Mobile Surface Charge Can Immobilize the Air/Water Interface.

Recent Surface Force Apparatus measurements on thin film drainage as a bubble approaches a surface are reinterpreted in terms of a new model for the air/water interface. In this model, surface charge at the interface can be convected and can diffuse along the surface as the film drains. This creates surface tension gradients, since surface tension includes a charge-dependent contribution from the double-layer free energy. Although this electrocapillary effect is relatively small, we show here that the gradients are large enough to create Marangoni effects that influence the hydrodynamic flow in real systems. The new model can account for observed changes in hydrodynamic boundary condition from mobile initially to immobile during early stages of film drainage, and back to partially mobile as drainage progresses. Experimental film profiles for a millimeter-size bubble in 1 mM KCl solution driven toward a mica surface at 27 µm/s are reasonably well described with this mobile surface charge model. At longer times, there are still features of the experimental data that remain to be explained, which suggests further modeling is warranted.

The influence of pH on the interfacial behaviour of Quillaja bark saponin at the air-solution interface.

The interfacial behavior of surfactants present in a natural extract from Quillaja saponaria Molina bark at the air-solution interface is studied by measurements of interfacial tension, interfacial elasticity, and interfacial reflectance FTIR spectroscopy. The active molecule, saponin, is observed directly at the air-solution interface (via reflectance FTIR spectroscopy) above and below the pKa of the molecule, and spectra confirm the altered charge of the interfacial layer at the two solution conditions. For all concentrations of saponin studied, and at pH values below and above pKa (i.e. pH 3 and 7), a reduction in interfacial tension as a function of time is observed, with some differences in early time-scale adsorption and with lower values of quasi-equilibrium interfacial tension for pH 3. The interfacial layer is seen to be elastic, as determined from measurements of hydrostatic expansion, with some variation at the two pH values, and as a function of concentration. In addition to interfacial layer characterisation, the interaction between two air-solution interfaces is probed using bubble collisions with an air-solution interface. This experiment allows for observation of thin film drainage kinetics and determination of the final foam film thickness for the case when one of the interfaces is at equilibrium while the dynamic adsorption layer is being established at the other. This is the first time when the interactions between such interfaces (i.e. only one being at equilibrium) have been studied. This is of particular importance for the formation stage of foams, during which time many of the interfaces are not at equilibrium. When two interfaces interact across a thin liquid film, pH is seen to significantly influence foam film thickness.

Variation of Local Surface Properties of an Air Bubble in Water Caused by Its Interaction with Another Surface.

Surface and hydrodynamic forces acting between an air bubble and a flat mica surface in surfactant-free water and in 1 mM KCl solution have been investigated by observing film drainage using a modified surface force apparatus (SFA). The bubble shapes observed with the SFA are compared to theoretical profiles computed from a model that considers hydrodynamic interactions, surface curvature, and disjoining pressure arising from electrical double layer and van der Waals interactions. It is shown that the bubble experiences double-layer forces, and a final equilibrium wetting film between the bubble and mica surfaces is formed by van der Waals repulsion. However, comparison with the theoretical model reveals that the double-layer forces are not simply a function of surface separation. Rather, they appear to be changed by one of more of the following: the bubble's dynamic deformation, its proximity to another surface, and/or hydrodynamic flow in the aqueous film that separate them. The same comments apply to the hydrodynamic mobility or immobility of the air-water interface. Together the results show that the bubble's surface is "soft" in two senses: in addition to its well-known deformability, its local properties are affected by weak external forces, in this case the electrical double-layer interactions with a nearby surface and hydrodynamic flow in the neighboring aqueous phase.

Effect of disjoining pressure on terminal velocity of a bubble sliding along an inclined wall.

The influence of salt concentration on the terminal velocities of gravity-driven single bubbles sliding along an inclined glass wall has been investigated, in an effort to establish whether surface forces acting between the wall and the bubble influence the latter's mobility. A simple sliding bubble apparatus was employed to measure the terminal velocities of air bubbles with radii ranging from 0.3 to 1.5 mm sliding along the interior wall of an inclined Pyrex glass cylinder with inclination angles between 0.6 and 40.1°. Experiments were performed in pure water, 10 mM and 100 mM KCl solutions. We compared our experimental results with a theory by Hodges et al. which considers hydrodynamic forces only, and with a theory developed by two of us which considers surface forces to play a significant role. Our experimental results demonstrate that the terminal velocity of the bubble not only varies with the angle of inclination and the bubble size but also with the salt concentration, particularly at low inclination angles of ∼1-5°, indicating that double-layer forces between the bubble and the wall influence the sliding behavior. This is the first demonstration that terminal velocities of sliding bubbles are affected by disjoining pressure.

Coalescence map for bubbles in surfactant-free aqueous electrolyte solutions.

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.

Inhibition of bubble coalescence: effects of salt concentration and speed of approach.

Bubble coalescence experiments have been performed using a sliding bubble apparatus, in which mm-sized bubbles in an aqueous electrolyte solution without added surfactant rose toward an air meniscus at different speeds obtained by varying the inclination of a closed glass cylinder containing the liquid. The coalescence times of single bubbles contacting the meniscus were monitored using a high speed camera. Results clearly show that stability against coalescence of colliding air bubbles is influenced by both the salt concentration and the approach speed of the bubbles. Contrary to the widespread belief that bubbles in pure water are unstable, we demonstrate that bubbles formed in highly purified water and colliding with the meniscus at very slow approach speeds can survive for minutes or even hours. At higher speeds, bubbles in water only survive for a few seconds, and at still higher speeds they coalesce instantly. Addition of a simple electrolyte (KCl) removes the low-speed stability and shifts the transition between transient stability and instant coalescence to higher approach speeds. At high electrolyte concentration no bubbles were observed to coalesce instantly. These observations are consistent with recent results of Yaminsky et al. (Langmuir 26 (2010) 8061) and the transitions between different regions of behavior are in semi-quantitative agreement with Yaminsky's model.