compressible flow solver. The initial size and number density of the bubbles are critical for their coherent dynamics in the cloud, yet three-dimensional simulations of clouds with various parameters are computationally demanding. For further reduced-order modeling, a new kernel is introduce into the model to regularize bubbles onto two-dimensional, axisymmetric grids. The evolution of the void fraction and the maximum pressure in the cloud simulated using the model agree with results of three-dimensional simulations, while the reduction in computational cost is a factor of $O(100)$. Finally, the model is applied to a parametric study of the coherent dynamics of bubbles. [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G21.00003: Ultrasound induced bubble clusters and tunnels in tissue-mimicking agar phantoms Pooya Movahed, Wayne Kreider, Adam D. Maxwell, Michael R. Bailey, Jonathan B. Freund Soft tissue fractionation induced by acoustic cavitation is desired for non-invasive tissue removal in histotripsy, while being a potential injury mechanism in other therapeutic ultrasound treatments such as lithotripsy. In this work, we investigate the formation of bubble clusters and tunnels in tissue-mimicking agar phantoms by focused ultrasound bursts to inform a class of damage models. Agar phantoms of different stiffness were subjected to a series of multi-cycle ultrasound bursts, using a burst wave lithotripsy (BWL) protocol [Maxwell \textit{et al.}, J. Urol., 193, 338-344 (2015)], and simultaneously imaged at 200 frames per second (1 image per ultrasound burst). Some bubbles become visible in images (\textasciitilde 200 microns) due to the negative pressure (\textasciitilde 7.5 MPa) in the initial bursts, and the number of visible bubbles increases continuously during the subsequent bursts. A Rayleigh---Plesset-type bubble dynamics model, which accounts for viscoelastic confinement of agar gels, is developed. Material fatigue leading to eventual irreversible fracture-like failure in this model is proposed to explain the key observations. In addition to isolated, approximately spherical bubbles, long tunnel-like features are observed, which are seemingly lines of joined bubbles along a possible fracture or defect. The geometry of these tunnel-like features is quantified, and a physical explanation for tunnel formation is proposed in terms of bubble expansion and unstable collapse. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G21.00004: Temperature and stress fields produced by ultrasound-induced cavitation in a viscoelastic medium Lauren Mancia, Eric Johnsen Ultrasound contrast agents can act as cavitation nuclei that mechanically damage surrounding tissue when they oscillate in diagnostic ultrasound. Encapsulated microbubbles have also been proposed as a means to improve the efficiency and efficacy of therapeutic ultrasound by increasing the rate of tissue heating. However, the thermal and mechanical effects of cavitation are difficult to distinguish from each other and to quantify experimentally as they often occur simultaneously. To address this challenge, we study the cavitation-induced temperature and stress fields produced by a spherical bubble oscillating in a Kelvin-Voigt viscoelastic medium with nonlinear elasticity using a model that also accounts for energy transport inside and outside the bubble. We find that the primary contribution to heating is viscous dissipation, which itself depends on both the material (viscosity) and the bubble dynamics. We examine the rate of viscous heating and the magnitude of stresses over a relevant range of tissue properties and waveform parameters to determine regimes where heating is expected to be dominant. A means of estimating the expected significance of viscous dissipation from given tissue properties and waveform parameters is proposed. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G21.00005: The hydrodynamic and ultrasound-induced forces on microbubbles under high Reynolds number flow representative of the human systemic circulation Alicia Clark, Alberto Aliseda Ultrasound contrast agents (UCAs) are micron-sized bubbles that are used in conjunction with ultrasound (US) in medical applications such as thrombolysis and targeted intravenous drug delivery. Previous work has shown that the Bjerknes force, due to the phase difference between the incoming US pressure wave and the bubble volume oscillations, can be used to manipulate the trajectories of microbubbles. Our work explores the behavior of microbubbles in medium sized blood vessels under both uniform and pulsatile flows at a range of physiologically relevant Reynolds and Womersley numbers. High speed images were taken of the microbubbles in an in-vitro flow loop that replicates physiological flow conditions. During the imaging, the microbubbles were insonified at different diagnostic ultrasound settings (varying center frequency, PRF, etc.). An in-house Lagrangian particle tracking code was then used to determine the trajectories of the microbubbles and, thus, a dynamic model for the microbubbles including the Bjerknes forces acting on them, as well as drag, lift, and added mass. Preliminary work has also explored the behavior of the microbubbles in a patient-specific model of a carotid artery bifurcation to demonstrate the feasibility of preferential steering of microbubbles towards the intracranial circulation with US. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G21.00006: Acoustic radiation force expansions in terms of partial wave phase shifts for scattering: Applications Philip L. Marston, Likun Zhang When evaluating radiation forces on spheres in soundfields (with or without orbital-angular momentum) the interpretation of analytical results is greatly simplified by retaining the use of s-function notation for partial-wave coefficients imported into acoustics from quantum scattering theory in the 1970s. This facilitates easy interpretation of various efficiency factors [1]. For situations in which dissipation is negligible, each partial-wave s-function becomes characterized by a single parameter: a phase shift allowing for all possible situations. These phase shifts are associated with scattering by plane traveling waves and the incident wavefield of interest is separately parameterized. (When considering outcomes, the method of fabricating symmetric objects having a desirable