Shear dependent network orientation in capillary suspensions

The addition of small amounts of an immiscible secondary fluid to a suspension can dramatically change the rheological behavior of the suspension by inducing particle gelation or by reinforcing a weak network [1]. This transition is caused by strong attractive capillary force between the particles and two fluids pro- ducing a sample-spanning network. Capillary suspensions exist in two states: a pendular state when the secondary fluid preferentially wets the particles, and a capillary state when the bulk fluid is preferentially wetting. These capillary suspension networks demonstrate a negative normal stress difference, from re-orientation of the flocs into the vorticity direction during shearing. Typically, systems with negative normal stress dif- ferences have either high volume concentrations (~58%) and are shear thinning, or are shear thickening with very low particle concentrations (~0.09%) [2, 3]. In contrast, the capillary suspensions we report here have a 25% solid concentration and are shear thinning; a combination that has never before been reported in literature. To investigate the network build-up systematically, we use strain rate sweep measurement with decreasing shear rate and report the corresponding normal stress differences. In the pendular state, the system undergoes a transition from a positive normal stress differences at the high shear rates to negative normal stress at the low shear rates. This phenomenon occurs due to the motion of particle networks, where the hydrodynamic force dominates and the flocs break-up at high shear rates. The remaining dimers and trimers tumble in the flow-gradient plane (Jeffrey orbits) and experience friction as they come into contact due to the capillary force. The network reforms and the size of the flocs grow as the shear rates decreases. The long, asymmetric flocs rotate to reorientate in the vorticity direction and this leads to a negative normal stress differences. Furthermore, we compare the rheological properties and images from confocal microscopy during shearing to gain insight into the network microstructure. [1] E. Koos, N. Willenbacher, Science, 2011, 331(6019): 897-900. [2] A. Montesi, et al., Physical review letter, 2004, 92, 58303. [3] C. Osuji, D.A. Weitz, Soft Matter, 2008, 4, 1388.status: publishe