Session A12: Bubbles: General I

Galloping Bubbles: spontaneous self-propulsion of vibrating bubbles along solid boundaries

Bubbles moving in a liquid are simple in appearance yet display a wealth of intriguing dynamics relevant to innumerable practical applications. Here, we demonstrate how confinement may prompt a symmetry-breaking of the harmonic shape oscillations of vibrating bubbles, leading to spontaneous self-propulsion along solid boundaries. We study the bubbles' spontaneous motion in terms of the key parameters of the system including bubble volume, driving frequency, and driving amplitude. Our results reveal that the dynamics of the self-propelling bubbles are intimately tied to their resonant responses to periodic forcing, which can be fine-tuned to produce a myriad of dynamics including rectilinear, circular, and zig-zag motions. Furthermore, we illustrate the ability of these bubbles to navigate complex geometries, as well as their emergent collective dynamics, including orbital states and self-assemblies.   

Experimental study of ultrasound and laser triggered droplet vaporization near a wall

Vaporization of volatile perfluorocarbon (PFC) droplets by ultrasound or laser has great potential for medical applications. However, few studies are reported for the acoustic/optical droplet vaporization (ADV/ODV) near solid boundaries of tissues. In this work, the ADV and ODV are experimentally investigated using an inverted microscope and a high-speed camera. The ADV experiment is performed using a focused ultrasound transducer with a center frequency of 3 MHz, and the ODV experiment is performed using an ND:Yag pulsed laser with a wavelength of 532 nm and gold nanoparticles for laser absorption. Based on the experimental results, the phase-changing droplets are more dynamic under the ADV condition, which affects the droplets and the surrounding liquid, than under the ODV condition, which only contributes to bubble nucleation. The effects of ambient temperature and solid properties on ADV and ODV are investigated.   

Direct Numerical Simulations of Vibrating Bubbles Self-Propelling Along Solid Boundaries

Recent experiments have revealed that vertically vibrated bubbles may spontaneously self-propel along horizontal walls. We use 3D direct numerical simulations to demonstrate that asymmetric vibration modes drive the propulsion in agreement with experiments, and to extend the study beyond the parameter regime accessible in the laboratory. We characterize the bubble motion in terms of the system parameters, including vibrational frequency and forcing, bubble size, viscosity, and gravity. We explore the influence of nonlinearities in the resonant frequencies predicted by the classic theory of Rayleigh for inviscid bubbles. Notably, we demonstrate that the same propulsion mechanism may be harnessed to propel sessile bubbles attached to solid boundaries. This generalization makes the problem amenable to theoretical analysis.   

Symmetry breaking and the emergence of the knife-edge shape of a bubble rising in viscoelastic solutions

The peculiar dynamics of rising bubbles in viscoelastic solutions has stirred a lot of interest in the scientific community. The three phenomena that have been identified over the ongoing 60 years of research are: (a) the velocity jump discontinuity [1,2], (b) the negative wake structure [2], and (c) the knife-edge bubble shape [3]. The first two have been thoroughly investigated. However, the notorious knife edge shape has not been reproduced numerically. Thus, we undertake a novel, fully 3D computational study to examine the buoyancy-driven rise of a bubble in a viscoelastic solution because we are interested in finding solutions that do not exhibit azimuthal symmetry. The governing equations are solved numerically using PEGAFEM-V [5]. For the first time, we capture numerically the so-called knife-edge shape obtaining excellent agreement with experiments. By varying the rheological properties of the material, we propose the physical mechanism that leads to the breakdown of axial symmetry and the appearance of the asymmetric bubble shape.  

[1] Astarita and Apuzzo, A.I.Ch.E. 11 (1965)

[2] Pilz and Brenn, J. Non-Newtonian Fluid Mech. 145 (2007)

[3] Liu et al., JFM 304 (1995)

[4] Fraggedakis et al. J. Fluid Mech. 789 (2016)

[5] Varchanis et al. J. Non-Newtonian Fluid Mech., 284 (2020)   

Stability Beyond Surfactants: The Remarkable Life of Thermal Marangoni Bubbles

Free-standing films are inherently fragile, unless stabilized with surfactants. Similarly, air bubbles on the surface of a pure liquid are ephemeral, which leads to the intriguing question: Can air bubbles be stabilized in the absence of surfactants? In pursuit of an answer, we discovered a compelling scenario – a pure, hot oil pool where bubbles at the surface exhibited surprisingly extended lifespans, lasting for tens of minutes or more. We attribute this remarkable longevity to the temperature gradients generated in this experiment. Indeed, oil was observed to be constantly drawn to the apex of the bubble, opposing its gravitational drainage. To investigate further, we developed a subtle measurement technique to quantify the ascending Marangoni flows. Our findings shed light on the mechanism behind the remarkable stability and persistence of thermal Marangoni bubbles, that is, how 'Marangoni' does not let bubbles die 'Young'...   

A sea state dependent gas transfer velocity for CO2 unifying theory, model and field data

Wave breaking induced bubbles contribute a significant part of air-sea gas fluxes. Recent modeling of the sea state dependent CO2 flux found that bubbles contribute up to ~40% of the total CO₂ air-sea fluxes (Reichl & Deike, 2020). In this study, we implement the sea state dependent bubble gas transfer formulation of Deike and Melville (2018) into a spectral wave model (WAVEWATCH III) incorporating the spectral modeling of the wave breaking distribution from Romero (2019). We evaluate the accuracy of the sea state dependent gas transfer parameterization against available measurements of CO₂ gas transfer velocity from 9 data sets (11 research cruises, Yang et al. (2022)). The sea state dependent parameterization for CO₂ gas transfer velocity is consistent with observations, while the traditional wind-only parameterization used in most global models slightly underestimates the observations of gas transfer velocity.  We produce a climatology of the sea state dependent gas transfer velocity using reanalysis wind and wave data spanning 1980-2017.  The climatology shows that the enhanced gas transfer velocity occurs frequently in regions with developed sea states (with strong wave breaking and high significant wave height). The present study provides a general sea state dependent parameterization for gas transfer, which can be implemented in global coupled models.   

Numerical study on bubbly flow in alkaline water electrolysis

Green hydrogen is considered a promising energy carrier for storing renewable energy such as solar and wind energy. Alkaline water electrolysis (AWE), one of the methods for producing green hydrogen, has been widely studied due to its characteristics of simple structure and low-cost electrode materials. In AWE, hydrogen bubbles generated at cathode increase electrical resistance, reducing the efficiency of water electrolysis and causing irregular hydrogen production. Despite several studies of hydrogen bubble formation in AWE, a comprehensive predictive model for electrolysis efficiency of AWE involving the interaction between potential field and bubble flow is lacking in the literature. In this work, we develop a numerical model to investigate the hydrogen bubble flow in alkaline water electrolysis using OpenFOAM. The two-fluid model for the continuous KOH electrolyte phase and the disperse hydrogen bubble phase is combined with a charge conservation equation for the electric potential. The numerical model is validated with the experimental results from the literature. We analyze the interaction between current and bubble distribution and investigate its effect on water electrolysis efficiency.