Experimental study on single bubble dynamics in liquids with varying viscosities: detachment behavior and predictive model modification

Single bubble dynamics under microgravity conditions in the presence of dissolved gas in the liquid

Airlift pumps characteristics for shear-thinning non-Newtonian fluids: An experimental investigation on liquid viscosity impact

Experimental investigation of liquid viscosity's effect on the flow behaviour and void fraction in a small diameter bubble column: How much do we know?

While ultrasound and cavitation based therapies have mushroomed over the years, there is a lack of online monitoring of the cavitation bubble dynamics in biological tissue. Here, we demonstrate that with ultrasonic plane wave imaging, the fast dynamics of single cavitation bubbles can be resolved non-invasively in a tissue mimicking material. Due to the high contrast of bubbles, plane wave compounding is not necessary and the frame rates of up to 74 kHz can been achieved with a research grade ultrasound scanner. Comparison with simultaneous high-speed imaging demonstrates excellent agreement of both measurement modalities in water and in a tissue mimicking material.

The microscopic understanding of the dramatic increase in viscosity of liquids when cooled toward the glass transition is a major unresolved issue in condensed matter physics. Here, we use machine learning methods to accelerate molecular dynamics simulations with first-principles accuracy for the glass-former toluene. We show that the increase in viscosity is intimately linked to the increasing number of dynamically correlated molecules N^{*}. While certain hallmark features of glassy dynamics, like physical aging, are linked to N^{*} as well, others, like relaxation stretching, are not.

Dynamics and morphology of single ellipsoidal bubbles in liquids

Many biomedical applications such as ultrasonic targeted drug delivery, gene therapy, and molecular imaging entail the problems of manipulating microbubbles by means of a high-intensity focused ultrasound (HIFU) pressure field; namely stable cavitation. In high-intensity acoustic field, bubbles demonstrate translational instability, the well-known erratic dancing motion, which is caused by shape oscillations of the bubbles that are excited by their volume oscillations. The literature of bubble dynamics in the HIFU field is mainly centered on experiments, lacking a systematic study to determine the threshold for shape oscillations and translational motion. In this work, we extend the existing multiphysics mathematical modeling platform on bubble dynamics for taking account of (1) the liquid compressibility which allows us to apply a high-intensity acoustic field; (2) the mutual interactions of volume pulsation, shape modes, and translational motion; as well as (3) the effects of nonlinearity, diffraction, and absorption of HIFU to incorporate the acoustic nonlinearity due to wave kinematics or medium—all in one model. The effects of acoustic nonlinearity on the radial pulsations, axisymmetric modes of shape oscillations, and translational motion of a bubble, subjected to resonance and off-resonance excitation and various acoustic pressure, are examined. The results reveal the importance of considering all the involved harmonics and wave distortion in the bubble dynamics, to accurately predict the oscillations, translational trajectories, and the threshold for inertial (unstable) cavitation. This result is of interest for understanding the bubble dynamical behaviors observed experimentally in the HIFU field.

Effects of surface tension on the dynamics of a single micro bubble near a rigid wall in an ultrasonic field

Three-dimensional (3D) transient mathematical models are developed to explore the single and multiscale bubble dynamics beneath the inclined downward-facing surface of the anode in an aluminum reduction cell. The single bubble motion is traced by the volume of fluid (VOF) method. For the multiscale bubble, the discrete phase model (DPM) combined with the VOF approach is used to track the dispersed microbubbles’ trajectories and trace the continuous macrobubbles’ surfaces, respectively. A discrete-continuum transition (DCT) model is proposed to convert the dispersed bubbles to continuous ones and deal with the multiscale motions under the inclined anode surface. The predicted results of the sliding bubble velocity and the gas coverage are in a good agreement with the experimental data from the literature. On increasing the inclination angle, the detachment time of the single bubble from the nucleation site decreases while the sliding velocity increases. The gas coverage, bubble size, and thickness of multiscale bubbles decrease on increasing the inclination angle.

This study introduces total internal reflection fluorescence (TIRF) as a method for investigating the dynamics of a single laser-induced cavitation bubble in water within a thin layer near a solid surface. TIRF, supported by a planar waveguide, was employed to image cavitation bubble collapses near the liquid–solid interface at 50 thousand frames per second (kfps). Simultaneously, traditional side-view background illumination at 150 kfps captured the bubble dynamics. Dilution series and thickness estimation were conducted to determine interfering fluorescence signal contributions. The results confirm that most fluorescence is excited by the evanescent field rather than surface scattering or bulk scattering. The lifecycle of cavitation bubbles with moderate non-dimensional stand-off distances, γ = 1.15, 1.35, and 1.6, are discussed. We find that a thin liquid film persists between the bubble and the solid surface throughout the entire collapse process, except for randomly distributed micron-sized bubbles attached to the surface between the first and second collapse. Shock waves were captured with TIRF during the second collapse, centered around regions of the toroidal cavity exhibiting locally stronger dynamics. These regions, termed gas-filled strong collapse areas (SCAs), were also where microcracks formed during the second collapse of a single cavitation bubble. While not all SCAs resulted in microcracks, every observed microcrack could be traced back to a preceding SCA. To our knowledge, this is the first application of TIRF to observe cavitation bubble collapses with sub-millisecond time resolution.

