Yielding of complex liquid foams

Abstract

Foamy complex matter is increasingly encountered in various application fields. Whereas final functional properties of hardened foams have been widely investigated, rheology of complex foamy materials has received less attention. Here, we consider two different types of complex matter to be mixed with aqueous foam and we investigate the resulting yield stress, which is known to control both static and flow properties of foams: (1) A concentrated emulsion possessing intrinsic yield stress properties and (2) granular suspensions which are known to exhibit particle size effects when incorporated into foam. Yield stress of emulsion foams is found to be governed by both the Bingham-capillary number (i.e., the emulsion yield stress scaled by the bubble capillary pressure) and the volume fraction of interstitial emulsion. Foams made with small solid particles reveal granular packings confined between foam bubbles and the resulting yield stress is shown also to be governed by the Bingham-capillary number, here based on Mohr-Coulomb criterion applied to the interstitial granular material. Yield stress of foams made with large particles is well described by the theory developed from micromechanics of particle-loaded yield stress materials, suggesting that for such size ratios, foam can be considered as a continuous matrix. Within the range of intermediate size ratio values, the normalized foam yield stress undergoes an exponential decay that seems to be related to the number of bubbles separating neighboring particles. Finally, reported results are used to estimate the thicknesses of a foamed material, such as isolating foamed mortars, that can be deposited on vertical substrates.