Session Q09: Bubbles: Dynamics and Rupture II

Where precisely does the fluid in a jet drop come from?

The rupture of small bubbles at an air-liquid interface can aerosolize droplets containing particulates from the liquid. If microscopic particles, such as bacteria and viruses, are scavenged by the rising bubble, they can be concentrated into the top jet drop. Although the bursting process is typically an inertio-capillary phenomenon, viscous stresses influence the dynamics within the jet, along with the size of the top jet drop. However, how viscosity affects the origin of the fluid that ultimately resides in the top jet drop, is still unclear. Here we use numerical simulations and Lagrangian particle tracking to identify which parcels of liquid around the bubble ultimately end up in the top jet drop. We find that the liquid that forms the top jet drop is not evenly distributed around the bubble, and that viscous stresses influence these distributions. Because these distributions can affect the pathogen loading in jet drops, we anticipate our results to be relevant to the transport of disease causing viruses and bacteria from contaminated fluids.   

Numerical investigation for primary breakup study of unperturbed air filament embedded in a steady environment

The dynamics of a quiescent finite-size air filament embedded in a denser medium with a range of viscosity are studied numerically. Surface tension causes the two edges of the filament to retract back. It is shown that for a small aspect ratio, the filament recoils in a single bubble while when the aspect ratio is large, it ruptures at both ends. In between these limits, the filament can avoid end pinch-off by reopening the neck and/or by undergoing a breakup in the middle region. The pinch-off of the air filament is followed by the formation of a thin thread of air, which later breaks up into microbubbles. Viscous effects are shown to not significantly impacting the fate of the air filament but can alter the shape of the end bulge and the neck region. For Ohnesorge numbers smaller than 0.01, the bulge adopts a prolate shape, while for Ohnesorge numbers larger than 1 an oblate shape is observed. Between these two limits, a spherical shape is obtained. Close to breakup, the neck radius is shown to decrease as a power-law function of the time with an exponent of 0.5 for small Ohnesorge numbers and 1 for large Ohnesorge numbers.

    

Bubble oscillations in a uni-axial extensional flow

The evolution of a bubble in a uniaxial straining flow is a reduced model for bubble dynamics in more complex flows, such as turbulent flows (Rodr\'iguez-Rodr\'iguez \& al., 2006).

We use direct numerical simulations to systematically analyze the dynamics of the first bubble oscillatory modes, as a function of both the Weber number, $W_e$, (ratio of kinetic and surface tension forces) and the Reynolds number, $R_e$, (ratio of kinetic and viscous forces). We show that the temporal evolution of each mode is accurately modeled by a damped forced harmonic oscillator.

The second mode oscillates with a pulsation in $\omega_2(1 - 0.07 W_e)$, where $\omega_2$ is the pulsation in a quiescent flow. We validate the theory of Kang \& Leal (1988) which originates from a coupling between the two first modes of oscillations.

Forcing and damping constants present non trivial dependences on $W_e$ and $R_e$, which are symptomatic of complex boundary layers dynamics. Our study is a rare example where flow-interface coupling can be measured quantitatively.   

Bubble collapse analysis and reentrant jet velocity measurements using wOFV

Experiments were performed to study the collapse of bubbles produced by a spark discharge in a water tank using high speed imaging. A bubble was generated by dischraging a bank of capacitors at a voltage of 10 kV in between two electrodes into liquid water to simulate an underwater explosion. The air pressure above the water was reduced to a partial vacuum in order to increase the time scales associated with the bubble dynamics and to properly scale explosion conditions in the ocean. The coaxial electrodes were placed on the wall of the water tank such that the bubble was hemispherical and so that the reentrant jet that forms as the bubble collapses could be observed. Wavelet-based optical flow velocimetry (wOFV) was applied to high speed video of the bubble collapse, which allowed the velocity profile of the reentrant jet to be measured and for features on the surface of the bubble and within the jet to be tracked in time. Jet centerline velocities were successfully measured in the experiments. These measurements are compared to CFD computations of the bubble dynamics.   

