Marguerite Léang, Frédéric Ott, Frédérique Giorgiutti-Dauphiné, Ludovic Pauchard, Lay-Theng Lee
[ Article ]
Colloidal silica dispersions dried under controlled conditions form solid gels that display similar mechanical properties as those observed in several practical processes. An understanding of their structural characteristics and liquid flow properties can therefore help establish these gels as an alternative family of model materials to study porous media systems. Neutron radiography, a non-destructive technique well-adapted to study hydrogen-rich domains in porous materials due to the high attenuation power of hydrogen, is applied to study water imbibition in gels prepared from silica nanoparticles of radii 5 to 40 nm. From static transmission measurements and quantitative evaluation of the water content in the solid gels, three types of porosities are determined : total porosity that describes all volume not occupied by the silica ; effective porosity that contributes to liquid flow ; residual porosity that contains bound residual water – this residual water increases with decrease in particle size and constitutes an important fraction of the gel. Direct visualization of the dynamics of water imbibition by real-time imaging shows that imbibition obeys the Lucas-Washburn law, where the liquid front propagates as √t. From these measurements, the effective pore radius, tortuosity and permeability are evaluated. Quantitative analysis of the propagation profile shows a broadening of the liquid front with time and a broadening exponent that suggests elongated pores with reduced correlated liquid menisci. We highlight the role of particle size on water retention and particle organization and their impact on the mechanical resistance of the gel.