Published: May 9th, 2021
A fast and reliable technique is proposed to control the shape oscillations of a single, trapped acoustic bubble that is based on coalescence technique between two bubbles. The steady-state, symmetry-controlled bubble shape oscillations allow analysis of the fluid flow generated in the vicinity of the bubble interface.
When located near biological barriers, oscillating microbubbles may increase cell membrane permeability, allowing for drug and gene internalization. Experimental observations suggest that the temporary permeabilization of these barriers may be due to shear stress that is exerted on cell tissues by cavitation microstreaming. Cavitation microstreaming is the generation of vortex flows which arise around oscillating ultrasound microbubbles. To produce such liquid flows, bubble oscillations must deviate from purely spherical oscillations and include either a translational instability or shape modes. Experimental studies of bubble-induced flows and shear stress on nearby surfaces are often restricted in their scope due to the difficulty of capturing shape deformations of microbubbles in a stable and controllable manner. We describe the design of an acoustic levitation chamber for the study of symmetry-controlled nonspherical oscillations. Such control is performed by using a coalescence technique between two approaching bubbles in a sufficiently intense ultrasound field. The control of nonspherical oscillations opens the way to a controlled cavitation microstreaming of a free surface-oscillating microbubble. High-frame rate cameras allow investigating quasi-simultaneously the nonspherical bubble dynamics at the acoustic timescale and the liquid flow at a lower timescale. It is shown that a large variety of fluid patterns may be obtained and that they are correlated to the modal content of the bubble interface. We demonstrate that even the high-order shape modes can create large-distance fluid patterns if the interface dynamics contain several modes, highlighting the potential of nonspherical oscillations for targeted and localized drug delivery.
In medicine, an administered drug must penetrate many obstacles in the living system before reaching the desired targets. However, most drugs are rapidly cleaned away from the blood stream. The targeting efficiency is low and they cannot easily cross cell membranes, leading to ineffective drug delivery. Currently, the use of microbubbles in combination with ultrasound has been proposed as an innovative method for noninvasive, precise and targeted delivery of drugs and genes to pathological tissues and cells1. In this approach, microbubbles can play a role as carriers where free drugs are either co-injected with a gas bubble suspension or loaded....
1. Design of the acoustic levitation chamber
A complete sequence of bubble coalescence leading to time-stable, symmetry-controlled nonspherical oscillations is presented in Figure 9. The approaching phase of two spherically-oscillating bubbles ends when the thin liquid film between the two bubbles is ruptured. It is worth noting that, at the last stage prior to the coalescence, the bubble interfaces deviate from sphericity. Both bubbles elongate on an ellipsoidal shape along the path of the rectilinear .......
The presented procedure consists of using bubble coalescence in order to trigger steady-state, symmetry-controlled bubble shape oscillations, allowing the study of the long-term fluid flow induced by these oscillations. The main challenge in the technique is the control of nonspherical oscillations for a bubble being trapped, far from any boundaries.
|Lens of focus 40 mm
|Continuous wave laser source
|DPSS laser of wavelength 532nm, energy 400 mW
|Cylindrical plano-concave lens
|lens of focus -25?4mm
|Cylindrical plano-concave lens
|lens of focus 250 mm
|Red polymer fluorescent microspheres
|Generator of function feeding the ultrasound transducer
|High-speed recording up to 1 Mfps
|Water for analysis
|Demineralized, undegassed water
|Softwate for simulating the acoustic field of the levitation chamber
|Nd:Yag pulsed laser
|New Wave Research
|5 ns pulse duration, λ=532 nm, 3.5 mm beam diameter, up to 50 mJ
|lens of focus 125 mm
|Spherical concave lens
|Bi-concave lens of focus -25mm
|Nominal frequency 31kHz, active area 35mm diameter
|Software for image processing and analysis in Java
|XY Linear stage
|Displacement stage with micrometric screw
|Z-axis linear stage
|Vertical displacement stage with micrometric screw
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