Explosion of electron bubbles attached to quantized vortices in liquid 4He

Electron bubbles in superfluid (4)He have been recently observed in low-temperature cavitation measurements under experimental conditions where quantized vortices are also present in the liquid, and which might be attached to the bubbles. We have calculated, within density functional theory, the structure and energetics of electron bubbles pinned to linear vortices in liquid (4)He at low temperature, and the pressure at which such structures become mechanically unstable. Our results are in semiquantitative agreement with the experiments. We discuss dynamical effects not included in the theoretical model used in the present calculations, and which could explain some discrepancies between our results and the experimental data.     

Effects of soluble surfactants on the deformation and breakup of stretching liquid bridges

Surfactants are routinely used to control the breakup of drops and jets in many applications such as inkjet printing, crop spraying, and DNA or protein microarraying. The breakup of surfactant-free drops and jets has been extensively studied. By contrast, little is known about the closely related problem of interface rupture when surfactants are present. Solutions of a nonionic surfactant, pentaethylene glycol monododecyl ether, or C12E5, in water and in 90 wt % glycerol/water are used to show the effects of surfactant and viscosity on the deformation and breakup dynamics of stretching liquid bridges. Equilibrium surface tensions for both solutions can be fitted with the Langmuir-Szyskowski equation. All experiments have been done at 24 degrees C. The critical micelle concentrations for C12E5 are 0.04 and 0.4 mM in water and the glycerol/water solution, respectively. With high-speed imaging, the dynamic shapes of bridges held captive between two rods of 3.15 mm diameter are captured and analyzed with a time resolution of 0.1-1 ms. The bridge lengths are 3.15 mm initially and about 5-7 mm at pinch-off. Breakup occurs after stretching for about 0.2-0.3 s, depending on the solution viscosity and the surfactant concentration. When the liquid bridges break up, the volume of the sessile drop left on the bottom rod is about 3 times larger than that of the pendant drop left on the top rod. This asymmetry is due to gravity and is influenced by the equilibrium surface tensions. Surfactant-containing low-viscosity water bridges are shown to break up faster than surfactant-free ones because of the effect of gravity. With or without surfactant, water bridges form satellite drops. Surfactant-containing high-viscosity glycerol/water bridges break up more slowly than surfactant-free ones because of strong viscous effects. Moreover, the shapes of the sessile drops close to breakup exhibit a "pear-like" tip; whether a satellite forms depends on the surface age of the bridge before stretching commences. These unexpected effects arising from the addition of surfactants are due to the capillary pressure reduction and Marangoni flows linked to dynamic surface tension.     

The pressure drop along rectangular microchannels containing bubbles

This paper derives the difference in pressure between the beginning and the end of a rectangular microchannel through which a flowing liquid (water, with or without surfactant, and mixtures of water and glycerol) carries bubbles that contact all four walls of the channel. It uses an indirect method to derive the pressure in the channel. The pressure drop depends predominantly on the number of bubbles in the channel at both low and high concentrations of surfactant. At intermediate concentrations of surfactant, if the channel contains bubbles (of the same or different lengths), the total, aggregated length of the bubbles in the channel is the dominant contributor to the pressure drop. The difference between these two cases stems from increased flow of liquid through the "gutters"-the regions of the system bounded by the curved body of the bubble and the corners of the channel-in the presence of intermediate concentrations of surfactant. This paper presents a systematic and quantitative investigation of the influence of surfactants on the flow of fluids in microchannels containing bubbles. It derives the contributions to the overall pressure drop from three regions of the channel: (i) the slugs of liquid between the bubbles (and separated from the bubbles), in which liquid flows as though no bubbles were present; (ii) the gutters along the corners of the microchannels; and (iii) the curved caps at the ends of the bubble.     

Surface tension measurements in freely suspended bubbles of thermotropic smectic liquid crystals

We present an experimental technique for the inflation of freely suspended films of thermotropic smectic liquid crystals to macroscopic bubbles. These bubbles have many properties in common with well known lyotropic soap bubbles, but on the other hand represent a completely different class of material with different physical properties and new phenomena. We describe the generation of such smectic bubbles and compare their physical properties with those of lyotropic soap bubbles. The measurement of inner pressure versus curvature is used for the determination of surface tensions of the smectic materials.     

