Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D11: Bubbles II: Cavitation, Acoustics and Biomedical |
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Chair: Simo Makiharju, University of Michigan Room: 335 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D11.00001: Original flocculation technique via acoustic cavitation bubbles driven by 20.3-kHz ultrasound in water Yuki Mizushima, Takayuki Saito Strange flocculation mechanism of particles (up to 1.0mm) driven by the acoustic field (20.3-kHz) is observed in water. It is not well-known particle formation in acoustic field, like \textit{dust striation}, but spherical agglomeration. Because kHz-order ultrasound is not acceptable for the separation technique due to its weak-directionality, applicable particle sizes are limited as similar size to a wavelength of the irradiated ultrasound or smaller than that. Hence, particles which are larger than mm-order in diameter are difficult to be manipulated with MHz-band ultrasound. However, our flocculation technique overcomes the limitation. It deeply relates to the motion of cavitation bubbles around the particles. First, in this study, we captured the particle motion and acoustic-cavitation-oriented bubble motion simultaneously by using a high-speed video camera. Second, we measured the distribution of the sound pressure in the water phase and discussed the relationship between that of the sound pressure and the motion of the particle and the acoustic cavitation bubble. Finally, we investigated the effects of the gravity force, the acoustic radiation force and the spatial heterogeneity of the pressure acting on the particle. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D11.00002: On the behavior of a bubble cloud under an ultrasound field Ana Medina-Palomo, Elena Igualada-Villodre, Javier Rodriguez-Rodriguez We present our latest numerical results on the determination of the resonance frequency of a bubble cloud excited by an acoustic wave. Thermal effects are incorporated to the Keller-Miksis equation by integration of an ode which models the heat transfer between the bubble and the liquid. It is found that thermal effects make the bubble oscillations damp out faster, which difficultates the resonance detection. We study how the parameters of the population, i.e.\ the mean and variance of the size distribution, affect the spectra, and thus, the detection of the resonance frequency. Spectra of monodisperse populations exhibit a peak at the resonance frequency that is easier distinguished compared to the case of polydisperse populations. To overcome the issue that the resonance peak is usually smaller than the central peak corresponding to the insonating wave, we focus on two strategies. Firstly, the use of a chirp, i.e.\ an acoustic pulse variable in frequency. This signal has a flat spectrum in a wide frequency band and is therefore more appropiate to excitate a population with different sizes. A second strategy consists in insonating the bubbles with a shock pressure wave. In the laboratory this is achieved by placing them in an open bottle that is suddenly hit at its mouth. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D11.00003: Three-dimensional features on oscillating microbubbles streaming flows Massimiliano Rossi, Alvaro G. Marin, Cheng Wang, Sascha Hilgenfeldt, Christian J. K\"ahler Ultrasound-driven oscillating micro-bubbles have been used as active actuators in microfluidic devices to perform manifold tasks such as mixing, sorting and manipulation of microparticles. A common configuration consists in side-bubbles, created by trapping air pockets in blind channels perpendicular to the main channel direction. This configuration results in bubbles with a semi-cylindrical shape that creates a streaming flow generally considered quasi two-dimensional. However, recent experiments performed with three-dimensional velocimetry methods have shown how microparticles can present significant three-dimensional trajectories, especially in regions close to the bubble interface. Several reasons will be discussed such as boundary effects of the bottom/top wall, deformation of the bubble interface leading to more complex vibrational modes, or bubble-particle interactions. In the present investigation, precise measurements of particle trajectories close to the bubble interface will be performed by means of 3D Astigmatic Particle Tracking Velocimetry. The results will allow us to characterize quantitatively the three-dimensional features of the streaming flow and to estimate its implications in practical applications as particle trapping, sorting or mixing. