Using capillary forces to control suspension rheology: Network structure, formation, and aging in capillary suspensions

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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 mixture are dramatically altered from a fluid-like to a gel-like state. Our research on this phenomenon was funded through DFG grant. We found that the yield stress and viscosity increase by several orders of magnitude as the volume fraction of the second fluid increases and we have shown that this transition is attributed to the capillary forces of the two fluids on the solid particles. In an analogy to wet granular materials, two distinct states are defined: the "pendular state" where the secondary fluid preferentially wets the particles; and the "capillary state" where the secondary fluid wets the particles less well than the primary fluid. We have found that both states are associated with a transition in the suspension from a fluid-like to gel-like state or from a weak gel to a strong gel. Our investigations have confirmed that capillary suspensions are a new class of materials that can be used to create tunable fluids, stabilize mixtures that would otherwise phase separate, and to create new materials such as low-fat foods or microporous foams. Recently, we have demonstrated that capillary suspensions offer a new, cost-efficient and versatile processing route for manufacturing porous ceramics with unprecedentedly high porosity at small pore sizes. This research proposal aims to answer fundamental questions about capillary suspensions and particularly about the nature of the sample-spanning networks formed in both the pendular and capillary state suspensions. This proposal intends to investigate the aging of capillary suspensions and changes to network structure on both the micro- and macroscopic scale. The microstructure of these networks including the particle-droplet configuration, floc (or cluster) size, pair distribution functions, and the dimension of the supposed fractal network structure will be characterized using imaging techniques. The strength of the macroscopic sample will be characterized using rheological techniques. The combination of these two techniques allows the direct correlation of changes in network morphology to the changes in network strength and flow behavior.