Session D7: Bubbles: Dynamics

Fast particle ejection by a growing laser-induced bubble.

We document experimentally four different interactions of a laser-induced bubble and a free-settling particle, with different combinations of the geometric and physical parameters of the system. In particular, we also discover the high-speed ejection of the particle, and a cavity behind the particle, in cases when initially the particle is in very close proximity to the bubble. These observations offer new insights into the causal mechanism for the enhanced cavitation erosion in silt-laden water.   

Bubbles as leaky pistons: extracting small particles from polydisperse suspensions

Moving air bubbles in liquid-filled geometries act as leaky pistons. When confined in a cylinder, a bubble moves relative to the liquid as a small fraction of the liquid leaks through a very thin annular gap between the bubble and the internal wall of the cylinder. At low velocities the thickness of this lubricating film formed around the bubble is set only by the liquid properties and the translating speed of the bubble, thus can be tuned in a simple fashion. Here, we use these air bubble pistons for extracting small particles from bidisperse microspheres suspensions. Particle extraction occurs when the thickness of the lubricating liquid film falls between the diameters of the two different particles. Initially, when the bubble interface is free of particles, both large and small particles can move into the liquid film region. While large particles will be collected at the bubble interface, small particles can leak through the thin film. As a result, the small particles are extracted from the bidisperse suspension and transferred to the back of the bubble. Findings of this study can potentially be useful to better understand the interaction between particles and liquid-gas interfaces.   

The Role of Confinement in Bubble Collapse in a Channel

In a variety of applications, cavitation bubbles collapse near solid objects give rise to damage in certain extreme cases. Numerous detailed computational studies have been conducted of a single bubble collapsing near a rigid wall. However, there are known situations where the bubbles collapse in a confined configuration, such as in biomedical applications and in the spallation neutron source. However, the effect of confinement (e.g., in the case of bubble collapse in a narrow channel) is poorly understood. In the present work, we quantify the effect of confinement on the bubble dynamics, pressures and temperatures produced from a single bubble collapsing in a channel. An in-house, solution-adaptive, high-order accurate shock- and interface-capturing method is used to solve the 3D compressible Navier-Stokes equations for gas/liquid flows. We demonstrate the conditions under which the channel walls strengthen/weaken the violence of the collapse and result in amplifying/reducing the wall pressures and temperatures. We further determine the smallest channel width, relative to the initial bubble radius, for which the presence of a second wall affects the collapse. Additional simulations of multiple bubbles collapsing in a channel are underway and will be discussed.   

Bubble Dynamics and Breakup in a T-junction at Moderate Reynolds Numbers.

The deformation and breakup of droplets and bubbles in an immiscible carrier liquid in microchannels has been extensively investigated in the literature. In this study, we address the case of bubbles and drops in a centimeter-scale T-junction at moderate Reynolds numbers, a problem that is relevant for fluidics and emulsion processing applications. The main features include complex oscillating transients, recirculation stabilization, and drop stabilization against breakup. In particular, very elongated drop shapes are observed, which would be unstable in the unbounded case and can be explained in terms of wall-induced distortion of the flow field. We show that wall effects can be exploited to obtain nearly monodisperse emulsions in confined flows. Surface tension also plays an important role on the breakup of the dispersed phase. Different drop sizes can be obtained depending on the Capillary number as well as the bubble initial size. A mechanism for finding the non-breakup and break-up regions depending on bubble size is found. It is found with different initial flow rates of the matrix flow, the non-breakup regime allows for the bubble to remain attached to the bottom wall of the T-junction. In the breakup regime, the elongation of the drop results in a significant delay for breakup, allowing for the study of the breakup time and location. Results are presented for different Ca and Re numbers.   

Dynamics of contact line pinning/depinning of sliding bubble on super-aerophobic surfaces

It is a great significance to understand the mechanism of pinning/depinning conditions of a bubble on super-aerophobic surfaces for many applications such as thermal/energy systems and microfluidics. Whereas the effect of surface morphology on droplet pinning/depinning or sliding were studied extensively, that on bubble has been limited. The aim of this study is to explore the effect of surface morphology on the dynamics of contact line pinning/depinning of a sliding bubble on micropatterned (i.e., pillar and pore) super-aerophobic surfaces considering key surface parameters (i.e., solid fraction of wetting and effective contact line). In this work, the effective contact line along a bubble boundary is visualized at receding and advancing sides of a bubble sliding on the micropatterned surfaces with systematically varied dimensions. Results show that the effective maximal three-phase contact line is a more relevant surface parameter than the solid fraction in defining the depinning force of a sliding bubble on inclined super-aerophobic surfaces, similar to the case of droplets.   

