The addition of a small amount of a nonwetting immiscible fluid to a suspension can drastically alter its rheological properties.

When a small amount (less than 1%) of a second immiscible liquid is added to the continuous phase of a suspension, the rheological properties of the admixture are dramatically altered and it can change from a fluid-like to a gel-like state. The rheological properties can be modified through changes in the particle size, temperature induced changes to the interfacial tension, and through the addition of surfactants. These changes are experimentally investigated in this current work, concentrating on the capillary (non-wetting) state with additional experiments in the pendular (wetting) state. The yield stress scales with the interfacial tension and reciprocal radius (Γ/r) as expected due to the capillary nature of the attractive force leading to network formation and gelation. The addition of surfactants leads to a drastic reduction in the yield stress and viscosity, which is attributed to a partial trapping of the secondary fluid within surfactant micelles.

The mechanical properties of a suspension can be dramatically altered by adding a small amount of a secondary fluid that is immiscible with the bulk phase. The substantial changes in the strength of these capillary suspensions arise due to the capillary force inducing a percolating particle network. Spatial information on the structure of the particle networks is obtained using confocal microscopy. It is possible, for the first time, to visualize the different types of percolating structures of capillary suspensions in situ. These capillary networks are unique from other types of particulate networks due to the nature of the capillary attraction. We investigate the influence of the three-phase contact angle on the structure of an oil-based capillary suspension with silica microspheres. Contact angles smaller than 90° lead to pendular networks of particles connected with single capillary bridges or clusters comparable to the funicular state in wet granular matter, whereas a different clustered structure, the capillary state, forms for angles larger than 90°. Particle pair distribution functions are obtained by image analysis, which demonstrate differences in the network microstructures. When porous particles are used, the pendular conformation also appears for apparent contact angles larger than 90°. The complex shear modulus can be correlated to these microstructural changes. When the percolating structure is formed, the complex shear modulus increases by nearly three decades. Pendular bridges lead to stronger networks than the capillary state network conformations, but the capillary state clusters are nevertheless much stronger than pure suspensions without the added liquid.