Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session M7: Bubbles: Cavitation, Acoustics and BiomedicalBubbles
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Chair: Kausik Sarkar, George Washington University Room: 407 |
Tuesday, November 21, 2017 8:00AM - 8:13AM |
M7.00001: Characterizing the collapse of a cavitation bubble cloud in a focused ultrasound field Kazuki Maeda, Tim Colonius We study the coherent collapse of clouds of cavitation bubbles generated by the passage of a pulse of ultrasound. In order to characterize such collapse, we conduct a parametric study on the dynamics of a spherical bubble cloud with a radius of $r=O(1)$ mm interacting with traveling ultrasound waves with an amplitude of $p_a=O(10^2 - 10^6)$ Pa and a wavelength of $\lambda=O(1-10)$ mm in water. Bubbles with a radius of $O(10)$ um 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 Cartesian grids that defines the Eulerian liquid phase. The flow field is solved using a WENO-based compressible flow solver. We identified that coherent collapse occurs when $\lambda\gg r$, regardless of the value of $p_a$, while it only occurs for sufficiently high $p_a$ when $\lambda\approx r$. For the long wavelength case, the results agree with the theory on linearized dynamics of d'Agostino and Brennen (1989). We extend the theory to short wave length case. Finally, we analyze the far-field acoustics scattered by individual bubbles and correlate them with the cloud collapse, for applications to acoustic imaging of bubble cloud dynamics. [Preview Abstract] |
Tuesday, November 21, 2017 8:13AM - 8:26AM |
M7.00002: Acoustic microstreaming due to an ultrasound contrast microbubble near a wall Nima Mobadersany, Kausik Sarkar In an ultrasound field, in addition to the sinusoidal motion of fluid particles, particles experience a steady streaming velocity due to nonlinear second order effects. Here, we have simulated the microstreaming flow near a plane rigid wall caused by the pulsations of contrast microbubbles. Although these microbubbles were initially developed as a contrast enhancing agents for ultrasound imaging, they generate additional therapeutic effects that can be harnessed for targeted drug delivery or blood brain barrier (BBB) opening. The microbubbles have a gas core coated with a stabilizing layer of lipids or proteins. We use analytical models as well as boundary element (BEM) simulation to simulate the flow around these bubbles implementing interfacial rheology models for the coating. The microstreaming flow is characterized by two wall bounded vortices. The size of the vortices decreases with the decrease of the separation from the wall. The vortex-induced shear stress is simulated and analyzed as a function of excitation parameters and geometry. These microstreaming shear stress plays a critical role in increasing the membrane permeability facilitating drug delivery or rupturing biological tissues. [Preview Abstract] |
Tuesday, November 21, 2017 8:26AM - 8:39AM |
M7.00003: Jetting of a ultrasound contrast microbubble near a rigid wall Kausik Sarkar, Nima Mobadersany Micron sized gas-bubbles coated with a stabilizing shell of lipids or proteins, are used as contrast enhancing agents for ultrasound imaging. However, they are increasingly being explored for novel applications in drug delivery through a process called sonoporation, the reversible permeabilization of the cell membrane. Under sufficiently strong acoustic excitations, bubbles form a jet and collapse near a wall. The jetting of free bubbles has been extensively studied by boundary element method (BEM). Here, for the first time, we implemented a rigorous interfacial rheological model of the shell into BEM and investigated the jet formation. The code has been carefully validated against past results. Increasing shell elasticity decreases the maximum bubble volume and the collapse time, while the jet velocity increases. The shear stress on the wall is computed and analyzed. A phase diagram as functions of excitation pressure and wall separation describes jet formation. Effects of shell elasticity and frequency on the phase diagram are investigated. [Preview Abstract] |
Tuesday, November 21, 2017 8:39AM - 8:52AM |
