1999-06-10
11:15 at CAB D28

Foam Microrheology: from honeycombs to random foams

A.M. Kraynik, D.A. Reinelt

Sandia National Laboratories, Albuquerque, New Mexico

 Foam covers a broad range of materials from shaving cream to the flexible polyurethane that cushions our seats. This lecture will focus on two decades of research on the rheology of liquid foam from a micromechanical point of view. These highly structured, multiphase fluids exhibit rich rheological response--shear modulus, yield stress, non-Newtonian viscosity--that can be related to geometry and mechanics at the cell level. Computer-generated animations based on the simple liquid honeycomb in 2D will be used to illustrate cell-level mechanisms that are also important in 3D. These include energy storage in expanding surfaces that cause elasticity, irreversible topological transitions within the foam structure that produce yield phenomena, and the interplay between cell distortion and film-level viscous flow that is responsible for viscoelastic relaxation. Static 3D structures ranging in complexity from the Kelvin cell to Weaire-Phelan and random polydisperse foams are calculated with the Surface Evolver, an interactive computer program developed by Brakke. Excellent agreement with experimental data on foam structure and shear modulus will be demonstrated. The geometry and rheological consequences of Plateau borders in wet foams will also be described. The structures of random liquid foam are being used as templates to develop micromechanical models of cellular solids. The fluid mechanics of bubbles growing in a viscous fluid reveals the evolution of foam structure that controls behavior in the solid state. The micromechanical approach that will be described has established a firm theoretical foundation for developing structure-property-processing relationships for foamed polymers. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under contract #DE-AC04-94AL85000.