Wet foam coarsening: More than film permeability

Foams with high liquid fractions, wet foams, are frequent in industrial applications. Understanding their stability is key to achieving control over foamy material design. Wet foams are difficult to study on ground as they rapidly destabilise through gravity-driven drainage. Experiments in microgravity avoid this drainage and allow to isolate the foam coarsening process, where bubble size grows as gas transfers from smaller to larger bubbles. We have studied the impact of stabiliser chemistry on coarsening with various surfactants and a protein, and over a range of liquid fractions. We measure coarsening rates that vary by almost a factor ten depending on the stabiliser. For moderately wet foams, we can rationalise the changing rates with surface tension and film permeability measured in independent experiments on ground. Once the foams become wetter and approach the volume fraction of random close packing, other stabiliser effects become evident. Molecules which exhibit strong adhesion between bubbles modify foam structure with larger films, resulting in faster than predicted coarsening rates. If the surface permeability is very low, coarsening is also faster than predicted, as some of the gas transfers outside of foam films. These results allow us to rationalise the multiple impacts of stabiliser chemistry on foam coarsening.