Influence of droplet breakup and particle agglomeration on the rheology of capillary suspensions

The rheological properties of a suspension can be dramatically altered by adding a small amount of a secondary fluid, which is immiscible with the bulk phase [1]. These capillary suspensions exist in two states: the pendular state where the secondary fluid preferentially wets the particles and the capillary state where the bulk fluid is preferentially wetting. In both cases, a percolating particle network arises leading to a vast increase in rheological properties due to network gelation. This phenomenon can be used to tune the rheological properties of suspensions, but is complicated by material properties and conditions during preparation. The mechanical properties depend on the size and distribution of secondary phase droplets during sample preparation, with smaller droplets leading to single particle bridging whereas larger droplets give rise to spherical particle agglomeration, which decreases the yield stress and shear modulus. Enhanced droplet breakup during sample preparation leads to stronger sample structures. In capillary state systems, this can be achieved by increasing the mixing intensity and time. In the pendular state, increased mixing intensity also leads to better droplet breakup but with longer times agglomeration is again favored. For each combination of particles and fluids, a strong network is formed in either the pendular or capillary state, but never both of them. This could be due to changes in droplet breakup caused by viscosity-mismatched fluids, but experiments with matched fluids show that this effect cannot be assigned to droplet breakup alone. In fact, the suspension properties without added secondary fluid also have a big influence on the possible structure buildup and the corresponding increase in rheological properties after secondary phase addition. [1] Koos, E., Willenbacher, N. Science 2011, 331, 897-900.status: publishe