Bubbles pinned on electrodes: Friends or foes of aqueous electrochemistry?

Electrochemists and engineers regard adherent gas bubbles as redox-inactive and therefore blocking entities. Adhesion of bubbles at electrodes generally carries an energy penalty. But this is not always the case: bubbles pinned on an electrode surface initiate the oxidation of water-soluble species under conditions where such reactions would normally be considered impossible. Here we critically review the recent literature that is beginning to unveil the novel concept of on-water electrochemistry. Harnessing electrochemical reactivity of the water–gas–electrode interface has the potential to become a game-changer in organic electrosynthesis, accelerating the transition toward a sustainable chemical industry by simplifying the direct integration of renewable electricity into the production of commodity chemicals.     

Pressure waves in a tube filled with a bubbly mixture with a nonuniform cross-sectional bubble distribution

The evolution of pressure waves in a tube filled with a gas-liquid medium with a stepped cross-sectional bubble distribution is investigated. The calculations are compared with experimental data. It is shown that in the case of a nonuniform bubble distribution, due to the appearance of transverse flows, the pressure pulse is damped faster than for a uniform distribution. The interaction of pressure waves with a bubble cluster in a tube filled with liquid is also analyzed.     

Dynamic fluid flow behaviour of a tank draining through a vertical tube

This paper describes the unsteady draining of a sealed tank partially filled with water. The water discharges via a vertical tube into an open tank at atmospheric conditions. The air inflow, compensating for the volume of the discharged liquid, enters the system in an oscillatory manner, much like the “gulping” seen in an upended beer bottle. A mathematical model, based closly on that derived by Dougall & Kathiresan [Chem. Engng Commun. 8, 289–304 (1981)], has been applied to predict the pressure fluctuations in the closed tank. The rate of water discharge from the tank has been predicted and gives a much closer agreement with experimental results than a prediction based on a steady counter-current flooding limitation approach. A drift flux model has been used to describe the two-phase flow effect in the tube and the Wallis flooding criterion has been modified for use in the slug flow regime to describe the boundary conditions at the bottom of the tube. The pressure fluctuations in the sealed tank have been measured and compared with results obtained from the mathematical prediction for a variety of tube diameters.     

The Motion of Long Bubbles in a Network of Tubes

The motion and interaction of discrete bubbles in porous materials is studied numerically using a network model. The goal is to extend analytical results for the motion of bubbles through a single straight tube to a more realistic geometry for porous materials, modeled here as a planar network of straight tubes of different radii. The problem is characterized by two dimensionless parameters, the capillary number (Ca) and the volume fraction of bubbles (); results are characterized by determining the effective permeability of the network and the mean residence time of bubbles in the material. The simulations indicate that at low volume fraction most of the bubbles follow a limited number of high-flow pathways through the network. In this case the predictions of our simulations can be approximated by a simple analytical model. Bubbles interact with each other because their presence changes the local resistance to flow in individual tubes. As increases, interactions between individual bubbles become important resulting in a wider range of residence times in the porous material.     

Pressure Waves in a Gas–Liquid Medium with a Stratified Liquid–Bubbly Mixture Structure

Evolution and decay of pressure waves of moderate amplitude in a vertical shock tube filled by a gas–liquid medium with a nonuniform (stepwise) distribution of bubbles over the tube cross section are studied experimentally. The gas–liquid layer has the form of a ring located near the tube wall or the form of a gas–liquid column located in the center of the tube. It is shown that the nonuniformity of bubble distribution over the tube cross section increases the attenuation rate of pressure waves.     

Waves in partially saturated porous media

The propagation of compressional waves in a porous medium is investigated in case the pore liquid contains a small volume fraction of gas. The effect of oscillating gas bubbles is taken into account by introducing a frequency-dependent fluid bulk modulus, which is incorporated in the Biot theory. Using a shock tube technique, new experimental data are obtained for a porous column subjected to a pressure step wave. An oscillatory behaviour is observed, consisting of two distinct frequency bands, which is predicted by the theoretical analysis.     

Secondary Acoustic Waves in a Polydisperse Bubbly Medium

Wave interaction of bubbles in a cavitation cluster is studied theoretically and experimentally. It is found that two groups of bubbles with different phases of oscillations and collapse time are formed at the early stage of bubblecluster formation. It is shown that small bubbles accumulating energy in the initial pulsed acoustic wave collapse in the field of internal positive pressure of the cluster. A secondary compression wave generated by inertial expansion of large bubbles is determined by means of numerical simulation and registered experimentally. The amplitude of this wave is comparable to the amplitude of the initial pulse. A decrease in the oscillation period and strengthening of the microbubble collapse are observed in the cluster under pulsed compression.     

Wetting and nonwetting fluid displacements in porous media

The understanding of simple laminar flow in tubes has often been used to interpret the more complicated flow in porous media. A study of the motion of two immiscible liquids in closed tubes with relatively large diameter (> 0.3 cm i.d), was conducted in order to examine the influence of wetting and nonwetting liquids on the flow behavior. The results indicate that the wetting properties of the fluids with regard to the tube wall have a major efffect on the formation and motion of long bubbles. A physically based model was used to predict the velocity and the conditions for no motion of bubbles and drops in tubes. These results were used to interpret the nature of oil and water flow in porous media. Experiments in which the wetting liquid was displaced by the nonwetting, or vice versa, were conducted by injecting the displacing liquid at a constant flux at the center of a two-dimensional chamber saturated with the displaced liquid. The influence of wetting-nonwetting characteristics on the quantity of liquid displaced, the shape of the interface between the two liquids, and the interpretation of the no motion radius in a closed tube to the case of a porous medium are discussed. It would appear that the no motion radius gives a good indication of the minimum width of a nonwetting penetrating finger and the maximum width of nonwetting ganglia left by drainage.     

Flow modulation of a planar free shear layer with large bubbles—direct numerical simulations

The flow of a planar free shear layer with cylindrical bubbles is simulated using a finite difference/front tracking scheme. This approach allows direct numerical simulation of the multiphase flow by wholly incorporating the local bubble flow field in conjunction with the large scale vortical structures of the liquid. The role of large bubbles in modifying low Reynolds number ( 250) shear flow structures is investigated, specifically for bubbles whose diameter approaches the scale of the largest liquid eddies. The results indicate that duration of eddy crossing is the main mechanism for flow modulation, which is typically characterized by decreased vortex coherency and size, modified fluctuation statistics and significant variations in pairing/merging phenomena. The comparison of fluctuating statistics and flow field visualization also allowed qualitative discrimination between the modulation of the non-linear eddy dynamics and fluctuations due simply to the random bubble induced perturbations.