Microplastic transport by bursting bubbles

The transport of microplastics – 500 to 10 microns in size – by means of bursting bubbles is studied. Single bubble bursts are recorded with high-speed photography and jet drops produced are caught to understand and quantify the mechanism by which microplastics are captured and ejected. As bursting bubble sea spray has previously been shown to be an effective transport mechanism for water, salts, and biological material out of the ocean into the atmosphere, this work extends that body of literature to quantify the transport of plastic pollution.   

Bubble bursting and sound generation in suspensions

Suspension rheology becomes solid-like as its particle volumetric fraction Φ increases, but how this affects bubble bursting and sound generation, is poorly known. Here we conduct lab experiments to investigate this effect by injecting a bubble (volume V) into a refractive index-matched suspension. We find that depending on Φ and V, sound with diverse waveforms are excited, covering a frequency band of f = O(10-10⁴) Hz. In a suspension of Φ=< 0.3 or in a suspension of Φ = 0.4 with a V smaller than critical, the bubble bursts after it forms a hemispherical cap at the surface and excites a high-frequency (HF) wave (f = O(10⁴ Hz)) with an irregular waveform, which likely originates from film vibration. However, in a suspension of Φ = 0.4 and with a V larger than critical, the bubble bursts as soon as it protrudes above the surface, and its aperture opens slowly, exciting Helmholtz resonance with f = O(10³) Hz. Superimposed on the waveform are an HF wave component excited upon bursting and a low-frequency (f = O(10)Hz) air flow vented from the deflating bubble, which becomes dominant at a large V. We interpret this transition as a result of the bubble film of a solid-like Φ = 0.4 suspension, being stretched faster than the critical strain rate such that it bursts by brittle failure.

    

Contraction of an air cone vs a liquid cone

The universality of the pinch-off of inviscid liquid threads is well known and scales with capillary-inertial dynamics. Recent work by Brasz et al. (2018)¹ investigates the repeated droplet formation from the tip of a retracting liquid cone, using a discrete self-similar formalism. Interchanging the air and liquid to form the contraction of an air-cone within a liquid pool is geometrically similar, but the dynamics will resemble more the purely inertial pinch-off of a bubble. Here we use the volume-of-fluid computational code Gerris to investigate the retraction of such an air cone over a range of different cone angles and initial perturbation levels, using refinement levels up to 20. We find that the parameter space for the air cone is significantly different from that of the liquid cone, showing regions of no pinch-off and regular pinch-off, with an irregular regime in between. The irregular pinch-off regime shows a much faster contraction speed and stronger jets emerging. The pinch-off radius follows inertial time-scaling with a power-law exponent close to 0.5, while overall discrete self-similarity is still approximately preserved.   

Bubble bursting at a viscoelastic surface: jet dynamics and bubble entrainment

Bubble bursting plays a key role in mediating the mass transport across the interface in nature and many industrial processes, such as sea spray aerosol generation and disease transmission from contaminant waters. A surface microlayer typically exists in natural bodies of water, which may contain proteins, bacteria, and viruses and behave as a viscoelastic surface. How the surface viscoelasticity affects the bubble bursting dynamics remains unclear. Here, we experimentally study the bursting of bubbles at the surface of bovine serum albumin (BSA) protein solutions. The increase in surface viscoelasticity decreases the jet velocity, jet height, and jet droplet number, and eventually kills the jet droplet ejection. In the regime of no jet droplet ejection, we observe the entrainment of a small gas bubble induced by the focusing of the capillary wave at the end of cavity collapse. The bubble entrainment is further discussed by investigating the effect of surface viscoelasticity on the capillary wave propagation during cavity collapse. These findings not only enrich the fundamental understanding of bubble bursting at a structurally complex surface, but also offer insights into the bubble-driven aerosolization of bulk substance such as pollutants.