Tuning suspension rheology using capillary forces

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. The yield stress and viscosity increase by several orders of magnitude as the volume of the second fluid increases. This transition is attributed to the capillary forces of the two fluids on the solid particles, and 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 find that both states are associated with a transition in the suspension from a fluid-like to gel-like state [1]. 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. Additionally, capillary suspensions have even recently been used to create porous ceramics with unprecedentedly high porosity at small pore sizes [2]. Since the capillary forces control the formation of the particle network, the strength of this network and hence the flow behavior of the suspensions can be widely tuned through the variation of the capillary force and the number of capillary bridges. Accordingly, the yield stress of the capillary suspensions scales with the inverse radius and the surface tension of the added liquid, which e.g. generally decreases with increasing temperature. The strength of these suspensions can be further adjusted through the addition of surfactants. Several nonionic surfactants with a range of HLB values, when added to the bulk fluid, all show that the yield stress decreases with increasing surfactant concentration much more than would be predicted through the reduction of the interfacial tension alone. Microscopic images reveal that with the addition of nonionic surfactants, the number of capillary bridges is reduced, decreasing the yield stress by several orders of magnitude. The addition of an ionic surfactant added to the disperse, aqueous phase is also able to decrease the yield stress by one order of magnitude. [1] E. Koos and N. Willenbacher, Science 331 (2011) 897. [2] J. Dittmann, E. Koos, N. Willenbacher and B. Hochstein, German Patent DE 10 2011 106 834.5 (2011).status: publishe