Session D4: Bubbles: Acoustics and Cavitation

Experimental study on the manipulation of microbubbles using ultrasound field

Non-contact manipulation techniques of microbubbles are developed by controlling the ultrasound filed. A plane standing wave-type, a ring-type and the focused-type ultrasound are used to manipulate microbubbles. It is shown that Primary Bjarknes force is well-utilized to control the position of microbubble. Microbubble clusters are observed in the actual experiments and they show the complicated behaviors as bubble clusters. These behaviors are discussed through the comparison of the experimental observation and theoretical estimation. It is experimentally shown that the size of bubble clusters gradually increases during the irradiation period of ultrasound. These clusters are captured in the central region of focused ultrasound. These clusters, however, suddenly disappear beyond the certain critical size. This type of phenomena will be discussed in the presentation.   

Hydrodynamic Forces on Microbubbles under Ultrasound Excitation

Ultrasound (US) pressure waves exert a force on microbubbles that can be used to steer them in a flow. To control the motion of microbubbles under ultrasonic excitation, the coupling between the volume oscillations induced by the ultrasound pressure and the hydrodynamic forces needs to be well understood. We present experimental results for the motion of small, coated microbubbles, with similar sizes and physico-chemical properties as clinically-available ultrasound contrast agents (UCAs). The size distribution for the bubbles, resulting from the in-house manufacturing process, was characterized by analysis of high magnification microscopic images and determined to be bimodal. More than 99{\%} of the volume is contained in microbubbles less than 10 microns in diameter, the size of a red blood cell. The motion of the microbubbles in a pulsatile flow, at different Reynolds and Womersley numbers, is studied from tracking of high-speed shadowgraphy. The influence of ultrasound forcing, at or near the resonant frequency of the bubbles, on the hydrodynamic forces due to the pulsatile flow is determined from the experimental measurements of the trajectories. Previous evidence of a sign reversal in Saffman lift is the focus of particular attention, as this is frequently the only hydrodynamic force acting in the direction perpendicular to the flow pathlines. Application of the understanding of this physical phenomenon to targeted drug delivery is analyzed in terms of the transport of the microbubbles.   

Algal cell disruption using microbubbles to localize ultrasonic energy for biofuel extraction

Cell disruption is a critical step in the production of algal-based biofuels, but current mechanical disruption methods require significant energy, typically more than actually available in the cell's oil. We propose and investigate an ultrasound disruption process using ultrasound contrast agents to localize the delivered energy. Experiments in a flow cell with focused ultrasound show a significant benefit. The degree of disruption increases with increasing peak rarefactional ultrasound pressure for pressures between 1.90 and 3.07 MPa and increasing microbubble concentration up to $12.5 \times 10^7$ bubbles/ml. Estimates suggest the energy of this method is less than one fourth of the energy of other industrial mechanical disruption techniques and comparable with theoretical disruption estimates. The increase in efficiency would make this technique viable for bioenergy applications.   

Theoretical Study on Propagation of Pressure Wave in a Rectangular Duct Containing Spherical Bubbles

Pressure waves propagating in bubbly liquids are affected by the motions of bubbles. Several mathematical models for bubbly liquids have been proposed in order to investigate the acoustic characteristics of bubble cloud. The models are classified into two types: one is the continuum model [e.g., van Wijngaarden, J. Fluid Mech. 33, 465-474 (1968)] and the other is the discrete model [e.g., Fujikawa and Takahira, Acustica 61, 188-199 (1986)]. The continuum model composed of the averaged equations for bubbly liquids treats the macroscopic behavior of bubble cloud. In contrary, the discrete model describes the motion of bubbles individually taking account of bubble/bubble interactions. The aim of our study is to investigate the validity of the continuum model by treating each bubble with the interaction. The present work theoretically investigates the propagation of pressure waves in a rectangular duct containing spherical bubbles with the discrete model [Takahira et al., JSME Int. J. Ser. B 38, 432-439 (1995)]. The results show that the propagation velocities of pressure waves obtained from the present study agree well with those obtained from the continuum models.   

Mass transfer effects in linear wave propagation through bubbly liquids

In this work we present a model to capture the influence of mass transfer effects on the effective acoustic properties of bubbly liquids. The solution of the conservation equations inside the bubble (e.g. continuity, momentum, energy and species) is coupled to the solution of the conservation equations in the liquid surrounding the bubble using local balances across the interface and a linearized version of the mass transfer flux obtained from the Hertz-Knudsen-Langmuir formula. The model is able to capture the transition from gas bubbles containing a non-soluble gas to gas/vapor bubbles with a given vapor/gas ratio. In addition to the influence of the enthalpy of vaporization, the velocity jump appearing at the interface is shown to have a significant influence in both, the effective phase velocity and the attenuation of the medium near the saturation line. The validity of common assumptions typically used in simplified models and limiting solutions obtained from the current approach are discussed in terms of characteristic non-dimensional numbers. Consistent with previously published data, the influence of mass transfer effect is specially notorious at low frequencies.   

