Thin Films and Bubble–Bubble Coalescence

Motivation

Thin liquid films are the structural elements of foams and froths. Their drainage, stabilization, and rupture determine whether neighboring bubbles coalesce or remain separated. In froth flotation, this directly affects bubble size, gas dispersion, froth lifetime, liquid drainage, and ultimately the grade and recovery of the concentrate.

A stable froth is needed to transport valuable particles, but excessive froth stability can reduce selectivity by limiting drainage of entrained gangue particles. Understanding the physics of thin liquid films therefore provides a route to control froth performance while reducing chemical dosage and the environmental footprint of flotation reagents.

Goals

• Establish quantitative methods to measure the thickness and thinning dynamics of individual liquid films.
• Investigate how surfactant type, surfactant concentration, and electrolyte concentration influence film drainage and stability.
• Link molecular-scale film stabilization mechanisms to macroscopic froth behavior.
• Identify the conditions that promote film rupture or stabilization and therefore influence bubble–bubble coalescence.
• Extend film analysis toward more complex film geometries, including dimples, particles, and non-uniform drainage.

Techniques

• Scheludko-type thin-film cell with inverted microscopy. 
• Optical white-light interferometry and multi-color interferometry.
• Image-processing algorithms for film-thickness reconstruction.
• Simulation-based validation of interference-order assignment.

Results

We established an interferometric methodology to measure the thickness of single liquid films over a broad range, from approximately 100 nm to several micrometers, complemented by classical Scheludko analysis for thinner films below 100 nm. This allows the drainage process to be followed over the full transition from relatively thick films to common and Newton black films.

Electrolytes play a particularly important role for ionic surfactants. Salt addition changes the electrostatic double layer and strongly influences thinning behavior below approximately 100 nm. This affects the transition between common black films and Newton black films, and therefore the likelihood of stabilization or rupture. In contrast, non-ionic surfactant systems are less sensitive to salt because they do not rely on electrostatic double-layer stabilization.

Under the presence of particles, the film thickness profiles strongly depend on the time for hydrophobic particles to adsorb at the interface. The film thickness becomes highly irregular in that case and the film ruptures with a solid-like behavior. The adsorbed particles can therefore create a steric/mechanical barrier to coalescence.

The developed analysis approach provides a foundation for connecting thin-film behavior to foam and froth stability. Future work will extend the method to films containing particles and to more complex film shapes, enabling a more direct connection between bubble–bubble coalescence, bubble–particle attachment, and froth performance.

Publications

• Eftekhari, M., Schwarzenberger, K., Karakashev, S. I., Grozev, N. A., & Eckert, K. (2024). Oppositely charged surfactants and nanoparticles at the air-water interface: Influence of surfactant to nanoparticle ratio. Journal of Colloid and Interface Science, 653, 1388-1401.
• Götzelt, R. Experimental study of liquid film thinning behavior using interferometry. Diplomarbeit, TU Dresden, 2024.