Rate of Bubble Coalescence Following Dynamic Approach: Collectivity-Induced Specificity of Ionic Effect (original) (raw)
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Air-bubble coalescence in aqueous electrolytic solutions, following quasi-static approach, was studied in order to understand its slow rate in purified water and high rate in electrolytic solutions. The former is found to be due to surface charges, originating from the speciation of dissolved CO 2 , which sustain the electric double layer repulsion. Rapid coalescence in electrolytic solutions is shown to occur via two different mechanisms: (1) neutralization of the carbonaceous, charged species by acids; or (2) screening of the repulsive charge effects by salts and bases. The results do not indicate any ion specificity. They can be explained within the DLVO theory for the van der Waals and electric double layer interactions between particles, in contrast to observations of coalescence following dynamic approach. The present conclusions should serve as a reference point to understanding the dynamic behavior. A ir-bubble coalescence is frequently encountered in nature as well as in industry 1-37. In some cases, rapid coalescence is preferred, e.g. to avoid clogging of a system. In others, a relative slow coalescence is required, in order, for example, to keep the total interfacial area of the bubbles as high as possible. An example of a natural process is the formation of bubbles during breaking of seawater waves. This foam stays stable for a short time, during which the bubbles enhance the dissolution of oxygen and other weakly soluble gases 7,8. Eventually, the coalescence of the bubbles with the sea-air interface leads to transfer of gas and ionic aerosols to the atmosphere 5-7. Industrial examples include mass transfer processes in bubble columns, distillation towers, gas-liquid contactors, bioreactors, electrochemical cells etc. 8,9. Relatively slow bubble coalescence (seconds or minutes) in aqueous solutions containing certain concentrations of electrolytes, was apparently first observed in 1929 10. This phenomenon has been widely discussed 10-37 , since it appears to be in qualitative contradiction to the DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory 38 , which describes the stability of colloidal particles in a medium, by calculating the combined effects of van der Waals (vdW) attraction and electric double layer (EDL) repulsion between the particles. It predicts accelerated bubble coalescence in aqueous solutions of electrolytes compared with purified water. So far, only partial explanations of this apparent contradiction have been given, based on surface elasticity 2,15,17,39,40 , surface diffusion 41 , ion partitioning at the tested liquid-air interface 37,42,43 hydration and structural forces 26,44,45 , effective/ partial immobilization of interfaces, and deformation effects 46-49. Various electrolytes have been investigated over the years, using a variety of methods to study bubble coalescence behavior. Since bubble coalescence is actually a process of thin film rupture, it has been studied by methods that apply to either whole bubbles or isolated thin films: a bubble column 11-20 , pairs of bubbles 12,19,21-26 , bubbles rising to the interface 27-30 , a liquid thin-film between two gas-phases 31-33 , and two bubbles in Atomic Force Microscopy (AFM) 34-36. These studies have revealed the effect of electrolyte type and concentration on the rate of bubble coalescence, mostly when the approach of the coalescing bubbles is dynamic (approach velocity higher than 10 mm/sec). The transition between rapid coalescence (order of magnitude of milliseconds or seconds) and relatively slow coalescence occurs only with certain electrolytes and at specific concentrations (termed ''transition concentrations'' 14). Craig et al., using the bubble column method, empirically classified the ions of the electrolytes as type a or b. They found that aa and bb electrolytes in aqueous solutions slow bubble coalescence, while ab and ba electrolytes have no meaningful effect compared with purified water 14. This classification was supported also by the results
Molecular dynamics simulation is applied to investigate the effect of two ionic liquids (IL) on the nucleation and growth of (nano-)cavities in water under tension and on the cavities’ collapse following the release of tension. Simulations of the same phenomena in two pure water samples of different sizes are carried out for comparison. The first IL, i.e., tetra-ethyl ammonium mesylate ([Tea][Ms]), is relatively hydrophilic and its addition to water at 25 wt% concentration decreases its tendency to nucleate cavities. Apart from quantitative details, cavity formation and collapse are similar to those taking place in water, and qualitatively follow the Rayleigh-Plesset (RP) equation. The second IL, i.e., tetrabutyl phosphonium 2,4-dimethylbenzene sulfonate ([P4444 ][DMBS]), is amphiphilic, and forms nanostructured solutions with water. At 25 wt% concentrations, [P4444 ][DMBS] favours the nucleation of bubbles, that tend to form at the interface between water-rich and IL-rich domains. ...
