Effect of Shear on Pumped Capillary Foams

Foam flow in many applications, like firefighting and oil recovery, requires stable foams that can withstand the stress and aging that result from both shear and thermodynamic instability. Events of drainage and coarsening drive the collapse of foams and greatly affect foam efficacy in processes relying on foam transport. Recently, it was discovered that foams can be stabilized by the synergistic action of colloidal particles and a small amount of a water-immiscible liquid that mediates capillary forces. The so-called capillary foams contain gas bubbles that are coated by a thin oil-particle film and integrated in a network of oil-bridged particles; the present study explores how this unique architecture impacts the foams' flow dynamics. We pumped capillary foams through millimeter-sized tubing (ID: 790 μm) at different flow rates and analyzed the influence of stress and aging on capillary foam stability. We find that the foams remain stable when pumped at higher flow rates but undergo phase separation when pumped at low flow rates. Our observations further show that the particle network is responsible for the observed stability in capillary foams and that network strength and stability of an existing foam can be increased by shearing.