set of phase shifts becomes a separate issue.) The existence of negative radiation force ``islands'' for beams reported in 2006 by Marston is manifested. This approach and consideration of conservation theorems [2] illustrate the unphysical nature of various claims made by other researchers. This approach is also directly relevant to objects in standing waves. [1] L. Zhang and P. L. Marston, J. Acoust. Soc. Am. 140, EL178-EL183 (2016). [2] P. L. Marston and L. Zhang, J. Acoust. Soc. Am. 139, 3139-3144 (2016). [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G21.00007: Revisiting the potential for bursting bubbles to damage cells below the free surface Peter Walls, James Bird The rapid motion associated with bubbles bursting at the surface of a liquid is known to cause damage to cells in a suspension, which is particularly problematic in bioreactors that require continuous injection of oxygen to sustain the cells. It is generally accepted that cells directly attached to the bubble's interface will experience lethal levels of damage. To prevent cells from initially attaching to the bubble's surface, surfactants are widely used. However, the potential for bursting bubbles to damage nearby, but not directly attached, cells is less clear. Previous numerical studies have predicted maximum energy dissipation rates (EDR) as high as $10^{10}$ $\textrm{W/m}^3$ for bubbles with radii less than 1 mm; lethal to the commonly used mammalian CHO cell. Here we show that these studies tend to underestimate the generated EDR levels by several orders of magnitude due to limited numerical mesh resolution. Furthermore, we demonstrate how a downward traveling jet can cause damage away from the interface. We validate our numerical model with high-speed bubble bursting experiments and relate the dynamics of this downward jet to the boundary layer equations. We anticipate our results will be an integral step towards developing more efficient aeration platforms. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G21.00008: Nonlinear oscillation and interfacial stability of an encapsulated microbubble under dual-frequency ultrasound Michael Calvisi, Yunqiao Liu, Qianxi Wang Encapsulated microbubbles (EMBs) are widely used in medical ultrasound imaging as contrast-enhanced agents. However, the potential damaging effects of violent, collapsing EMBs to cells and tissues in clinical practice have remained a concern. Dual-frequency ultrasound is a promising technique for improving the efficacy and safety of sonography. The EMB system modeled consists of the external liquid, membrane, and internal gases. The microbubble dynamics are simulated using a simple nonlinear interactive theory, considering the compressibility of the internal gas, viscosity of the liquid flow, and elasticity of the membrane. The radial oscillation and interfacial stability of an EMB under single and dual-frequency excitations are compared. The simulation results show that the dual-frequency technique produces larger backscatter pressure at higher harmonics of the primary driving frequency. This enriched acoustic spectrum can enhance blood-tissue contrast and improve sonographic image quality. The results further show that the acoustic pressure threshold associated with the onset of shape instability is greater for dual-frequency driving. This suggests that the dual-frequency technique stabilizes the EMB, thereby improving the efficacy and safety of contrast-enhanced agents. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G21.00009: Effects of non-condensable gas on the dynamic oscillations of cavitation bubbles Yuning Zhang Cavitation is an essential topic of multiphase flow with a broad range of applications. Generally, there exists non-condensable gas in the liquid and a complex vapor/gas mixture bubble will be formed. A rigorous prediction of the dynamic behavior of the aforementioned mixture bubble is essential for the development of a complete cavitation model. In the present paper, effects of non-condensable gas on the dynamic oscillations of the vapor/gas mixture bubble are numerically investigated in great detail. For the completeness, a large parameter zone (e.g. bubble radius, frequency and ratio between gas and vapor) is investigated with many demonstrating examples. The mechanisms of mass diffusion are categorized into different groups with their characteristics and dominated regions given. Influences of non-condensable gas on the wave propagation (e.g. wave speed and attenuation) in the bubbly liquids are also briefly discussed. Specifically, the minimum wave speed is quantitatively predicted in order to close the pressure-density coupling relationship usually employed for the cavitation modelling. Finally, the application of the present finding on the development of cavitation model is demonstrated with a brief discussion of its influence on the cavitation dynamics. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G21.00010: Bubble absorption by an air-filled helically-supported capillary channel Negar Beheshtipour, David Thiessen Gas-liquid phase separation under microgravity conditions where buoyancy is not active represents a challenge for two-phase liquid-continuous space systems. Similar challenges are present in micro-scale electrochemical systems on Earth that generate gas bubbles in geometries where surface tension prevails over gravity. A possible ground-based application would be the removal of carbon dioxide bubbles from large aspect ratio channels in a direct-methanol fuel cell that could otherwise occlude the channel. In this study we use a 3-mm diameter stretched stainless-steel spring coated with a superhydrophobic layer to create a helically-supported capillary channel. Such a channel that is submerged in water and filled with air while vented to the atmosphere was found to absorb a stream of 2.5-mm diameter air bubbles at a rate of at least 36 bubbles/s. An optical detector and high-speed imaging system have been used to study bubble absorption dynamics. A significant finding is that the initial attachment of the bubble to the channel that involves the rupture of a thin film of water happens in less than 1 ms. The rapid rupture of the water film separating the bubble from the channel might be attributed to the roughness of the hydrophobic coating. [Preview Abstract] |
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