arXiv: Soft Condensed Matter, 2020
Inhibition of bubble coalescence in electrolyte solutions enables the formation of oceanic whitecaps and affects the heat and mass transfer in many bubble related engineering processes. For different electrolytes, the ability to inhibit bubble coalescence correlates to the ion specificity at the air water interface at an abnormal cation-anion pair relationship, rather than the typically expected cation or anion series that was widely reported in atmospheric, bio- and chemical processes. Here we show that the inhomogeneous interfacial flow, at a different electrolyte concentration from the solution because of the surface specificity of both cation and anion, contributes to the bubble coalescence inhibition behavior in electrolyte solutions. The interfacial flow, achieved with the mobile air-water interface, contributes to the continuous change of electrolyte concentration within the liquid film formed between two colliding bubbles, thereby resulting in a concentration gradient of ele...
Bubble coalescence in electrolytes: Effect of bubble approach velocity
Chemical Engineering Journal, 2021
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Coalescence map for bubbles in surfactant-free aqueous electrolyte solutions
Advances in Colloid and Interface Science, 2011
Factors influencing bubble coalescence in surfactant-free aqueous electrolyte solutions are considered in this compilation of literature results. These factors include viscous and inertial thin film drainage, surface deformation, surface elasticity, mobility or otherwise of the air-water interface, and disjoining pressure. Several models from the literature are discussed, with particular attention paid to predictions of transitions between regions where behaviour is qualitatively different. The transitions are collated onto a single chart with salt concentration and bubble approach speed as the axes. This creates a map of the regions in which different mechanisms operate, giving an overall picture of bubble coalescence behaviour over a wide range of concentration and speed. Only mm-size bubbles in water and NaCl solutions are discussed in this initial effort at creating such a map. Data on bubble coalescence or non-coalescence are collected from the literature and plotted on the same map, generally aligning well with the predicted transitions and thus providing support for the theoretical reasoning that went into creating the coalescence map.
Electrolytes that Show a Transition to Bubble Coalescence Inhibition at High Concentrations
Journal of Physical Chemistry C, 2008
We have studied the coalescence of bubbles in electrolyte solutions by measuring the fraction of contacting bubble pairs that coalesce as a function of electrolyte concentration. At low concentrations, we have reproduced earlier results in the literature, but by extending our measurements to higher electrolyte concentrations, we have found that some electrolytes previously thought not to inhibit bubble coalescence do show a transition to coalescence inhibition at higher (>1 M) concentrations. These results suggest that coalescence inhibition should be studied over wider concentration ranges if full insight into the factors governing coalescence inhibition is to be obtained.
A quantitative review of the transition salt concentration for inhibiting bubble coalescence
Advances in colloid and interface science, 2014
Some salts have been proven to inhibit bubble coalescence above a certain concentration called the transition concentration. The transition concentration of salts has been investigated and determined by using different techniques. Different mechanisms have also been proposed to explain the stabilizing effect of salts on bubble coalescence. However, as yet there is no consensus on a mechanism which can explain the stabilizing effect of all inhibiting salts. This paper critically reviews the experimental techniques and mechanisms for the coalescence of bubbles in saline solutions. The transition concentrations of NaCl, as the most popularly used salt, determined by using different techniques such as bubble swarm, bubble pairs, and thin liquid film micro-interferometry were analyzed and compared. For a consistent comparison, the concept of TC95 was defined as a salt concentration at which the "percentage coalescence" of bubbles reduces by 95% relative to the highest (100% in ...
Coalescence of Bubbles with Mobile Interfaces in Water
Physical Review Letters
The fluid flow inside a thin liquid film can be dramatically modified by the hydrodynamic boundary condition at the interfaces. Aqueous systems can be easily contaminated by trace amounts of impurities, rendering the air-liquid interface immobile, thereby significantly resisting the fluid flow. Using high speed interferometry, rapid thinning of the liquid film, on the order of the collision speed, was observed between two fast approaching air bubbles in water, indicating negligible resistance and a fully mobile boundary condition at the air-water interface. By adding trace amounts of surfactants that changed the interfacial tension by 10 −4 N=m, a transition from mobile to immobile was observed. This provides a fundamental explanation why the bubble coalescence time can vary by over 3 orders of magnitude.