Etude d'une assemblée de bulles microfluidiques excitées par une onde ultrasonore : transmission acoustique et phénomène de streaming

Abstract : Because of the important compressibility of gas bubbles in water, inducing a very low resonance frequency, one can find interest in studying bubbles from an acoustic and a fluid mechanics point of view. Using microfluidics techniques in order to produce assemblies of acoustically driven bi-dimensional bubbles, we are studying their influence on both acoustic waves and the surrounding fluid.Bubbles being sub-wavelength resonators, we show that a micro-bubbles assembly interacts with acoustical waves which wavelengths that are substantially bigger than the bubbles size. Developing a way to extract bubbles contribution to the acoustic signal, we show that their resonance frequency follows a law slightly different from the one Minnaert had found for spherical bubbles. The impact of this medium on the acoustical wave has been studied and we show that a decrease in the acoustical transmission happens in a range of frequencies above the resonance. This decrease can be adjusted in amplitude and in frequency making our system an easily tunable metameterial.Because of the strong response of bubbles induced by acoustical waves, the bubbles surface oscillates with a great amplitude in the surrounding fluid. This oscillation, working together with a coupling present between the bubbles, can drive a strong steady streaming in the fluid. Systems of several bubbles are studied, and a theory is proposed in order to predict the flow they induce. The interaction between the streaming phenomenon and an external flow is also presented, showing that exclusion zones can be present under certain circumstances in these systems. These exclusion zones can be useful in micro-fluidics in order to trap particles or chemicals.
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Thomas Combriat. Etude d'une assemblée de bulles microfluidiques excitées par une onde ultrasonore : transmission acoustique et phénomène de streaming. Dynamique des Fluides [physics.flu-dyn]. Université Grenoble Alpes, 2018. Français. ⟨NNT : 2018GREAY052⟩. ⟨tel-02057669⟩

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