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Apeldoorn, Netherlands

Zeravcic Z.,Leiden University | Lohse D.,Physics of Fluids Group | Van Saarloos W.,Leiden University
Journal of Fluid Mechanics | Year: 2011

In this paper the collective oscillations of a bubble cloud in an acoustic field are theoretically analysed with concepts and techniques of condensed matter physics. More specifically, we will calculate the eigenmodes and their excitabilities, eigenfrequencies, densities of states, responses, absorption and participation ratios to better understand the collective dynamics of coupled bubbles and address the question of possible localization of acoustic energy in the bubble cloud. The radial oscillations of the individual bubbles in the acoustic field are described by coupled linearized Rayleighâ€" Plesset equations. We explore the effects of viscous damping, distance between bubbles, polydispersity, geometric disorder, size of the bubbles and size of the cloud. For large enough clusters, the collective response is often very different from that of a typical mode, as the frequency response of each mode is sufficiently wide that many modes are excited when the cloud is driven by ultrasound. The reason is the strong effect of viscosity on the collective mode response, which is surprising, as viscous damping effects are small for single-bubble oscillations in water. Localization of acoustic energy is only found in the case of substantial bubble size polydispersity or geometric disorder. The lack of localization for a weak disorder is traced back to the long-range 1/r interaction potential between the individual bubbles. The results of the present paper are connected to recent experimental observations of collective bubble oscillations in a two-dimensional bubble cloud, where pronounced edge states and a pronounced low-frequency response had been observed, both consistent with the present theoretical findings. Finally, an outlook to future possible experiments is given. © 2011 Cambridge University Press. Source

Agency: Narcis | Branch: Program | Program: Completed | Phase: Technology | Award Amount: | Year: 1997


Agency: Narcis | Branch: Project | Program: Completed | Phase: Technology | Award Amount: | Year: 2007

The study of microbubble dynamics in ultrasound under microscopic flow conditions has potential applications to diagnostic imaging. Microbubbles are routinely injected in the circulation to enhance the contrast in ultrasound medical imaging. This project it aimed at studying the dynamics of microbubbles excited by an ultrasound field inside an artificial microscopic channel. This model system allows for a detailed study of the balance between viscous shear flow and acoustic radiation pressure. Furthermore, the walls can be functionalized to study molecular targeting. A precise quantification of these forces has been hampered by the difficulty in controlling the position of bubbles due to buoyancy, and in repeating experiments on the same bubble. Here we propose a method based on optical micromanipulation using optical tweezers to precisely control the position of microbubbles. We also propose to extend the well-established optical tweezers-based force measurement technique to the case of microbubbles, which is non-trivial, due to the complex trapping configuration. With the proposed setup it will be possible to quantify acoustical and fluid dynamics forces on individual microbubbles with controlled and repeatable boundary conditions. This study may directly optimize targeted molecular imaging strategies.

Prakash V.N.,Physics of Fluids Group | Tagawa Y.,Physics of Fluids Group | Calzavarini E.,Lille Laboratory of Mechanics | Mercado J.M.,Physics of Fluids Group | And 4 more authors.
New Journal of Physics | Year: 2012

We report the results of the first systematic Lagrangian experimental investigation in a previously unexplored regime of very light (air bubbles in water) and large (D/η ≫ 1) particles in turbulence. Using a traversing camera setup and particle tracking, we study the Lagrangian acceleration statistics of ∼3mm diameter (D) bubbles in a water tunnel with nearly homogeneous and isotropic turbulence generated by an active grid. The Reynolds number (Re λ) is varied from 145 to 230, resulting in size ratios, D/η, in the range of 7.3-12.5, where η is the Kolmogorov length scale. The experiments reveal that gravity increases the acceleration variance and reduces the intermittency of the probability density function (PDF) in the vertical direction. Once the gravity offset has been subtracted, the variances of both the horizontal and vertical acceleration components are about 5 ± 2 times larger than those measured in the same flow for fluid tracers. Moreover, for these light particles, the experimental acceleration PDF shows a substantial reduction of intermittency at growing size ratios, in contrast with neutrally buoyant or heavy particles. All these results closely match numerical simulations of finite-sized bubbles with the Faxén corrections. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. Source

Stevens R.J.A.M.,Johns Hopkins University | Stevens R.J.A.M.,Physics of Fluids Group | Stevens R.J.A.M.,University of Twente | Van Der Poel E.P.,Physics of Fluids Group | And 4 more authors.
Journal of Fluid Mechanics | Year: 2013

The unifying theory of scaling in thermal convection (Grossmann & Lohse, J. Fluid. Mech., vol. 407, 2000, pp. 27-56; henceforth the GL theory) suggests that there are no pure power laws for the Nusselt and Reynolds numbers as function of the Rayleigh and Prandtl numbers in the experimentally accessible parameter regime. In Grossmann & Lohse (Phys. Rev. Lett., vol. 86, 2001, pp. 3316-3319) the dimensionless parameters of the theory were fitted to 155 experimental data points by Ahlers & Xu (Phys. Rev. Lett., vol. 86, 2001, pp. 3320-3323) in the regime 3 × 107 ≤ Ra ≤ 3 × 109 and 4≤ Pr ≤ 34 and Grossmann & Lohse (Phys. Rev. E, vol. 66, 2002, p. 016305) used the experimental data point from Qiu & Tong (Phys. Rev. E, vol. 64, 2001, p. 036304) and the fact that Nu(Ra, Pr) is independent of the parameter a, which relates the dimensionless kinetic boundary thickness with the square root of the wind Reynolds number, to fix the Reynolds number dependence. Meanwhile the theory is, on the one hand, well-confirmed through various new experiments and numerical simulations; on the other hand, these new data points provide the basis for an updated fit in a much larger parameter space. Here we pick four well-established (and sufficiently distant) Nu(Ra, Pr) data points and show that the resulting Nu(Ra, Pr) function is in agreement with almost all established experimental and numerical data up to the ultimate regime of thermal convection, whose onset also follows from the theory. One extra Re(Ra, Pr) data point is used to fix Re(Ra, Pr). As Re can depend on the definition and the aspect ratio, the transformation properties of the GL equations are discussed in order to show how the GL coefficients can easily be adapted to new Reynolds number data while keeping Nu(Ra, Pr) unchanged. © 2013 Cambridge University Press. Source

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