Foam Rheology: III. Measurement of Shear Flow Properties

Abstract

The experimental study of foam is complicated by its inherently unstable nature and by the presence of a liquid film slip layer at the wall. A method is described for generating reproducible and stable foam capable of retaining its structure for prolonged times. An experimental technique has also been devised which eliminates problems associated with wall slippage and allows measurement of material functions without use of any empiricisms for the wall region. In a steady‐shear flow, foam behaves like a Bingham plastic with a viscosity inversely proportional to shear rate indicating the presence of a yield stress τ0. The value of the viscosity, which is significantly higher than the parent liquid viscosity, is an increasing function of gas volume fraction φ. Yield stress values obtained by extrapolating viscosity versus shear stress data agree with direct τ0 measurements obtained by using a stress relaxation technique. The yield stress is also found to increase with φ. Small amplitude oscillatory shearing experiments show foam to behave as an elastic solid for small deformations. This is evident from its frequency independent moduli, very small phase shift between strain input and stress output, and the elastic modulus's being much larger than the loss modulus. Stress growth experiments at the start‐up of steady‐shear flow reveal stress overshoots not larger than 30%. The transient viscosity divided by the steady‐state viscosity is found to be insensitive to shear rate, but is an increasing function of φ.