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D11.00004: Shear Stress induced Stretching of Red Blood Cells by Oscillating Bubbles within a Narrow Gap Fenfang Li, Milad Mohammadzadeh, Claus-Dieter Ohl The flow pattern, especially the boundary layer caused by the expanding/contracting bubble in a narrow gap (15 $\mu$m) and the resultant stretching of red blood cells is investigated in this work. High speed recordings show that a red blood cell (biconcave shape, thickness of 1-2 $\mu$m) can be elongated to five times its original length by a laser-induced cavitation bubble within the narrow gap. However, flexible cancer cells in suspension (RKO, spherical shape, diameter of 10-15 $\mu$m) are hardly elongated under the same experimental condition. We hypothesize that the shear stress at the boundary layer is crucial for this elongation to occur. Therefore, in order to resolve the related fluid dynamics, we conducted numerical simulations using the finite element method (Fluent). The rapidly expanding/contracting vapor bubble is successfully modeled by employing viscosity and surface tension. The transient pressure inside the bubble and the velocity profile of the flow is obtained. We observe strong shear near the upper and lower boundary during the bubble oscillation. The flow fields are compared with analytical solutions to transient and pulsating flows in 2D. In the experiment the red blood cells sit within the lower boundary layer, thus are probably elongated by this strong shear flow. In contrast, the spherical cancer cells are of comparable size to the gap height so that they are lesser affected by this boundary layer flow. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D11.00005: Using ultrasound to steer ultrasound contrast agents: Implications for targeted drug delivery Alicia Clark, Alberto Aliseda Ultrasound can be used to manipulate ultrasound contrast agents (UCAs), micron-sized bubbles used in ultrasound imaging to increase image contrast. The Bjerknes force, resulting from the lagged response of the microbubbles to the oscillatory ultrasound pressure field, can be utilized to steer the microbubbles to a targeted area in the vasculature, with the microbubbles serving as drug delivery vectors and injectors. The response of microbubbles to ultrasound in a sheared flow has shown a complex coupling of ultrasound-induced volume oscillations with hydrodynamic forces: Saffman lift and the Bjerknes force. In this work, the relative influence of these two forces acting in the across-streamlines direction is determined as a function of the Reynolds and Womersley and the excitation to bubble natural frequency ratio. We use in-vitro experiments to study the behavior of microbubbles in physiologically-realistic pulsatile flows. Quantitative information about microbubble trajectories in physiological conditions is necessary to develop models in order to control ultrasound steering of bubble-based drug delivery vectors in the human vasculature. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D11.00006: Analytical and experimental analyses of the translation of microbubbles under short acoustic pulses Elena Igualada-Villodre, Ana Medina-Palomo, Javier Rodriguez-Rodriguez The translation of bubbles as a result of the primary Bjerknes force is studied both analytically and experimentally. In particular, we focus on the translational dynamics of bubbles under the effect of short acoustic pulses, i.e. pulses whose duration is of the order of the characteristic viscous time based on the bubble size. The experiments developed show that existing models widely used in the literature do not allow to properly reproduce the bubble velocity history. Given the comparison between analytical and experimental results, we can conclude that the viscous drag cannot be approximated by a constant drag coefficient when the time scale is of the order of the characteristic viscous time, as the history force becomes dominant. In other words, the history force is needed to correctly reproduce the experimental results. In this talk we will show analytical solutions of the bubble translational dynamics equation for both piece-wise constant and oscillatory forcing. This work has been supported by Spanish Ministries of Science and of Economy and Competitiveness through grants: DPI2008-06369 and DPI2011-28356-C03-02. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D11.00007: Shock-Induced Bubble Collapse in a Vessel: Implications for Vascular Injury in Shockwave Lithotripsy Vedran Coralic, Tim Colonius We numerically investigate the shock-induced collapse (SIC) of a preexisting bubble in a blood vessel and evaluate the potential of such an event to contribute to onset of vascular injury in shockwave lithotripsy (SWL). Previously, we utilized a 3D, high-order accurate, shock- and interface-capturing, multicomponent flow algorithm to carry out a large-scale parametric study of this problem [V. Coralic and T. Colonius, Eur. J. Mech. B-Fluid 40, 64-74 (2013)]. The results indicated that the influence of the blood vessel on the bubble dynamics was negligible and confirmed with experiments that the vessel would freely deform under the forcing imparted by the collapse. As a result, in this study, we perform simulations of the SIC of a preexisting bubble in a free field and couple them to a freely deforming Lagrangian mesh so to characterize the deformations in the fluid surrounding the bubble, which, as our previous results suggest, may be interpreted as the vessel and surrounding tissue. We report the fully 3D and time-dependent Green-Lagrange strains and compare them to the ultimate strains obtained in uniaxial compression/tension tests in tissue. Our findings suggest that the SIC of preexisting bubbles in blood vessels is a viable mechanism by which injury may be initiated in SWL. