Microfluidic Study of Gravity-Driven Drainage and Coalescence of Aqueous Two Dimensional Foams

Foams, a two-phase dispersion, are staples of the cosmetic, personal care, petroleum, pharmaceutical, and other industries. Central to these applications is the stability of the dispersion against separation. Foams break down by two mechanisms: the first is bubble coalescence, which is driven by the gravity drainage of the continuous phase. The drainage acts to push the bubbles against each other, and leads to the formation of thin lamellae, which break and cause the coalescence. The second is the mass transfer of the dispersed phase through the continuous phase, which is caused by the difference in pressures between the bubbles and droplets due to their size differences. This causes the smaller bubbles to disappear and the larger bubbles to grow (Oswald ripening or coarsening). Coalescence can be significantly retarded by the use of surfactants which adsorb onto the interfaces of the phases and create a disjoining pressure, stabilizing the thin lamellae. Surfactants have a much smaller effect on coarsening. However, colloidal particles, which also adsorb onto the surfaces of the bubbles and droplets, have been reported to retard both coalescence and coarsening. We developed a system using microfluidic flow-focusing devices to generate mono-disperse, vertical 2D foams and capture the drainage, coalescence and coarsening processes in high resolution. Our analysis yields a quantitative look at both the size gradient in plateau borders as the continuous phase drains and at the reduction in coalescence and coarsening rates in foams. This microfluidic system can be modified to be applied to particle-stabilized foams, emulsions, and Pickering emulsions