Velocimetry in both phases of a cavitating flow by fast X-ray imaging

A promising method to measure velocity fields in a cavitating flow is presented. Dynamics of the liquid phase and of the bubbles are both investigated. The measurements are based on ultra fast X-ray imaging performed at the APS (Advanced Photon Source) of the Argonne National Laboratory. The experimental device consists of a millimetric Venturi test section associated with a transportable hydraulic loop. Various configurations of velocity, pressure, and temperature have been investigated. Radio-opaque particles are used as tracers for the liquid phase, in association with a multi-pixels sensor to record the successive positions of the particles. The use of X-rays instead of light solves the problem of light reflection and dispersion on phase boundaries, since X-rays penetrate a gas/liquid flow in straight lines. Images contain simultaneously the information related to the particles (for PIV analysis in the liquid), to the vapor bubbles (for PIV in the gas). The slip velocity between vapor and liquid is calculated everywhere both velocities can be obtained.   

Cavitation you can hold in your hand... for a moment

In a popular party trick a glass bottle is filled with water and firmly struck at the top, breaking the bottle with nothing but bare hands. We present evidence that this trick is caused by cavitation formed by the acceleration of the fluid. Traditional velocity based methods for determining cavity formation do not successfully predict cavitation onset, however, a dimensionless cavitation equation derived from the Navier-Stokes equation predicts cavitation as a function of pressure head and acceleration. Our experiments utilized accelerometers and high-speed photography to observe cavitation with good agreement between experiments and predictions. Elucidating the onset of cavitation based on these simple parameters will help those who attempt this trick appreciate the physical complexity of this phenomenon and improve their bottle breaking skills.   

Cavitation stuctures formed during the collision of a sphere with an ultra-viscous wetted surface

We investigate the inception of cavitation and associated structures when a sphere collides with a solid surface covered with a layer of ultra-viscous non-Newtonian liquid with kinematic viscosities, $v$, of up to 20 million cSt (at nominal low shear). Using a synchronized dual-view high-speed imaging system, we confirm that there is no shear-induced cavitation even in such highly favorable conditions. We show that liquids with high visco-elastic properties can enable the sphere to rebound without any prior contact with the solid wall. A decrease in sphere impact velocity for such non-contact rebound cases results in a systematic delay in cavity inception by depressurization from the time of achieving the minimum gap distance. We find vastly different bubble entrapment characteristics on the sphere surface during entry into the liquid layer for low and high-viscosity liquids. These were found to play an important role in the formation of cavitation structures in non-contact cases. In contrast, when contact occurs, we observe a cylindrical structure attached to the wall having undulations along the cavity interface which were further investigated using high-speed particle image velocimetry (PIV) techniques.   

Discussion on the Applicability of Rayleigh-Plesset Equation for a Nano-scale bubble using Molecular Dynamics Simulation

Multi-phase flows such as cavitation and boiling have much variety on the scale in time and space compared with single phase flows. It is necessary to recognize the multi-scale structure accurately to construct a sophisticated numerical method for the prediction of various multi-phase flow phenomena. In this point of view, clarification of the valid range of continuum mechanics would be very important. Here, an interesting problem in the case of cavitation is, to what extent Rayleigh-Plesset (R-P) equation, which describes the radius change of a spherical bubble under a pressure given at far from the bubble, can express the behavior of a tiny bubble quantitatively. In this work, we discussed the validity of the application of R-P equation to a nano-scale bubble using Molecular Dynamics (MD) simulation. In the simulation, liquid argon at a decompressed state in a cubic domain was simulated. As a result, a nano-scale bubble was generated after a waiting time, and it rapidly grew to several nanometers, and it reached to an equilibrium state showing a transient behavior. We compared the bubble radius change observed in the MD simulation with the numerical result of R-P equation, and confirmed that R-P equation can well predict the behavior of such tiny bubble.   

Supersonic microjets induced by hemispherical cavitation bubbles

In recent years methods to produce fast microjets have received significant attention due to their potential to be employed in needle-free injection devices that can provide mass inoculation. In this talk we present a novel technique capable of producing jets that can reach up to 400 m/s. The jets are produced by a device that consists only of two electrodes on a plastic substrate and a tapered hole of 13 $\sim$ 20 $\mu$m between them. A short pulse of electric current is applied to the electrodes, then a spark bridges between the electrodes creating a cavitation bubble. Liquid is accelerated through the hole during the expansion and collapse of the bubble producing two separate jets. We found that as the exit velocity of the jet increases the jets become unstable. The second jet exiting the hole, usually faster than the first jet exits as a spray. The effect of viscosity was also studied with silicone oils up to 100 cSt. Finally, we also demonstrate that the jets can penetrate into soft material, thus they have the potential to be used in a needle-free drug-delivery application.