The addition of small amounts of a secondary fluid to a suspension can, through the attractive capillary force, lead to particle bridging and network formation [1]. The capillary bridging phenomenon can be used to stabilize particle suspensions and precisely tune their rheological properties. This effect can even occur when the secondary fluid wets the particles less well than the bulk fluid. The properties of capillary suspensions can depend on the fluid and solid properties with even a small change to the contact angle leading to a drastic change in the rheological properties as a network transitions from contacts dominated by the strong capillary force to the weak van der Waals force. The network changes are examined using confocal microscopy and directly compared to macroscopic measurements of the shear modulus in the linear regime. A model capillary suspension composed of glass spheres in an index-matched oil with added aqueous glycerol is used in this study where the three-phase contact angle is varied through silanization. For this model suspension, capillary bridges between individual particles that create a strong space-spanning network are observed for low contact angles. At high contact angles, no network is observed and the droplets, while still attached to individual particles, do not form bridges or clusters. Transitional and cluster behavior can be observed at intermediate contact angles. The floc structure also varies with the mixing conditions where large droplets can become jammed with particles and a space-spanning capillary suspension is not formed.
[1] E. Koos and N. Willenbacher, Science 331:6019, 897-900 (2011).
erin.koos@kit.edu