Micropumping of liquid by directional growth and selective venting of gas bubbles

We introduce a new mechanism to pump liquid in microchannels based on the directional growth and displacement of gas bubbles in conjunction with the non-directional and selective removal of the bubbles. A majority of the existing bubble-driven micropumps employs boiling despite the unfavorable scaling of energy consumption for miniaturization because the vapor bubbles can be easily removed by condensation. Other gas generation methods are rarely suitable for micropumping applications because it is difficult to remove the gas bubbles promptly from a pump loop. In order to eradicate this limitation, the rapid removal of insoluble gas bubbles without liquid leakage is achieved with hydrophobic nanopores, allowing the use of virtually any kind of bubbles. In this paper, electrolysis and gas injection are demonstrated as two distinctively different gas sources. The proposed mechanism is first proved by circulating water in a looped microchannel. Using H(2) and O(2) gas bubbles continuously generated by electrolysis, a prototype device with a looped channel shows a volumetric flow rate of 4.5-13.5 nL s(-1) with a direct current (DC) power input of 2-85 mW. A similar device with an open-ended microchannel gives a maximum flow rate of approximately 65 nL s(-1) and a maximum pressure head of approximately 195 Pa with 14 mW input. The electrolytic-bubble-driven micropump operates with a 10-100 times higher power efficiency than its thermal-bubble-driven counterparts and exhibits better controllability. The pumping mechanism is then implemented by injecting nitrogen gas bubbles to demonstrate the flexibility of bubble sources, which would allow one to choose them for specific needs (e.g., energy efficiency, thermal sensitivity, biocompatibility, and adjustable flow rate), making the proposed mechanism attractive for many applications including micro total analysis systems (microTAS) and micro fuel cells.     

Controlling the formation of capillary bridges in binary liquid mixtures

We study the formation of capillary bridges between micrometer-sized glass spheres immersed in a binary liquid mixture using bright field and confocal microscopy. The bridges form upon heating due to the preferential wetting of the hydrophilic glass surface by the water-rich phase. If the system is cooled below the demixing temperature, the bridges disappear within a few seconds by intermolecular diffusion. Thus, this system offers the opportunity to switch the bridges on and off and to tune precisely the bridge volume by altering the temperature in a convenient range. We measure the bridge geometry as a function of the temperature from bright field images and calculate the cohesive force. We discuss the influence of the solvent composition on the bridge formation temperature, the strength of the capillary force, and the bridge volume growth rate. Furthermore, we find that the onset of bridge formation coincides with the water-lutidine bulk coexistence curve.     

Nonequilibrium bubbles in a flowing langmuir monolayer

We investigate the nonequilibrium behavior of two-dimensional gas bubbles in Langmuir monolayers. A cavitation bubble is induced in liquid expanded phase by locally heating a Langmuir monolayer with an IR-laser. At low IR-laser power the cavitation bubble is immersed in quiescent liquid expanded monolayer. At higher IR-laser power thermo capillary flow around the laser-induced cavitation bubble sets in. The thermo capillary flow is caused by a temperature dependence of the gas/liquid line tension. The slope of the line tension with temperature is determined by measuring the thermo capillary flow velocity. Thermodynamically stable satellite bubbles are generated by increasing the surface area of the monolayer. Those satellite bubbles collide with the cavitation bubble. Upon collision the satellite bubbles either coalesce with the cavitation bubble or slide past the cavitation bubble. Moreover we show that the satellite bubbles can also be produced by the emission from the laser-induced cavitation bubbles.     

Rupture kinetics of liquid bridges during a pulling process: a kinetic density functional theory study

Capillary bridge is a common phenomenon in nature and can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. Especially, it plays a crucial role in the operation of atomic force microscopy (AFM) and influences in the measured force. In the present work, we study the rupture kinetics and transition pathways of liquid bridges connecting an AFM tip and a flat substrate during a process of pulling the tip off. Depending on thermodynamic conditions and the tip velocity, two regimes corresponding to different transition pathways are identified. In the single-bridge regime, the initial equilibrium bridge persists as a single one during the pulling process until the liquid bridge breaks. While, in the multibridge regime the stretched liquid bridge transforms into an intermediate state with a collection of slender liquid bridges, which then break gradually during the pulling process. Moreover, the critical rupture distance at which the bridges break changes with the tip velocity and thermodynamic conditions, and its maximum value occurs near the boundary between the single-bridge regime and the multibridge regime, where the longest range capillary force is produced. In this work, the effects of tip velocity, tip size, tip-fluid interaction, and humidity on rupture kinetics and transition pathways are also systematically studied.     

