Session ZC13: Dynamics and Rupture II

The four stages of a shock-induced high-speed liquid microjet

Two decades ago high-speed liquid jets resulting from the interaction of micro bubbles with low amplitude therapeutic shock waves had first been promoted as a mechanism for microinjection of cells, with great potential in applications such as targeted drug delivery. Here, experimental high-speed x-ray phase contrast images of laser-induced shock waves interacting with micro bubbles, complemented by numerical simulations, brings us within reach of controlling such jets. This work focuses on the limit at which jets form, presented in four distinct stages. The first phase suggests that the mean acceleration of the proximal point on the bubble interface plays a major role in defining wether a jet will form or not. The second phase indicates that, once developped, the jet travels through the bubble at an almost constant speed, and when lacking momentum, may succumb to hydrodynamic instabilities, resulting in gas-encapsulated droplets. If the jet reaches the distal side of the bubble, and the Weber number associated with the jet is greater than 100, the third phase shows the entrainment of gas by the jet on the distal side of the bubble. The final stage consists of a hollow cylindrical gas ejecta of a few nanoliters detaching from the bubble, suggesting a controlled way of injecting cells with small quantities of liquid and gas.   

Breakup dynamics of initially non-spherical rising bubbles

The recent emergence of new immersed energy storage systems such as the underwater compressed air energy storage (UWCAES) system requires the development of risk analysis studies associated with the potential accidents that can occur. For instance, a rupture of the pipe connecting the energy conversion platform to the storage tanks would generate a bubble plume rising rapidly to the surface by means of gravity. The dynamics of the plume itself depends on the interaction mechanisms between the bubbles composing it, including breakup and coalescence. 

To improve the understanding of the breakup phenomenon, DNS simulations of single air rising bubble in water are performed using the in-house ARCHER solver. Different initial shapes of bubble are investigated (spheroid, torus, and spherical cap), so that the variety of bubble shapes in the plume is considered. Focus is made on large bubbles belonging to the central breakup regime.

The bubble dynamics is analyzed in terms of shape evolution and related to the breakup process. It is shown that the initial shape influences the breakup dynamics. The breakup time and distribution of daughter bubbles are also extracted to help design a model for the breakup due to interfacial instability that will be included in a Population Balance Model.    

Detachment characteristics of hydrogen bubbles in water electrolysis

Hydrogen production from renewable energy through water electrolysis is a key technology for the energy transition. One present bottleneck is a lack of understanding of the dynamics of the electrolytic bubbles near the electrode surfaces as their presence can significantly impact electrolyzer efficiency, e.g. by blocking active sites. In this work, we experimentally investigate the detachment characteristics of the hydrogen bubbles formed in water electrolysis on a Pt electrode. We observe that for acid concentrations of 0.1 M and beyond, contact line spreading plays a dominant role in bubble behaviour. The buoyancy driven departure of the spreading bubble is not well described by the widely used Fritz model, which assumes a static contact angle. Instead, our data shows that the initial spreading is followed by contact line pinning, which can be understood when accounting for dynamic wetting effects. In addition to buoyancy driven bubble detachment, we also observe coalescence-triggered bubble departure and analyse the contact line dynamics for this case. Our results suggest that the radius ratio is an important, but not the only parameter determining the detachment of coalescing bubbles.   

Distinct dynamics of jetting produced by bubble bursting at a structurally compound interface

Bursting of bubbles at a liquid surface is ubiquitous in a wide range of physical, biological, and geological phenomena, as a key source of aerosol droplets for mass transport across the interface. However, how a structurally compound interface, widely present in nature, mediates the bursting process remains largely unknown. Here, we present some of our recent studies considering the following configuration: 1) When a bare bubble bursting at an air-oil-water compound interface, we show that the coupling of oil spreading and cavity collapse dynamics results in a multi-phase jet and the follow-up jetted droplet size change (Nat. Commun. 2021). The jet radius and velocity are altered with even a thin oil layer, and oily aerosol droplets are produced. The coupling of oil spreading and cavity collapse dynamics results in a multi-phase jet and the follow-up droplet size change. Corresponding scaling laws are proposed to quantify the jetting dynamics; 2) When an oil-coated bubble bursting at an air-water interface, we show that jet drops can be a few microns, while prior studies report that the bursting of such a bare bubble produces jet drops with a size of the order of 100 µm (Nat. Phys. 2023). The faster and smaller jet drops result from the singular dynamics of the oil-coated cavity collapse, and the air-oil-water compound interface offers a distinct damping mechanism to smooth out the precursor capillary waves effectively; 3) when a protein-coated bubble bursts at a viscoelastic surface of a bovine serum albumin solution, we document an unexpected phenomenon that a daughter bubble is entrapped with no subsequent jet drop ejection, contrary to bare bubble bursting dynamics at a Newtonian surface (Phys. Rev. Lett. 2023). We show that the strong surface dilatational elastic stress from the viscoelastic surface retards the cavity collapse and effectively damps out the precursor waves. The onset of daughter bubble entrainment is well predicted by an interfacial elastocapillary number comparing the effects of surface dilatational elasticity and surface tension. Our work not only advances the fundamental understanding of bubble bursting dynamics, but also help reducing model uncertainty for bubble-mediated aerosol flux and airborne transmission of bulk substances.   

