This review focuses on the current knowledge regarding (i) the mechanisms governing foamability and foam stability, and (ii) models for the foam column kinetics. Although different length scales of foam structure, such as air-water interface and liquid film, have been studied to elucidate the mechanisms that control the foamability and foam stability, many questions remain unanswered. It is due to the collective effects of different mechanisms involved and the complicated structures of foam sub-structures such as foam films, Plateau borders and nodes, and foam networks like soft porous materials. The current knowledge of the effects of solid particles on liquid film stability and foam drainage is also discussed to highlight gaps in our present level of understanding foam systems with solid particles. We also critically review and summarize the models that describe macroscopic foam behaviors, such as equilibrium foam height, foam growth and collapse, within the context of the mechanisms involved.

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We reviewed eight commonly used equilibrium adsorption models and examined their underlying assumptions, fitting qualities, and parameter stabilities. We compared several objective functions that have been applied to curve fitting analysis and a few statistics tests that have been performed to evaluate regression quality. The iteratively reweighted least squares algorithm was selected as the most suitable regression method for adsorption models in the presence of heteroscedasticity. The fraction of unexplained variance was selected to indicate the model fitting quality. Two sources of parameter instability were identified: residue instability and function instability. While the definition of the instability caused by residue is well established, we are the first to consider the instability caused by an adsorption model. The models discussed in this article can be applied to many surfactants, such as normal alcohols, polyglycol ethers, and sodium dodecyl sulfate at different salt concentrations. Our results show that both the model fitting quality and parameter instability increase with the number of parameters subject to curve fitting. For the Frumkin-type of reorientation model, the parameter instability can be as high as 25%. The high degree of instability in some complicated adsorption models may invalidate the estimated parameters.Therefore, additional measurements or simulations are required for complicated models to extract reliable model parameters.

Interfacial gas enrichment (IGE) of dissolved gases in water is shown to govern the strong attraction between solid hydrophobic surfaces of an atomic force microscopy (AFM) colloidal probe and a solid substrate. However, the role of IGE in controlling the attraction between fluid-fluid interfaces of foam films and emulsion films is difficult to establish by AFM techniques due to the extremely fast coalescence. Here, we applied droplet-based microfluidics to capture the fast coalescence event under the creeping flow condition and quantify the effect of IGE on the drainage and stability of water films between coalescing oil droplets. The amount of dissolved gases is controlled by partially degassing the oil phase.When the amount of dissolved gases (oxygen) in oil decreases (from 7.89 mg/L to 4.59 mg/L), the average drainage time of coalescence significantly increases (from 19 ms to 50 ms). Our theoretical quantification of the coalescence by incorporating IGE into the multilayer van der Waals attraction theory confirms the acceleration of film drainage dynamics by the van der Waals attractive force generated by IGE. The thickness of the IGE layer decreases from 5.5 nm to 4.9 nm when the amount of dissolved gas decreases from 7.89 mg/L to 4.59 mg/L. All these results establish the universal role of dissolved gases in governing the strong attraction between particulate hydrophobic interfaces.

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