Rheological and Microstructural Investigation of Capillary Suspensions under Shear

The addition of small amounts of an immiscible secondary fluid to a suspension can lead to particle bridging and network formation. This is caused by the attractive capillary force due to liquid bridges formed between the particles. Thus, this kind of suspensions is called a ``capillary suspension''. The capillary bridging phenomenon can be used to stabilize particle suspensions and precisely tune their rheological properties. Capillary suspensions can be created whether the secondary fluid preferentially wets the particles or not. A pendular state is created when the secondary fluid preferentially wets the particles and a capillary state is obtained when the bulk fluid is preferentially wetting. In recent years, knowledge about the behavior and microstructure of capillary suspensions at the quiescence has been advancing. However, the dynamic behavior of capillary suspension is still poorly understood, despite such understanding being crucial for the processing. The objective of this thesis is, therefore, to investigate the rheological behavior and the correlated microstructure of capillary suspensions under shear.

Capillary suspensions with a low particle particle concentration (φsolid=0.25), two different wetting properties, and various amounts of added secondary fluid were studied. The investigated samples have two different wetting conditions, characterized by the ternary contact angle θ, representing the capillary suspensions in the pendular and capillary states. Two central questions are the objective of this thesis. First, how does the microstructure of capillary suspensions change under shear? Second, how do the low level particle-particle interactions influence the rheological response in the asymptotic nonlinear regime?

To investigate the network build-up systematically, a strain rate step measurement under decreasing shear rates was executed. The corresponding viscosity and normal stress differences were reported. 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 leading to shear thinning. 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 grows as the shear rates decrease. The long, asymmetric flocs rotate to reorientate in the vorticity direction leading to negative normal stress differences. Analogue experiments were also conducted for the capillary state. This system showed similar shear thinning, but only a negative normal stress difference. As confirmed with confocal microscopy, more networks are formed due to the droplet breakup at high shear rates in the capillary state.

Using oscillatory shear rheology, the asymptotic deviation from the linear viscoelastic behavior of capillary suspensions was studied. A non-integer power law stress scaling was observed as a function of imposed strain, in contrast to the expected cubical scaling reported in literature for most materials. Preliminary experiments with a capillary state sample provide the first definitive proof that such non-cubic scalings are not an experimental artefact. By comparing the results from this system with other reports for atypical scaling, I formulated a hypothesis that particle collisions, particularly in non-colloidal kinetically trapped systems, are the cause of the non-integer scaling. This hypothesis was tested with other samples where non-integer scaling can be traced back to Hertzian-like particle-particle contacts in the capillary suspensions networks.

Furthermore, the contribution of concave (low contact angle) and convex menisci (high contact angle) as well as the secondary fluid concentration is discussed. While the experiments in this thesis were conducted specifically with capillary suspensions, the obtained results may be applicable for other material systems that exhibit strong attractive interactions.