The motion of a bubble sliding against an inclined wall

The motion of a bubble sliding over an inclined wall is studied experimentally for a wide range of liquid properties and bubbles sizes, considering wall inclination angles from nearly horizontal to nearly vertical. All experiments are restricted to sliding behavior, below the transition to steady bouncing motion (Barbosa et al., 2016). We study both the shape of the bubble and its drag coefficient. For small angles, the bubble shape is dominated by gravitational effects resulting in a flattened shape against the wall; for large angles, the bubble remains in constant contact with the wall but adopts a shape similar to that observed for an inertia-dominated free rising bubble. We model this transition of shape considering balances among surface tension, gravitational and inertial forces; we observe good agreement with experiments. We found that the drag coefficient is strongly influenced by the shape that the bubble adopts as it slides over the wall. By considering the flow in the film and around the bubble, we propose an empirical correlation to predict the drag coefficient. The proposed expression agrees well with the experimental measurements.   

Removing biofilm using bubbles

Scrubbing dynamics of a bubble impacting a tilted wall has been studied through a combination of experimental observations and computational validation. We experimentally characterize the impact and slide of a single bubble against a solid substrate. A force balance that includes buoyancy, hydrodynamic inertia \& drag, bubble deformation \& rotation, and film force has been modeled numerically at different tilted angles. We found that experimental observations were in good agreement with results from the computational model. Moreover, shear (or scrubbing) force on the wall has been calculated and compared with bacterium adhesion forces in order to evaluate the potential of a bubble-impacting method for removing biofilm from different surfaces.   

Physical cleaning by bubbly streaming flow in an ultrasound field

Low-intensity ultrasonic cleaning with gas-supersaturated water is a promising method of physical cleaning without erosion; we are able to trigger cavitation bubble nucleation by weak ultrasound under gas supersaturation and thus clean material surfaces by mild bubble dynamics. Here, we perform particle image velocimetry (PIV) measurement of liquid flow and cavitation bubble translation in an ultrasonic cleaning bath driven at 28 kHz and then relate it to cleaning tests using glass slides at which silica particles are attached. The ultrasound pressure amplitude at the cleaning spot is set at 1.4 atm. We select the supersaturation level of dissolved oxygen (DO) as a parameter and control it by oxygen microbubble aeration. It follows from the PIV measurement that the liquid flow is enhanced by the cavitation bubble translation driven by acoustic radiation force; this trend becomes clearer when the bubbles appear more densely as the DO supersaturation increases. In the cleaning tests, the cleaned areas appear as straight streaks. This suggests that physical cleaning is achieved mainly by cavitation bubbles that translate in ultrasound fields.   

Gas Bubble Dynamics under Mechanical Vibrations

The scientific community has a limited understanding of the bubble dynamics under mechanical oscillations due to over simplification of Navier-Stockes equation by neglecting the shear stress tensor and not accounting for body forces when calculating the acoustic radiation force. The current work experimental investigates bubble dynamics under mechanical vibration and resulting acoustic field by measuring the bubble size and velocity using high-speed imaging. The experimental setup consists of a custom-designed shaker table, cast acrylic bubble column, compressed air injection manifold and an optical imaging system. The mechanical vibrations resulted in accelerations between 0.25 to 10 times gravitational acceleration corresponding to frequency and amplitude range of 8 - 22Hz and 1 - 10mm respectively. Throughout testing the void fraction was limited to \textless 5{\%}. The bubble size is larger than resonance size and smaller than acoustic wavelength. The amplitude of acoustic pressure wave was estimated using the definition of Bjerknes force in combination with Rayleigh-Plesset equation. Physical behavior of the system was capture and classified. Bubble size, velocity as well as size and spatial distribution will be presented.   

Dynamics of the central entrapped bubble during drop impact

When a drop impacts onto a liquid surface, it entraps a thin central air disk. The air is then brought towards the axis of symmetry by surface tension. This contraction dynamics is very challenging to capture, due to the small length scales (a few micrometers thin air disk) and time scales (contracting in a few hundred microseconds). We use the open source two-phase flow codes Gerris and Basilisk to study this air entrapment phenomenon. The effects of liquid properties such as viscosity and surface tension, and of the impact velocity were investigated. We focus on the morphology of the contracting air disk. The bubble is expected to contract into a single spherical bubble. However, in some cases, the air can be stretched vertically by the liquid inertia and split into two smaller bubbles. The convergence of capillary waves on the air disk towards the axis of symmetry can also make it rupture at the center, thus forming a toroidal bubble. In other cases, vorticity shedding can deform the contracting bubble, leading to more complex structures. A parameter space analysis based on the Reynolds and Weber numbers was then done to classify the different regimes and explain the transitions.