Session QQ: Bubbles IV

Understanding the spontaneous brittle-to-ductile transition in foam fracture

A single layer of aqueous foam bubbles in a Hele-Shaw cell, when exposed to compressed air, can fail in both a brittle and a ductile fracture mode. Unlike fracture in hard crystalline matter, where either mode can be induced through external parameter changes, we observe that cracks in foam can transition {\it spontaneously} from the brittle to the ductile stage. The transition occurs dynamically and is accompanied by the cessation of film rupture and an abrupt change in crack speed obeying a gap of forbidden velocities. The spontaneous transition can be understood through the continuous action of dissipation through air flow in the expanding fracture channel. An accompanying theory based only on fluid dynamics and bubble geometry explains quantitatively the mechanism and location of this transition, as well as the dependence of the phenomenon on experimental parameters. The new insights are applicable beyond foam physics to fields like hydraulic fracture.   

Deformation of soap films pushed through tubes at high velocity

The behaviour of soap films pushed through tubes at large velocities, up to several m/s, is investigated. The film shape deviates from its equilibrium configuration perpendicular to the walls and gets curved downstream. A simple model relates the radius of curvature of the film to the friction in the lubrication films touching the wall. For highly soluble surfactants above the cmc, the scaling proposed by Bretherton in 1961 holds up to surprisingly high velocities, at which the capillary and Weber numbers are no longer small parameters. A limit of stability of the film, beyond which the films burst or evolve unsteadily, is predicted, and captures quantitatively the observations. In contrast, an unsteady motion is obtained for unsoluble surfactant, governed by strong Marangoni effects. The new questions raised by our results on the dissipation in soap films are discussed and comparison with numerical simulations are proposed.\\[4pt] [1] {\it Deformation of soap films pushed through tubes at high velocity}, B. Dollet, I. Cantat. J. Fluid. Mech. {\bf 652} p.529-539 (2010).   

The micro-macro link for liquid foam stability

The experimental study of bubble rearrangements triggered artificially in both bubble clusters and liquid foams allow us to established a link between the stability of the system and the dynamical behaviour of foam films. More precisely, the amount of liquid available locally in the system has to be larger than a critical value to ensure the formation of transient films within dynamical conditions. This has been clearly attributed to a dynamical thickening effect of the fresh film, whose thickness is strongly dependent on the rheological properties of interfaces. A simple model is proposed to capture these new findings and shows good quantitative agreement with measurements for the critical liquid fraction of foam collapse, providing new insight in the understanding of the complex coupling between internal dynamics and stability of foams.   

Accurate detection of pierced position/angle in bubble measurements by a Single-Tip Optical fiber Probe

An optical fiber probe (OFP) is very useful and reliable to measure bubble diameters, velocities, and local void fractions simultaneously in bubbly flow systems. One of the authors already developed the Single-Tip Optical-fiber Probe (S-TOP), which is practically employed in small-size-bubble measurement in industrial plants. Its sensing tip is smoothly ground into a wedge shape. In optical fiber probing, errors due to S-TOP tip randomly touching the bubble surface were unavoidable thus far. To overcome this problem, we newly propose a pre-signal method, which is powerful yet simple process. It occurs intensively, only when the well-tuned wedge shape tip touches vertically the center region of the bubble. In this study, at first, we quantify the relationship between the intensity of the pre-signal and the contact position/angle. Second, we discuss the outbreak mechanism of the pre-signal. Third, based on the relationship, a pre-signal method is established. Finally, it is confirmed that the pre-signal method is effective in bubbly flow measurement.   

Explosive boiling incipience on a thin wire

When a metastable liquid is superheated above its saturation temperature, a phase transition occurs via a nucleation process leading to the creation of at least one vapor bubble that grows rapidly. If the surrounding liquid is subcooled, the bubble will eventually undergo a violent collapse. A further characterization of the thermodynamic properties of this explosive phase change, (temperature at the onset of nucleation as well as pressure inside the first nuclei), together with the following bubble dynamics, is necessary for a better comprehension of boiling phenomena. Thanks to dedicated experiments in which a platinum micrometer--size wire is heated in a liquid at ambient pressure and temperature, we will report that the onset temperature is close to the spinodal temperature but slightly depends on the heating rate. Using high--speed video imaging of the bubble dynamics together with the Rayleigh--Plesset equation, we will show how the heating rate, as well as the heater size governs the nucleation process (bubble lifetime, maximum radius reached, expansion velocity and cooling of the wire at the onset).   

