Session G7: Bubbles: General II

What happens on the vortex structures when the rising bubble transits from zigzag to spiral?

It has been demonstrated by many experiments that in certain liquids, a single millimeter-sized bubble will rise within an unstable path, which transits from zigzag to spiral sometimes.After performing several groups of direct numerical simulations,the present work gives a theoretical explanation to reveal the mechanism causing the transition,and the results are presented in two parts.In the first part,where a freely rising bubble is simulated, equal-strength vortex pairs are observed to shed twice during a period of the ziazag motion, which is triggered by the amounts of streamwise vorticities accumulated on the bubble interface. However,when the balance between the counter-rotating vortices is broken, an angular velocity is induced between the asymmetric vortex pairs,driving the bubble rising in an opposite spiral path.Therefore,although there is no preference of the spiral direction as observed in experiments,it is actually determined by the sign of the stronger vortex thread.In the second part, external vertical magnetic fields are imposed onto the spirally rising bubble,in order to weak the imbalance between the vortex pairs. As a consequence,as the vortex pairs become more symmetric,the rotating radius of the spirally rising bubble decreases.   

Atomistic study of the graphene nanobubbles.

A two-dimensional (2D) heterostructures can be created using 2D crystals stacking method. Substance can be trapped between the layers which leads to formation of the surface nanobubbles. We study nanobubbles trapped between graphene layers with argon atoms inside using molecular dynamics approach. For bubbles with radius in range 7-34 nm the solid close-packed state of argon is found, although according to bulk argon phase diagram the fluid phase must be observed. The universal shape scaling (constant ratio of height to radius), which is found experimentally and proved by the theory of elasticity of membranes, is also observed in our atomistic simulations. An unusual pancake shape (extremely small height to radius ratio) is found for smallest nanobubble with radius 7 nm. The nanobubbles with similar shape were experimentally observed at the interface between water and hydrophobic surface.   

Pinning effects from substrate and AFM tip surfaces on interfacial nanobubbles

Measurement accuracy of atomic force microscopy (AFM) is vital to understand the mechanism of interfacial nanobubbles. In this study, we report the influence of pinning derived from both substrate and AFM tip surfaces on the measured shape of interfacial nanobubbles in peak force tapping mode. First, we pushed the nanobubbles using the AFM tip with high peak force setpoint. As a result, the deformed nanobubbles kept their flat shape for several tens of minutes. We quantitatively discuss the pinning force from substrate surface, which retains the flat shape enhancing the stability of nanobubbles. Next, we prepared three AFM tips with different wettability and measured the nanobubbles with an identical setpoint. By comparing the force curves obtained during the measurements, it seems that the (middle-)hydrophobic tips penetrated the liquid/gas interface and received repulsive force resulting from positive meniscus formed by pinning at the tip surface. In contrast, hydrophilic tip didn't penetrate the interface and received the force from the deformation of the interface of the nanobubbles. In addition, the measurements using the (middle-)hydrophobic tips led to the underestimation of the nanobubbles profile corresponding to the pinning position at the tip surfaces.   

Mass production of monodisperse microbubbles for real applications avoiding microfluidics

In this presentation we report experiments showing the effect on the controlled generation of microbubbles of the pressure gradient imposed by the relative flow of a liquid stream around an airfoil-shaped solid. Taking advantage of the conclusions in Evangelio et al. JFM 2015, 778, 653-668, where the local pressure gradient was identified as the mechanism responsible of the generation of microbubbles in microfluidic devices and, with the purpose of overcoming the low production rates associated with these kind of microdevices, we have used the same physical principle but have applied it to a totally different geometry: a rectangular planar wing composed by symmetrical airfoils. The relative velocity field is imposed either submerging the static wing within a flowing hydraulic channel or by rotating the wings within a reservoir containing the otherwise quiescent liquid mass. We provide physical insight on the bubbling process and deduce a scaling law which expresses the diameters of the bubbles formed as a function of the gas flow rate, relative liquid velocity and the angle of attack of the incident flow. In spite of the geometry is totally different, we recover the same results obtained using microfluidic devices but with much higher production rates.   