M7.00004: Shape instability of a bubble in a viscoelastic medium Kazuya Murakami, Renaud Gaudron, Eric Johnsen Bubble dynamics play an important role in therapeutic ultrasound and other medical applications. For this reason, we investigate the shape instability of a single gas bubble in a viscoelastic, tissue-mimicking medium. The non-spherical bubble surface is expressed by superposition of spherical harmonics. Cauchy's equation of motion is reduced to two ordinary differential equations: the Rayleigh-Plesset type equation for the mean bubble radius and the equation for the mode amplitude, which are solved simultaneously. For a given bubble radius and frequency, the parametric instability is determined by the n-th order natural frequency. In addition, the amplitude threshold of the mean radius is analytically found by the stability theory for Mathieu equation. Our analysis is validated against experimental results, in which non-spherical bubble oscillations are observed under ultrasound irradiation. Finally, the pressure threshold for the shape instability is numerically examined. [Preview Abstract] |
Tuesday, November 21, 2017 8:52AM - 9:05AM |
M7.00005: Modeling Encapsulated Microbubble Dynamics at High Pressure Amplitudes Jan F Heyse, Sanjeeb Bose, Gianluca Iaccarino Encapsulated microbubbles are commonly used in ultrasound contrast imaging and are of growing interest in therapeutic applications where local cavitation creates temporary perforations in cell membranes allowing for enhanced drug delivery. Clinically used microbubbles are encapsulated by a shell commonly consisting of protein, polymer, or phospholipid; the response of these bubbles to externally imposed ultrasound waves is sensitive to the compressibility of the encapsulating shell. Existing models approximate the shell compressibility via an effective surface tension (Marmottant et al. 2005). We present simulations of microbubbles subjected to high amplitude ultrasound waves (on the order of $10^6$ Pa) and compare the results with the experimental measurements of Helfield et al. (2016). Analysis of critical points (corresponding to maximum and minimum expansion) in the governing Rayleigh-Plesset equation is used to make estimates of the parameters used to characterize the effective surface tension of the encapsulating shell. [Preview Abstract] |
Tuesday, November 21, 2017 9:05AM - 9:18AM |
M7.00006: Steering Microbubbles in Physiologically Realistic Flows Using the Bjerknes Force Alicia Clark, Alberto Aliseda Ultrasound contrast agents (UCAs) are lipid-coated microbubbles that are used to increase contrast in ultrasound imaging due to their ability to scatter sound. Additionally, UCAs can be used in conjunction with ultrasound in medical applications such as targeted drug delivery and thrombolysis. These applications utilize the Bjerknes force, an ultrasound-induced force caused by the phase difference between the incoming ultrasound pressure wave and the microbubble volume oscillations. The dynamics of microbubbles under ultrasound excitation have been studied thoroughly in stagnant fluid baths; however, understanding of the fundamental physics of microbubbles in physiologically realistic flows is lacking. An \textit{in vitro }experiment that reproduces the dynamics (Reynolds and Womersley numbers) of a medium-sized blood vessel was used to explore the behavior of microbubbles. Using Lagrangian tracking, the trajectory of each individual bubble was reconstructed using information obtained from high speed imaging. The balance of hydrodynamic forces (lift, drag, added mass, etc.) against the primary Bjerknes force was analyzed. The results show that an increase in ultrasound pulse repetition frequency leads to a linear increase in the Bjerknes force and the increase in the force is quadratic with the amplitude of the excitation. [Preview Abstract] |
Tuesday, November 21, 2017 9:18AM - 9:31AM |
M7.00007: Asymmetric bubble collapse and jetting in generalized Newtonian fluids Ratnesh K. Shukla, Jonathan B. Freund The jetting dynamics of a gas bubble near a rigid wall in a non-Newtonian fluid are investigated using an axisymmetric simulation model. The bubble gas is assumed to be homogeneous, with density and pressure related through a polytropic equation of state. An Eulerian numerical description, based on a sharp interface capturing method for the shear-free bubble-liquid interface and an incompressible Navier-Stokes flow solver for generalized fluids, is developed specifically for this problem. Detailed simulations for a range of rheological parameters in the Carreau model show both the stabilizing and destabilizing non-Newtonian effects on the jet formation and impact. In general, for fixed driving pressure ratio, stand-off distance and reference zero-shear-rate viscosity, shear-thinning and shear-thickening promote and suppress jet formation and impact, respectively. For a sufficiently large high-shear-rate limit viscosity, the jet impact is completely suppressed. Thresholds are also determined for the Carreau power-index and material time constant. The dependence of these threshold rheological parameters on the non-dimensional driving pressure ratio and wall stand-off distance is similarly established. Implications for tissue injury in therapeutic ultrasound will be discussed. [Preview Abstract] |
