Coalescence Studies

Droplet or bubble coalescence is a highly dynamic process involving different length scales. Adsorbed surfactants or the presence of particles strongly affect the hydrodynamics and film stability during coalescence.

Motivation

Coalescence is a prevalent effect in industrial processes, for example in bubble columns or in the settler unit of liquid-liquid extraction. Bubble coalescence influences bubble size distribution, velocity, gas holdup, and interfacial transfer rate. Despite extensive research, the impact of different adsorbed substances and particles on film drainage and bubble coalescence remains unclear.

Goals

• Controlled film stability and rupture experiments
• Spatiotemporal film thickness evolution for flotation-relevant surfactant systems
• Robust interferogram evaluation
• Stability and rupture of a liquid film surrounded by air under the presence of hydrophobic particles

Techniques

• Thin-film interferometry
• Scheludko cells
• Bubble collision setup

Results

In foam fractionation of proteins, foam coalescence accelerates the liquid drainage leading to higher protein enrichment. 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.

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.

Keshavarzi, B., Krause, T., Sikandar, S., Schwarzenberger, K., Eckert, K., Ansorge-Schumacher, M. B., & Heitkam, S. (2022). Protein enrichment by foam fractionation: experiment and modeling. Chemical Engineering Science, 256, 117715.