Negative normal stress differencesN₁–N₂in a low concentration capillary suspension

Abstract

Here, negative normal stress differences are reported in capillary suspensions, i.e. particle suspensions in a two-fluid system that creates strong capillary attractions, at a solid concentration of 25%, and a volume fraction that has heretofore been considered too low to show such normal stress differences. Such capillary suspensions have strong particle networks and are shear thinning for the entire range of shear rates studied. 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. In the pendular state, the system undergoes a transition from a positive normal stress difference at high shear rates to a negative stress difference at low shear rates. These results are an indication of flexible flocs in the pendular state that are able to rotate to reorientate in the vorticity direction under shear. Analogous experiments were also conducted for the capillary state, where only a negative normal stress difference occurs. The capillary state system forms more network contacts due to droplet breakup at higher shear rates, which enhances the importance of hydrodynamic interactions in the non-colloidal suspension.

Particle contact dynamics as the origin for noninteger power expansion rheology in attractive suspension networks

“…Indeed, the yield stress of PMMA-glycerol-paraffin oil based capillary suspensions in the capillary state are reported to be in the same order of the NP3 in silicone oil normal suspension, indicating a weak attractive capillary force [26]. In this previous work, no apparent yield stress is reported for the PMMA in glycerol normal suspension, which justifies the assumption that no attractive van der Waals force acts on the particles.…”

“…The details of each sample used in this work are given in Table 1. The method of sample preparation for each model system was described in Natalia et al [26]. and all reported measurements were executed at least three times to check their reproducibility.…”

Using an added liquid to suppress drying defects in hard particle coatings

“…Capillary suspensions, on the other hand, dry very differently, as depicted in Figure 5b. Previous studies of pendular state capillary suspensions using confocal microscopy have shown that the secondary fluid creates bridges between two particles or, if the amount of liquid is higher, funicular clusters [26,[34][35][36]. In a consolidated coating, this corresponds to the secondary fluid being located at the throats of porous media.…”

The behavior of capillary suspensions at diverse length scales: From single capillary bridges to bulk

“…During flow of pendular state suspensions, isolated clusters, present at high shear rates, follow Jeffery orbits giving rise to a positive normal stress difference N 1 − N 2 . As the shear rate is decreased, these clusters collide and begin to align with the flow to form vorticity rolls, resulting in a negative normal stress difference N 1 − N 2 [38]. The magnitude of this stress difference, however, depends on the ability of the flocs to reorientate in the flow, and the pendular state clusters with small bridges show a more negative N 1 − N 2 than the more rigid clusters with higher secondary fluid volumes and coalesced bridges.…”