Session F7: Bubbles: General I

Underwater smelling by the star-nosed mole

The star-nosed mole can sniff underwater objects by rapidly blowing and inhaling bubbles. How these mammals manipulate bubbles without losing them is poorly understood. In this experimental study, we show that the peculiar shape of the mole's nose can stabilize bubbles. We laser-cut a series of star-shaped plastic templates and measure the largest angle they can be titled before bubbles are released. The arms of the star anchor the bubbles in place by enabling the buoyancy forces between the arms to counter the effects of tilt. Based on this finding, we design and construct a mole-inspired underwater sniffing device that uses oscillation of bubbles to feed a metal oxide chemical sensor, a first step in expanding machine olfaction to underwater applications   

The life and death of film bubbles

Following its burst, the fragmentation of a large bubble (film bubble) at the air-water interface can release hundreds of micrometer-sized film-drops in the air we breathe. This mechanism of droplet formation is one of the most prominent sources of sea spray. Indoor or outdoor, pathogens from contaminated water are transported by these droplets and have also been linked to respiratory infection. The lifetime and thickness of bubbles govern the number and size of the droplets they produce.     Despite these important implications, little is known about the factors influencing the life and death of surface film bubbles.  In particular,   the fundamental physical mechanisms linking bubble aging,  thinning, and lifetime  remain poorly understood.   To address this gap,  we present the results of an extensive investigation of the aging of film-drop-producing bubbles   in various ambient air,  water composition,  and temperature conditions. We present and validate a  generalized physical picture and model of bubble cap thickness evolution. The model and physical picture are linked to  the lifetime of bubbles via a series of cap rupture mechanisms  of increasing  efficiency.   

Shapes and paths of an air bubble rising in quiescent liquids

We investigate shapes and paths of an air bubble rising inside a liquid via experiments and direct numerical simulations. Close to three hundred experiments are conducted in order to generate a phase plot in the Gallilei and Eotvos numbers plane. The plot separates distinct regimes, in terms of bubble behavior; specifically, an axisymmetric, skirted, wobbling, and three types of break-up behaviors are observed in our experiments. A wide range of Gallilei and Eotvos numbers is obtained by using aqueous glycerol solutions of different concentrations as the surrounding fluid and by varying the bubble size. The dynamics of the bubbles in each regime is investigated in terms of shapes, topological changes and trajectories followed by the bubbles. The present study complement the numerical simulation results presented by Tripathi et al. (Nature Communications, 2015).   

Non-isothermal bubble rise dynamics in a self-rewetting fluid

The motion of a gas bubble in a square channel with linearly-increasing temperature in the vertical direction is investigated via three-dimensional numerical simulations. The channel contains a so-called “self-rewetting” fluid whose surface tension exhibits a parabolic dependence on temperature with a well-defined minimum. An open-source finite-volume fluid flow solver (Basilisk) is used with a dynamic adaptive grid refinement technique. We find that in contrast to `ordinary' fluids with linear dependence of surface tension on temperature, the buoyancy-induced upward motion of the bubble may be enhanced or retarded by a thermocapillary-driven flow depending on the location of the bubble with respect to the position where the surface tension becomes minimum. These phenomena are observed at sufficiently small Bond numbers.   

Numerical and experimental investigations of an air bubble rising in a Carreau-Yasuda shear-thinning liquid

The dynamics of an air bubble rising in a quiescent shear-thinning fluid modelled using a simplified Carreau-Yasuda rheological model is investigated numerically and experimentally. For the parameter values considered in the present study, a rising bubble in a shear-thinning fluid exhibits three-dimensional behaviour. Both path instabilities (zigzagging/spiralling motion) and topological changes are observed for an air bubble rising in a shear-thinning fluid. However, for a Newtonian surrounding fluid, a bubble maintains azimuthal symmetry and rises in a straight path for the same set of parameters. The mechanism of this three-dimensional behaviour is investigated by inspecting the variation of vertical vorticity component and viscosity. Experiments have also been conducted using a high speed camera to visualise the bubble rise behaviour in both Newtonian and shear-thinning fluids as surrounding media. The shapes and trajectories of the bubble obtained from experiments show qualitative agreement with those obtained from numerical simulations.   