Mechanisms of equilibrium shape transitions of liquid droplets in electrowetting

Liquid droplets bridging the gap between two dielectric-coated horizontal electrode plates suffer breakup instabilities when a voltage applied between the electrodes exceeds a threshold. Interestingly enough, broken liquid bridges (i.e. a pair of a sessile and a pendant drop) can spontaneously rejoin if the voltage is still applied to the electrodes. Here we study the electro-hydrostatics of the liquid bridges in the joined or broken state and we illuminate the mechanisms of the shape transitions that lead to bridge rupture or droplet joining. The governing equations of the capillary electro-hydrostatics form nonlinear and free boundary problems which are solved numerically by the Galerkin/finite element method. On one hand, we found that capillary bridges become unstable at a turning point bifurcation in their solution space. The solutions past the turning point are unstable and the instability signals the bridge rupture. On the other hand, the separate droplets approach each other as the applied voltage increases. However, solutions become unstable past a critical voltage at a turning point bifurcation and the droplets join. By studying the relative position of the turning points corresponding to bridge rupture and droplet joining, respectively, we define parameter regions where stable bridges or separate droplets or oscillations between them can be realized.     

A conductance study of reducing volume liquid bridges

This work demonstrates how electrical conductance measurements can be employed for the study of liquid bridge behavior when their volume varies with time while their separation distance remains constant. The liquid bridges are edge pinned between two vertical, identical rods (r-bridges) at varying separation distances. Liquid evaporation is used as a means of reducing the bridge volume in a continuous smooth fashion. A zero-order continuation sequence with respect to Bond number and liquid bridge volume is combined with the shooting method for the solution of the Young-Laplace equation to give the liquid bridge shape as a function of its instantaneous volume. A novel, very efficient computational scheme is developed based on singular perturbation expansion for the solution of the Laplace equation in the liquid bridge to compute its electrical conductance that proved faster by orders of magnitude compared to other alternative approaches. The potential for estimating the liquid bridge characteristics or the evaporation rate by matching the experimental and theoretical results is discussed extensively.     

Holographic generation of bubble gratings at liquid—glass interfaces and the dynamics of bubbles on surfaces

Intense diffraction from a periodic array of microscopic bubbles is reported. The bubbles are generated by 100 ps, 1.06 μm pulses from a Nd:YAG laser which are crossed at a liquid—dielectric interface. The time dependence of the diffraction yields information on surface bubble expansion, contraction, and migration. The mechanism for the production of the bubble gratings is described.     

Ionic physisorption on bubbles induced by pulsed ultra-sound

Ion flotation processes involve the use of bubbles in order to separate ionic species from a mixed solution. Due to bubble interfaces we may assume null curvature at the molecular scale, where selective ion adsorption might be more easily investigated than with liquid-liquid extraction. In contrast to a classical flotation set-up, where bubbles are introduced via a glass frit, we use here a controlled sono-device generating cavitation bubbles which are initially absolutely clean. Moreover we have a faster process with a smaller device. The liquid phase resulting from the coalescence of the overflowing foam is enriched in some ions versus the initial brine. We show here that this effect follows the Hofmeister series and can be attributed to a weak adsorption of hydrated ions at the surfactant-water interface. The selectivity of alkali metals physisorbed at interfaces is analysed through the concentrations of competing ions remaining in solution by inductively coupled plasma optical emission spectrometry. Cationic selectivity, which is independent of the method for obtaining a foam, is discussed via the Gibbs free energy difference for bulk to hydrated surfactant monolayer. Relative values of effective adsorption energies are determined versus sodium ions taken as reference and correspond to 1-3% of the total hydration free energy.     

Gas bubbles in viscous liquids and melts

The basic set of equations and boundary conditions for viscous liquids and melts with gas has been derived. The flux of bubbles in the space of sizes, distribution of bubbles with respect to size, number of bubbles in the unit volume, and swelling of the system of viscous gas with liquid have been found. The initial and intermediate stages of the diffusion decomposition of the metastable system of viscous liquid with gas are considered.     

Temperature-controlled 'breathing' of carbon dioxide bubbles

We report a microfluidic (MF) approach to studies of temperature mediated carbon dioxide (CO(2)) transfer between the gas and the liquid phases. Micrometre-diameter CO(2) bubbles with a narrow size distribution were generated in an aqueous or organic liquid and subsequently were subjected to temperature changes in the downstream channel. In response to the cooling-heating-cooling cycle the bubbles underwent corresponding contraction-expansion-contraction transitions, which we term 'bubble breathing'. We examined temperature-controlled dissolution of CO(2) in four exemplary liquid systems: deionized water, a 0.7 M aqueous solution of NaCl, ocean water extracted from Bermuda coastal waters, and dimethyl ether of poly(ethylene glycol), a solvent used in industry for absorption of CO(2). The MF approach can be extended to studies of other gases with a distinct, temperature-dependent solubility in liquids.     