Challenges of entrapping microbubbles in near-wall streamwise vortices

It is well-known that bubbles and light particles tend to cluster inside vortices (low-pressure, high-enstrophy regions), even in complicated flows such as homogeneous isotropic turbulence. However, this trend does not appear to hold as well for sparse microbubbles in the presence of quasi-streamwise vortices of wall-bounded turbulence. To understand the fundamental physics of this phenomenon, we use DNS to study very small bubbles in simplified flow cases, such as a single vortex pair, and extend this to a 3D turbulent channel flow with a moderate Reynolds number of . We also simulate other small particles with different densities to understand the role of density in particles' behavior near quasi-streamwise vortices in wall-bounded turbulence. All particles are on the order of the Kolmogorov length scale of the turbulent channel and their paths are integrated using modified Maxey-Riley equations, and then correlated to Eulerian vortex structures—identified using the swirl criterion—to determine what limits bubble clustering in these structures.   

Unsteady Flow Field Simulation of Free Bubble Rising in Quiescent Water

The dynamic behavior of free air bubble rising in quiescent water is investigated numerically at Reynolds number of 1045, Weber number of 3.49 and Eotvos number of 1.77. Both the classic empirical classifications (Clift, et al. 1978) and the new experimental observations (Moreto et al. 2022) indicate that the bubble with the values mentioned above in the non-dimensional parameter space exhibits a wobbling motion during the ascending process. By simulating the instantaneous 3-D flow field around free rising bubbles, the complex mechanism that results in the wobbling motion, especially lateral oscillations, can be better understood. The numerical method uses level set phase boundaries immersed in a spatially structured and normalized grid. The governing equations are discretized using a finite-volume approach. The velocity-pressure coupling is solved using the SIMPLE algorithm. Strong flow quantity variations can be found near the bubble surface and across the vortex rings shed from the bubble in the wake. To handle the challenges posed by the highly unsteady and thin boundary layer that is formed on the bubble surface, highly resolved grids are needed to fully characterize the time-dependent variation of the skin friction force acting upon the rising bubble surfac[XL1] e. Close comparison of the experimental data with the numerical simulations is implemented, to offer a reliable way of verification and validation of both results.*Sponsored by ONR (N00014-21-1-2392)   

Early time spreading of nanobubble-dispersed aqueous drops on oil-solid interfaces

Nanobubbles suspensions play an important role in numerous applications, including fabrication of functional materials, drug delivery, water treatment, carbon dioxide capture, and surface decontamination. The dynamic wetting of drops containing nanobubbles on solid surfaces is key to design the mass transport, adsorption, and reaction processes. Generally, the presence of surface-active materials in a drop may generate Marangoni stresses which can reduces the rapid motion of three phase contact line at the early time of the spreading process. Herein, we experimentally characterize the early time spreading dynamics of nanobubble-laden surfactant drops on a hydrophilic glass, submerged in an oil phase. We report that nanobubbles, when dispersed in the surfactant solution, can reduce the duration of the early time retardation regime, for instance from 0.2 to 0.02 s. The retardation time is found to be a decreasing function of nanobubble concentration.   

Mapping shear stress in inclined bubble impact

Bubbles are used in a range of applications related to cleaning, from ultrasonic baths to wastewater treatment. The cleaning is typically attributed to shear stresses in the fluid or scavenging of particles. The case of injected bubbles rising along a surface has gained interest recently for its potential in preventing biofouling and cleaning produce. We conduct experiments and numerical simulations of a bubble rising underneath an inclined surface held at various angles. From simulations, we calculate snapshots of shear stress induced at the wall, showing that the largest shear stresses occur where the wall-bubble film thickness is minimum. We combine these snapshots to obtain maximum shear stress maps, allowing us to determine the area of the wall subject to a specified stress level during a single bounce of a bubble. We find that the shear stress maps predict the cleaning patterns from experiments remarkably well except for the steepest case, where bubble trajectories in experiments vary considerably due to the presence of algae in the water. Additionally, we compare this to the similar case of a drop impacting an inclined surface.   

Coalescence driven dynamics of hydrogen bubbles during water electrolysis

The evolution of electrogenerated gas bubbles during water electrolysis significantly hampers the overall process efficiency. It is therefore beneficial to promote their detachment. For a single bubble, a departure from the electrode surface occurs when buoyancy wins over the downward-acting forces (e.g. contact, Marangoni, and electric forces). In this work, the dynamics of a pair of H₂ bubbles produced during water electrolysis in 0.5 mol/L H₂SO₄ at dual microelectrode is systematically studied by varying the cathodic potential. By combining high-speed imaging and electrochemical methods, we demonstrate the importance of bubble-bubble interactions for the detachment process. We show that bubble-bubble coalescence on the one hand may lead to a substantially earlier departure than that one defined by buoyancy, resulting in significantly higher reaction rates at constant potential. On the other hand, intensive coalescence events may reverse the direction of a departed bubble, driving it back toward the surface. The latter leads to the resumption of bubble growth near the electrode surface followed by buoyancy-driven detachment. We experimentally explore the phase diagram for these different behaviors as a function of the electrode distance and the applied potential and provide a simple model explaining the observed trends.   

Direct numerical simulations of bubble-particle interactions

The interaction between a freely rising deformable bubble and a settling particle in a Newtonian background fluid is studied. The density and viscosity ratios between the bubble and the background fluid are kept constant at 10³  and 10², respectively. The study considers the effect of varying the bubble Bond number (Bo) and Galileo number (Ga) on the interaction. The simulations are implemented using three-dimensional particle-resolved DNS using Level Contour Reconstruction Method (LCRM); this is a hybrid level-set front-tracking method to accurately capture the motion and interaction of the bubble and particle. The transient numerical simulations show that the deformation of the bubble varies depending on whether the interaction is dominated by surface tension or gravitational forces. When the bubble approaches the particle, a thin film of liquid is trapped between the rising bubble and the settling particle and begins to drain. After drainage, the thin liquid film undergoes rupture forming a three-phase contact line. We present results of the spatio-temporal evolution of the bubble-particle interaction and the associated velocity, vorticity, and pressure fields of all three phases.