Rising dynamics of a bubble confined in vertical cells with rectangular cross-sections

Recently, the drag friction acting on a fluid drop in confined space has been actively studied [1]. Here, we investigate the rising velocity of a bubble in a vertical cell with a rectangular cross-section, both theoretically and experimentally, in which understanding of the drag force acting on the rising bubble is crucial. Although the drag force in such confined space could involve several regimes, we study a special case in which the bubble is long and the aspect-ratio of the rectangular cross-section of the cell is high. As a result, we found new scaling law for the rising velocity and the drag force, and confirmed the laws experimentally. Crossover to the rising dynamics in a Hele-Shaw cell [2] will be also discussed. [1] K. Okumura, Adv. Colloid Interface Sci. (2017, in press) [2] A. Eri and K. Okumura, Soft Matter 7 (2011)   

The shape and motion of gas bubbles in a liquid flowing through a thin annulus

We study the shape and motion of gas bubbles in a liquid flowing through a horizontal or slightly-inclined thin annulus. Experimental data show that in the horizontal annulus, bubbles develop a unique “tadpole” shape with an elliptical cap and a highly-stretched tail, due to the confinement between the closely-spaced channel walls. As the annulus is inclined, the bubble tail tends to decrease in length, while the geometry of the cap remains almost invariant. To model the bubble evolution, the thin annulus is conceptualised as a ``Hele-Shaw” cell in a curvilinear space. The three-dimensional flow within the cell is represented by a gap-averaged, two-dimensional model constrained by the same dimensionless quantities. The complex bubble dynamics are solved using a mixed control-volume finite-element method combined with interface-capturing and mesh adaptation techniques. A close match to the experimental data is achieved, both qualitatively and quantitatively, by the numerical simulations. The mechanism for the elliptical cap formation is interpreted based on an analogous irrotational flow field around a circular cylinder. The shape regimes of bubbles flowing through the thin annulus are further explored based on the simulation results.   

Dynamics of the liquid film around elongated bubbles rising in vertical capillaries

We performed a theoretical, numerical and experimental study on elongated bubbles rising in vertical tubes in co-current liquid flows. The flow conditions were characterized by capillary, Reynolds and Bond numbers within the range of $\mathrm{Ca}=0.005-0.1$, $\mathrm{Re}=1-2000$ and $\mathrm{Bo}=0-20$. Direct numerical simulations of the two-phase flows are run with a self-improved version of OpenFOAM, implementing a coupled Level Set and Volume of Fluid method. A theoretical model based on an extension of the traditional Bretherton theory, accounting for inertia and the gravity force, is developed to obtain predictions of the profiles of the front and rear menisci of the bubble, liquid film thickness and bubble velocity. Different from the traditional theory for bubbles rising in a stagnant liquid, the gravity force impacts the flow already when $\mathrm{Bo}<4$. Gravity effects speed up the bubble compared to the $\mathrm{Bo}=0$ case, making the liquid film thicker and reducing the amplitude of the undulation on the surface of the bubble near its tail. Gravity effects are more apparent in the visco-capillary regime, i.e. when the Reynolds number is below 1.   

Freezing Bubbles

The two-stage freezing process of a liquid droplet on a substrate is well known; however, how bubbles freeze has not yet been studied. We first deposited bubbles on a silicon substrate that was chilled at temperatures ranging from --10 \textdegree C to --40 \textdegree C, while the air was at room temperature. We observed that the freeze front moved very slowly up the bubble, and in some cases, even came to a complete halt at a critical height. This slow freezing front propagation can be explained by the low thermal conductivity of the thin soap film, and can be observed more clearly when the bubble size or the surface temperature is increased. This delayed freezing allows the frozen portion of the bubble to cool the air within the bubble while the top part is still liquid, which induces a vapor pressure mismatch that either collapses the top or causes the top to pop. In cases where the freeze front reaches the top of the bubble, a portion of the top may melt and slowly refreeze; this can happen more than just once for a single bubble. We also investigated freezing bubbles inside of a freezer where the air was held at --20 \textdegree C. In this case, the bubbles freeze quickly and the ice grows radially from nucleation sites instead of perpendicular to the surface, which provides a clear contrast with the conduction limited room temperature bubbles.   

Nonspherical liquid droplet falling in air

The dynamics of an initially nonspherical liquid droplet falling in air under the action of gravity is investigated via three-dimensional numerical simulations of the Navier-Stokes and continuity equations in the inertial regime. The surface tension is considered to be high enough so that a droplet does not undergo break-up. Vertically symmetric oscillations which decay with time are observed for low inertia. The amplitude of these oscillations increases for high Gallilei numbers and the shape asymmetry in the vertical direction becomes prominent. The reason for this asymmetry has been attributed to the higher aerodynamic inertia. Moreover, even for large inertia, no path deviations/oscillations are observed.