Dissolution of carbon dioxide bubbles and microfluidic multiphase flows

We experimentally study the dissolution of carbon dioxide bubbles into common liquids (water, ethanol, and methanol) using microfluidic devices. Elongated bubbles are individually produced using a hydrodynamic focusing section into a compact microchannel. The initial bubble size is determined based on the fluid volumetric flow rates of injection and the channel geometry. By contrast, the bubble dissolution rate is found to depend on the inlet gas pressure and the fluid pair composition. For short periods of time after the fluids initial contact, the bubble length decreases linearly with time. We show that the initial rate of bubble shrinkage is proportional to the ratio of the diffusion coefficient and the Henry's law constant associated with each fluid pair. Our study shows the possibility to rapidly impregnate liquids with CO(2) over short distances using microfluidic technology.     

Shearing of nanoscopic bridges in two-component thin liquid layers between chemically patterned walls

Bridge phases associated with a phase transition between two liquid phases occur when a two-component liquid mixture is confined between chemically patterned walls. In the bulk the liquid mixture with components A, B undergoes phase separation into an A-rich phase and a B-rich phase. The walls bear stripes attractive to A. In the bridge phase A-rich and B-rich regions alternate. Grand canonical Monte Carlo studies are performed with the alignment between stripes on opposite walls varied. Misalignment of the stripes places the nanoscopic liquid bridges under shear strain. The bridges exert a Hookean restoring force on the walls for small displacements from equilibrium. As the strain increases there are deviations from Hooke's law. Eventually there is an abrupt yielding of the bridges. Molecular dynamics simulations show the bridges form or disintegrate on time scales which are fast compared to wall motion and transport of molecules into or from the confined space. Some interesting possible applications of the phenomena are discussed.     

Enhancement of sonochemical reaction rate by addition of micrometer-sized air bubbles

The sonochemical reaction rate has been enhanced by the introduction of tiny air bubbles. The bubbles including micrometer-sized ones are produced by method of atomization and are introduced into aqueous luminol solution under 141-kHz sonication in order to investigate the enhancement of sonochemical reaction rate by introduction of tiny bubbles through the intensity measurement of sonochemiluminescence (SCL). It is shown that the introduction of tiny bubbles under sonication accomplishes the large SCL intensity compared to the cases of sonication only and liquid flow under sonication. It is also shown that it is important to adjust the configuration of tiny-bubble addition to the sound field. Through the investigations on the intensity and the spatial pattern of luminol-SCL, it has been clarified that tiny bubbles added into the sonicated liquid not only cause the liquid flow but also increase the number of collapsing bubbles active for sonochemical reaction. It is also shown that the tiny-bubble addition enhances the reaction rate of KI oxidation under sonication. Therefore, the present method of introduction of tiny bubbles is effective for enhancement of sonochemical reaction rate.     

Sliding variability of droplets on a hydrophobic incline due to surface entrained air bubbles

The ability of a liquid droplet to move on an incline has important ramifications in discrete volume fluidic devices. By taking advantage of the spontaneous and copious formation of visible air bubbles within water droplets on a polytetrafluoroethylene (PTFE) surface, we uncovered a direct correlation between their presence and the ability of droplets to slide down an incline. We forward two possible mechanisms to account for this behavior. The first is attributed to the air bubbles creating regions where additional solid-liquid-vapor phase interfaces are present; wherein due to the buoyancy force acting upwards, the orientation of the contact angles of each bubble (which should also be in hysteresis but in the opposite direction of the hysteresis at the droplet rim contact lines) dictate that the net force of the bubbles in the droplet act down an incline. We show here that this mechanism cannot fully account for the bubble enhanced sliding behavior. The second mechanism is based on the occurrence of the droplet front advancing first, causing the droplet to elongate and thus allowing the receding contact line to partially sweep inwards over the bubbles. This causes a series of point-wise disruptions on the contact line that permits the droplet to slide down more readily. The relatively short time of ～180s during which these micron sized bubbles decrease in size indicates a possibility of this mechanism contributing to a transient means to reduce the retention force of droplets that reside on hydrophobic surfaces.     

Particles driven up the wall by bursting bubbles

The phenomenon of particles being "driven up the wall" of a vessel by bursting bubbles at an air-water interface covered with hydrophobic nanoparticles is reported. Experiments have shown that the bubbles bursting at the interface give rise to the local surface pressure gradient, which pushes the particles to climb and coat the walls of the vessel. A theoretical model based on the lubrication approach to estimate the height and speed at which the particle layers climb up the walls yields values that are in fair agreement with the experimental measurements. The effects of the liquid viscosity, electrolyte strength, and particle wettability are also examined.