Effect of Dynamic Surface Tension on Foam Flow Through Fibrous Materials

Abstract

A new experimental technique and method of analysis have been developed to study the flow of liquid foams in fiber networks. Foam is injected into the center hole of a compressed mat and the resulting radial spreading pattern is observed through the top transparent plate. The existence of a minimum critical pressure gradient for foam flow in porous media is demonstrated as the foam front spreads, slows down, and eventually stops. The radial position at which it stops is a function of the inlet pressure. A two-parameter fluid model is proposed that incorporates "critical pressure gradient" and fluid mobility parameters to describe the interaction of the foam with the porous material. From the model, radial pressure profiles are generated that predict a qualitative difference between the pressure distribution and spreading rate of a fluid with yield behavior (that is, a minimum critical pressure gradient) compared to the flow of a Newtonian fluid. From the position at which the foam front stops, the critical pressure gradient for foam mobilization is quantified. By numerically integrating the equation for the frontal advance rate and using experimental data, a value is obtained for the foam mobility. The mobility is observed to increase with increasing foam quality and remain relatively independent of the bubble size of the foam injected into the mat. The effects of dynamic surface tension on foam flow are also shown. Foams were generated from aqueous surfactant solutions at different concentrations, each with the same equilibrium surface tension but vastly different dynamic surface tensions. As the dynamic surface tension increased, foam breakdown occurred, resulting in a significant region of liquid saturation within the mat. This effect was attributed to the inability of the solutions with a higher dynamic surface tension to prevent film rupture and foam collapse.