Bubble growth as a means to measure dissolved nitrogen concentration in aerated water

Controlling the amount of dissolved gases in water is important, for example, to food processing; it is essential to quantitatively evaluate dissolved gas concentration. The concentration of dissolved oxygen (DO) can be measured by commercial DO meters, but that of dissolved nitrogen (DN) cannot be obtained easily. Here, we propose a means to measure DN concentration based on Epstein-Plesset-type analysis of bubble growth under dissolved gas supersaturation. DO supersaturation in water is produced by oxygen microbubble aeration. The diffusion-driven growth of bubbles nucleated at glass surfaces in contact with the aerated water is first observed. The observed growth is then compared to the extended Epstein-Plesset theory that considers Fick's mass transfer of both DO and DN across bubble interfaces; in this comparison, the unknown DN concentration is treated as a fitting parameter. Comparisons between the experiment and the theory suggest, as expected, that DN can be effectively purged by oxygen microbubble aeration.   

Collective dissolution of microscopic bubbles

The dissolution of a single microscopic gas bubble in the surrounding liquid environment is a classical problem: the capillary pressure inside the bubble and the chemical equilibrium of the two phases (Henry's law) drive the diffusion of the excess dissolved gas away from the bubble, leading to its collapse and dissolution in finite time. This diffusion process conditions the lifetime of the bubble and is sensitive, among other effects, to the chemical and mechanical environment of the bubble. In this work, we investigate specifically the influence of other diffusing bubbles on the dissolution process and characterize these collective dynamics: each bubble acts as a chemical source and therefore slows down the dissolution of its neighbours. The effect of bubble density and spatial arrangement on their collective dissolution and life time will be discussed.   

Characterization of the Dynamics of Vapor Bubble Collapse

A numerical/theoretical analysis is presented to characterize the dynamics of a spherical vapor bubble, collapsing at different degrees of severity, controlled and quantified by the nondimensional number, B (introduced by Florschuetz and Chao, 1965). The numerical framework is exercised and validated over the three regimes of bubble collapse, namely, thermal, intermediate, and inertial collapse in increasing order of B. The conventional Rayleigh-Plesset perspective used to discriminate between different regimes, is extended to include the bubble energy balance and jump conditions. It is discovered that the time history of vapor velocity more clearly illustrates the distinctions between the three regimes. For thermal collapse, vapor velocity suffers an initial transient and then equilibrates to nearly zero. At intermediate rates of collapse, the time scales of the process are such that an imbalance occurs between the condensation rate and interface regression rate, which results in a significant magnitude of vapor velocity. At even larger rates of collapse, the time scales become exceedingly small such that the thermal boundary layer is sufficiently thin to provide the necessary heat conduction to balance the large condensation rate, resulting in negligible vapor velocity.   

Bubble clustering in a glass of stout beer

To clarify why the texture in stout beer poured into a pint glass descends, we investigated local time development of the void fraction and velocity of bubbles. The propagation of the number density distribution, i.e. the texture, appearing near the inclined wall is observed. We visualized individual advective bubbles near the inclined wall by microscope and measured the local void fraction using brightness of images while the velocity of bubbles by means of Particle Tracking Velocimetry. As the result of measurements, we found the local void fraction and the bubbles advection velocity increase and decrease repeatedly with a time delay. We conclude the texture pattern is composed of fluid blobs which contain less bubbles; extruding and suction flows respectively toward and from the interior of the container form respectively in front and back of the blobs.   

Bubble streams rising beneath an inclined surface

Bubbles released beneath a submerged inclined surface can tumble along the wall as they rise, dragging the surrounding fluid with them. This effect has recently regained attention as a method to mitigate biofouling in marine environment, such as a ship hull. It appears that the efficacy of this approach may be related to the velocity of the rising bubbles and the extent that they spread laterally as they rise. Yet, it is unclear how bubble stream rise velocity and lateral migration depend on bubble size, flow rate, and inclination angle. Here we perform systematic experiments to quantify these relationships for both individual bubble trajectories and ensemble average statistics.