An acoustic levitation technique for measuring the rheology of foam near the order–disorder transition

Abstract

Understanding the rheological behavior of foams is important as a basic problem in fluid physics, and as a practical problem in many industries. Foams are tremendously important in a variety of applications. The most important quality of a foam in many of these applications is its response to imposed strain, or its rheological behavior. Yet there exists almost no experimental data on the rheological properties of real 3-D foams. This is due in large part to the earth-based requirements for contact containment, and to the fact that gravity-induced drainage quickly destroys all but the ‘‘driest’’ foams. A unique method to provide noncontact control and manipulation of foam samples, via acoustic levitation, is described. This levitation technique, coupled with low-gravity conditions, will enable investigation of dry (ordered) and wet (disordered) foams. The analysis which will allow determination of a foam’s yield stress, effective bulk modulus, and effective bulk viscosity from the observed response of a foam sample to quasistatic, step function, and periodic acoustic and ambient pressure changes is outlined. Experimental results will be presented for foams experiencing small deformations. The results will be analyzed to determine if the foam may be modeled as a viscoelastic solid. [Work supported by NASA.]