Liquid and particles retention in foamed suspensions

This article describes a new strategy to modify the drainage behaviour of aqueous foams with solid nanoparticles. While for high particulate concentrations the liquid flow can be slowed down owing to viscosity effects, we show that much less concentrated systems, but possessing peculiar physicochemical properties, can also strongly modify the drainage behaviour. Taking advantage of the agglomerated state of pyrogenic silica in aqueous media, highly porous macro-particles are specifically prepared in the initial foaming solutions. It is shown that their typical size and their intrinsic yield-stress properties enable them to be captured by the bubbles during the foam generation stage and subsequently to be retained in the foam channels, according to a geometrical criterion based on the particle/bubble sizes ratio. The drainage curves of the foams display significant retention rates for the suspension during several hours, an effect that is enhanced for higher particles concentrations. This has been related to the high internal porosity and hydrophilic character of the macro-particles, acting as liquid-traps in the foam microstructure.