Influence of liquid density variation on the bubble and gas dynamics of a single acoustic cavitation bubble

When a bubble is excited by an acoustic field, in addition to the volume oscillation, the bubble may deviate from the spherical shape and undergo shape oscillations. The shape oscillations can be directly caused by the anisotropic pressure or they can be caused by the energy transfer from the volume mode through nonlinear modal couplings. The shape oscillations influence the bubble dynamics in many ways. First of all, shape oscillations can limit the volume responses of the bubble since the onset of shape oscillations can lead to the development of localized large curvature and hence the breakup of the bubble. Second, the resonance coupling between the volume mode and a shape mode can give rise to the occurrence of chaos even when the acoustic forcings are so weak that the bubble oscillates close to its equilibrium radius. Third, the coupling between two or more shape oscillation modes can lead to translational motions of the bubble centroid; this can indirectly affect the bubble coalescence in an acoustic field. Finally, the shape oscillations may influence the coalescence through modifying the short-range attractive forces between two neighboring bubbles.

An efficient thermal management system is desirable for improving the performance of key components of electric vehicle (EV), such as battery packs and insulated-gate bipolar transistors (IGBTs). This paper investigates the application of single bubble nucleate boiling heat transfer in battery and IGBT component cooling pack. A semi-mechanistic flow boiling model, which combines four main submodels i.e., phase change model, micro-region model, Marangoni model, and contact angle model, is developed to get the insight of various subprocesses like bubble inception, growth, departure, scavenging effect while the bubble departs, and condensation. For model validation, simulations are carried out for single bubble flow boiling in a vertical rectangular channel and compared against the experimental data available in the literature. Thereafter, simulations are carried out for the battery and IGBT cooling pack to understand the physical phenomena associated with nucleate boiling in such systems. The choice of a single vapor bubble vis-à-vis multiple bubbles has been based on the objective of validating the developed numerical model. An enhancement of ∼30% in heat transfer is achieved for both battery and IGBT components when the system is subjected to a nucleate boiling cooling regime as compared to a conventional single-phase convection cooling system. Nusselt number variation due to the single bubble movement along the coolant path is studied in detail for both serpentine-shaped cooling path in a battery and straight flow path in an IGBT. Moreover, the influence of Reynolds number over bubble dynamics is analyzed.

The possibility of increasing the calculation efficiency by the joint use of two models of the dynamics of a single weakly-nonspherical vapor bubble under its strong collapse in liquid is studied. In both models the motion of liquid and vapor is split into a spherical component and its small nonspherical perturbation. The models differ in the description of the spherical component. In the first (simplified) model, it is described by a system of ODE together with partial differential equations in temperature, derived under the assumption of weak compressibility of liquid and bubble homobaricity. In the second model, one-dimensional gas dynamics equations are applied. The advantage of the simplified model consists in determining a numerical solution with much-less computer time costs in comparison with what is required for the numerical integration of gas dynamics equations. The assumptions used in the simplified model in the final stage of collapse become incorrect, and, as a result, the numerical solution errors increase. Therefore, the simplified model is applied at the beginning of bubble collapse, whereas the gas dynamics equations are used at its end. Within this approach, the numerical solution in the final stage of collapse is dependent on the moment of transition to the gas dynamics equations. It is shown that satisfactory description of evolution of bubble sphericity distortion is achieved when the transition is made under the condition that the Mach number M of vapor in the vicinity of the bubble surface is less than 0.4. Satisfactory resolution of the shock wave in the bubble is attained when the transition is performed at M<0.2.

Single bubble dynamics in the vicinity of a solid boundary submerged in water were studied experimentally. Single bubble inside a water tank was generated by a spark discharge of capacitor into a couple of copper wires closing a simple circuit. A circular polycarbonate plate was placed horizontally above the bubble creation site. Polycarbonate plates with two different thicknesses were tested by changing the distance between the plate and the creation site. The effects of distance to the wall and wall thickness on the bubble motion is observed by considering the fluid-structure interaction. It is shown that motion of the two boundaries during the bubble generation differ from each other. Jetting behavior of two cases is also different.