Coexistence of liquid and solid phases in flowing soft-glassy materials.

Magnetic-resonance-imaging rheometrical experiments show that concentrated suspensions or emulsions cannot flow steadily at a uniform rate smaller than a critical value (gamma(c)). As a result, a "liquid" region (sheared rapidly, i.e., at a rate larger than gamma(c)) and a "solid" region (static) coexist. The behavior of the fluid in the liquid region follows a simple power-law model, while the extent of the solid region increases with the degree of jamming of the material.

Capillary condensation in a fractal porous medium.

Small-angle x-ray and neutron scattering are used to characterize the surface roughness and porosity of a natural rock which are described over three decades in length scales and over nine decades in scattered intensities by a surface fractal dimension D = 2.68+/-0.03. When this porous medium is exposed to a vapor of a contrast-matched water, neutron scattering reveals that surface roughness disappears at small scales, where a Porod behavior typical of smooth interfaces is observed instead. Water-sorption measurements confirm that such interface smoothing is due predominantly to the water condensing in the most strongly curved asperities rather than covering the surface with a wetting film of uniform thickness.

A NMR investigation of adsorption/desorption hysteresis in porous silica gels.

The purpose of this study is to investigate possible differences in geometry and connectivity of the liquid phase in a partially water-satured porous medium between the adsorption and the desorption branches, using a series of silica gels. This was done by comparing the T1 and T2 relaxation times (in 1H and 2H nuclear magnetic resonance (NMR) relaxation) as well as the water self-diffusion coefficient D (in 1H) along the two branches of the adsorption/desorption isotherms. The results show that the two relaxation times and the diffusion coefficient are strongly dependent on the water content (saturation level). When plotted in normalized coordinates, the T1 and T2 vs. P/Po curves fit closely the adsorption/desorption isotherms, which validates the two-population, fast-exchange model. Furthermore, because at equivalent saturation levels, there is no difference between the relaxation times and diffusion coefficients obtained along the adsorption branch and those obtained along the desorption branch, one is led to the conclusion that despite different equilibrium conditions, the geometry and connectivity of the liquid phase are statistically the same along the two branches.