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D11.00008: Dynamics of bubble collapse under vessel confinement in 2D hydrodynamic experiments Galina Shpuntova, Joanna Austin One trauma mechanism in biomedical treatment techniques based on the application of cumulative pressure pulses generated either externally (as in shock-wave lithotripsy) or internally (by laser-induced plasma) is the collapse of voids. However, prediction of void-collapse driven tissue damage is a challenging problem, involving complex and dynamic thermomechanical processes in a heterogeneous material. We carry out a series of model experiments to investigate the hydrodynamic processes of voids collapsing under dynamic loading in configurations designed to model cavitation with vessel confinement. The baseline case of void collapse near a single interface is also examined. Thin sheets of tissue-surrogate polymer materials with varying acoustic impedance are used to create one or two parallel material interfaces near the void. Shadowgraph photography and two-color, single-frame particle image velocimetry quantify bubble collapse dynamics including jetting, interface dynamics and penetration, and the response of the surrounding material. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D11.00009: Characterization of Acoustic Droplet Vaporization Using MRI David Li, Steven Allen, Luis Hernandez-Garcia, Joseph Bull Acoustic droplet vaporization (ADV) is the selective vaporization of liquid droplets to form larger gas bubbles. The ADV process is currently being researched for biomedical applications such as gas embolotherapy, drug delivery, and phase-change contrast agents. In this study an albumin encapsulated dodecafluoropentane (DDFP, CAS: 678--26--2) microdroplet suspension was vaporized using a single element focused (f/2, D$=$19 mm) 3.5 MHz transducer (Panametrics A321S, Olympus, Waltham, MA). The resulting DDFP bubble clouds were imaged using both bright field microscopy and MRI (Varian 7T, Agilent Technologies Inc., Santa Clara, CA). Field distortions due to DDFP bubble generation were characterized against the bright field images as a function of acoustic power and bubble cloud size. Experimentally a direct correlation between bubble cloud dimensions generated and field distortions seen in the MRI was observed. Additionally, MR velocimetry was used to measure the flow field resulting from ADV. The field distortions due to the bubbles were further characterized by modeling Maxwell's equations using COMSOL (COMSOL Inc., Burlington, MA). The ability to characterize ADV with alternative imaging modalities may prove useful in further development of ADV based biomedical therapies. [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D11.00010: Acoustic droplet vaporization is initiated by superharmonic focusing Oleksandr Shpak, Martin Verweij, Rik Vos, Nico de Jong, Detlef Lohse, Michel Versluis Acoustically sensitive emulsion microdroplets composed of a low boiling point liquid perfluorocarbon have the potential to be a highly efficient system for local drug delivery, embolotherapy or for tumor imaging. The physical mechanisms underlying the acoustic activation of these phase-change emulsions into vapor bubbles, termed acoustic droplet vaporization, have not been well understood. The droplets have a very high activation threshold, its frequency dependence does not comply with homogeneous nucleation theory and focusing spots have been observed while the wavelength is at least an order larger than the droplet size. Here we show that acoustic droplet vaporization is initiated by a combination of two effects: highly nonlinear distortion of the acoustic wave before it hits the droplet, and focusing of the distorted wave by the droplet itself. At high excitation pressures, nonlinear distortion causes significant superharmonics with wavelengths below the diameter of the droplet. The proposed model is validated with experimental data captured with an ultra high-speed camera on the exact locations of the nucleation spots. Moreover, the presented mechanism explains the hitherto counterintuitive dependence of the nucleation threshold on the ultrasound frequency. [Preview Abstract] |
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