Actively Enhanced Boiling Heat Transfer using Acoustic Interfacial Actuation

Acoustic actuation is used to enhance boiling heat transfer on a submerged surface by controlling the formation and evolution of vapor bubbles and inhibiting instabilities that lead to film boiling and critical heat flux. The receptivity of a vapor bubble that forms at a prescribed nucleation site to acoustic interfacial excitation and to acoustically induced Bjerknes body forces is investigated on a surface-embedded hot spot with emphasis on the acoustic effects on nucleation, growth, contact-line motion, condensation, and detachment. The investigation also considers arrays of vapor bubbles that form on a prescribed grid of surface-engineered nucleation sites and the interactions between adjacent vapor bubbles. It is shown that acoustic actuation enables dissipation of higher heat fluxes at a given surface temperature, and a significant delay of the critical heat flux with reduction of the vapor mass above the surface. Supported by ONR.   

Infrared Visualisation of Nucleate Boiling from an Isolated Site in a Hele-Shaw Cell

Nucleation, growth and detachment of HFE-7000 confined vapour bubbles are investigated locally using simultaneously an infrared camera with 10 $\mu $m spatial resolution and a visible camera. Bubbles are created from a 50 $\mu $m artificial nucleation site on a 100 $\mu $m Inconel film in a Hele-Shaw cell. A single bubble nucleation is investigated. Experiments are performed by varying both the convective inlet liquid mass flow rate, and the heat flux supplied at the wire. Bubble detachment diameters at the single artificial nucleation site and the associated effects on the heat transfer by the confinement influence are investigated at low Reynolds number. The experimental set-up enables observation in the 2D Hele-Shaw cell of the flow and of the bubble growth; this is achievable since the cell has one face transparent to infrared radiation. The infrared video resolution enables us to observe the presence of a bubble and its magnitude of thermal disturbance on the flow. The temperature profile of the bubble as it nucleates, grows and detaches from the wall can be measured from the infrared videos. The temperature change at the nucleation site highlights the frequency of the bubble detachment based on the temperature signal. From analysis of the visible and infrared videos, the nucleation site surface temperature, bubble detachment diameter and bubble nucleation frequency can be calculated.   

Drag force acting on an ellipsoidal bubble with fore-aft asymmetry

We evaluate the drag force acting on an ellipsoidal clean bubbles rising steadily by experiments and numerical analysis. Flow fields and bubble shapes are obtained using a numerical simulation, which is based on a finite-difference solution of the equation s of motion on an orthogonal curvilinear coordinate system. Bubble motion in eight different clean liquids are also observed using high-speed photography. Photochromic dye is used to visualize the rear vortex structure. The degree of fore-aft asymmetric bubble shape is quantitatively evaluated using Legendre polynomials. It is confirmed that the existence of standing eddy hardly changes the drag even in the case with fore-aft asymmetry of bubble shape. It is also found that non-dimensional steady drag acting on a bubble has a liner relation with aspect ratio. The discrepancy of drag coefficients between analytical theory and experimental results is discussed.   

Conditions for microthread formation in viscous coflows

We have performed numerous experiments with coaxial coflowing fluids in microfluidic devices at low Reynolds numbers and have compare them against BEM numerical simulations, finding excellent agreement. The conditions determining the generation of liquid threads with diameters below $1\%$ the diameter of the injection tube have been analyzed in detail and the crucial role of the inner to outer viscosity ratio for the generation of such tiny jets has been elucidated. Thanks to our numerical results, we deduce a simple model that predicts, as a function of the control parameters, the conditions under which this type of liquid jets are generated as well as the diameters of the resulting drops.   

Novel high bandwidth wall shear stress sensor for ultrasonic cleaning applications

Ultrasonic cleaning is due to the action of cavitation bubbles. The details of the cleaning mechanisms are not revealed or confirmed experimentally, yet several studies suggest that the wall shear stresses generated are very high, i.e. of the order of several thousand Pascal. Ultrasonic cleaning applications span a wide range from semiconductor manufacturing, to low pressure membrane cleaning, and the in the medical field cleaning of surgical instruments. We have developed a novel sensor to monitor and quantify cleaning activity which is (1) very sturdy, (2) has a high bandwidth of several megahertz, (3) is cheap in manufacturing costs, and (4) of very small size. We analyze the sensor signal by comparing its response time correlated to single laser induced cavitation bubbles using high-speed photography. Additionally, we will present first measurements in ultrasonic cleaning bathes using again high-speed photography. A preliminary discussion on the working mechanism of the sensor will be presented.