Tuesday, November 21, 2017 9:31AM - 9:44AM |
M7.00008: Transient thermal driven bubble's surface and its potential ultrasound-induced damage Pooya Movahed, Jonathan B. Freund Ultrasound-induced bubble activity in soft tissues is well-known to be a potential injury mechanism in therapeutic ultrasound treatments. We consider damage by transient thermal effects, including a hypothetical mechanism based on transient thermal phenomena, including viscous dissipation. A spherically symmetric compressible Navier-Stokes discretization is developed to solve the full governing equations, both inside and outside of the bubble, without the usual simplifications in the Rayleigh-Plesset bubble dynamics approach. Equations are solved in the Lagrangian framework, which provides a sharp and accurate representation of the interface as well as the viscous dissipation and thermal transport effects, which preclude reduction to the usual Rayleigh-Plesset ordinary differential equation. This method is used to study transient thermal effects at different frequencies and pressure amplitudes relevant to therapeutic ultrasound treatments. High temperatures achieved in the surrounding medium during the violent bubble collapse phase due to the viscous dissipation in the surrounding medium and thermal conduction from the bubble are expected to cause damage. [Preview Abstract] |
Tuesday, November 21, 2017 9:44AM - 9:57AM |
M7.00009: Influence of cavitation bubble growth by rectified diffusion on cavitation-enhanced HIFU Kohei Okita, Kazuyasu Sugiyama, Shu Takagi, Yoichiro Matsumoto Cavitation is becoming increasingly important in therapeutic ultrasound applications such as diagnostic, tumor ablation and lithotripsy. Mass transfer through gas-liquid interface due to rectified diffusion is important role in an initial stage of cavitation bubble growth. In the present study, influences of the rectified diffusion on cavitation-enhanced high-intensity focused ultrasound (HIFU) was investigated numerically. Firstly, the mass transfer rate of gas from the surrounding medium to the bubble was examined as function of the initial bubble radius and the driving pressure amplitude. As the result, the pressure required to bubble growth was decreases with increasing the initial bubble radius. Next, the cavitation-enhanced HIFU, which generates cavitation bubbles by high-intensity burst and induces the localized heating owing to cavitation bubble oscillation by low-intensity continuous waves, was reproduced by the present simulation. The heating region obtained by the simulation is agree to the treatment region of an in vitro experiment. Additionally, the simulation result shows that the localized heating is enhanced by the increase of the equilibrium bubble size due to the rectified diffusion. [Preview Abstract] |
Tuesday, November 21, 2017 9:57AM - 10:10AM |
M7.00010: Manipulation of Microbubble Clusters Using Focused Ultrasound Hironobu Matsuzaki, Taichi Osaki, Kei Kawaguchi, Johan Unga, Mitsuhisa Ichiyanagi, Takashi Azuma, Ryo Suzuki, Kazuo Maruyama, Shu Takagi In recent years, microbubbles (MBs) are expected to be utilized for the ultrasound drug delivery system (DDS). For the MB-DDS, it is important to establish a method of controlling bubbles and bubble clusters using ultrasound field. The objective of this study is to clarify behaviors of bubble clusters with various physical conditions. MBs in the ultrasound field are subjected to the primary Bjerknes force. The force traps MBs at the focal region of the focused ultrasound field. The trapped MBs form a bubble cluster at the region. A bubble cluster continues growing with absorbing surrounding bubbles until it reaches a maximum size beyond which it disappears from the focal region. In the present study, two kinds of MBs are used for the experiment. One is Sonazoid with average diameter of 2.6 um and resonant frequency of 5 MHz. The other is developed by Teikyo Univ., with average diameter of 1.5 um and presumed resonant frequency of 4 MHz. The bubble cluster's behaviors are analyzed using the high-speed camera. Sonazoid clusters have larger critical size than the other in every frequency, and its cluster size is inversely proportional to the ultrasound frequency, while Teikyo-bubble clusters have different tendency. These results are discussed in the presentation. [Preview Abstract] |
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