How Confinement-Induced Structures Alter the Contribution of Hydrodynamic and Short-Ranged Repulsion Forces to the Viscosity of Colloidal Suspensions

Abstract

Confined systems ranging from the atomic to the granular are ubiquitous in nature. Experiments and simulations of such atomic and granular systems have shown a complex relationship between the microstructural arrangements under confinement, the short-ranged particle stresses, and flow fields. Understanding the same correlation between structure and rheology in the colloidal regime is important due to the significance of such suspensions in industrial applications. Moreover, colloidal suspensions exhibit a wide range of structures under confinement that could considerably modify such force balances and the resulting viscosity. Here, we use a combination of experiments and simulations to elucidate how confinement-induced structures alter the relative contributions of hydrodynamic and short-range repulsive forces to produce up to a tenfold change in the viscosity. In the experiments we use a custom-built confocal rheoscope to image the particle configurations of a colloidal suspension while simultaneously measuring its stress response. We find that as the gap decreases below 15 particle diameters, the viscosity first decreases from its bulk value, shows fluctuations with the gap, and then sharply increases for gaps below 3 particle diameters. These trends in the viscosity are shown to strongly correlate with the suspension microstructure. Further, we compare our experimental results to those from two different simulations techniques, which enables us to determine the relative contributions of hydrodynamic and short-range repulsive stresses to the suspension rheology. The first method uses the lubrication approximation to find the hydrodynamic stress and includes a short-range repulsive force between the particles while the second is a Stokesian dynamics simulation that calculates the full hydrodynamic stress in the suspension. We find that the decrease in the viscosity at moderate confinements has a significant contribution from both the hydrodynamic and shortrange repulsive forces whereas the increase in viscosities at gaps less than 3 particle diameters arises primarily from short-range repulsive forces. These results provide important insights into the rheological behavior of confined suspensions and further enable us to tune the viscosity of confined suspensions by changing properties such as the gap, polydispersity, and the volume fraction.

Numerical study on the inertial effects of particles on the rheology of a suspension

“…The decrease ratios in viscosity for the corresponding confinement were found to be 0.35 for f = 38% and 0.8 for f = 52%, respectively. 28 Although the concentrations of their suspensions were higher than that of ours, these tendencies are compatible.…”

Viscosity Estimation of a Suspension with Rigid Spheres in Circular Microchannels Using Particle Tracking Velocimetry

“…It has been reported in previous research that wall shear stress decreases locally in micro-vessels due to the motion of blood cell and hydrodynamic interactions [47,48]. Furthermore, in numerical simulations, the relative viscosity could be below 1 locally for dense (highly concentrated) suspensions with rigid particles [16]. These findings are important for considering blood flow in bioengineering.…”

“…Fukui et al [12] found that particles rotate to achieve a kinetic balance with the surrounding hydrodynamic forces, which results in a decrease in fluid resistance. Moreover, some researchers reported that viscosity significantly decreased for strongly confined conditions [13,14,15,16]. The above phenomena are reflected in the strong correlation between suspension viscosity and microstructure defined by spatial particle arrangements.…”

Aqueous Colloid + Polymer Depletion System for Confocal Microscopy and Rheology

“…Finally, colloidal gels deform non-linearly [33] and yield under shear flow [25,34]. Single-particle imaging also aids in elucidating the contribution of hydrodynamic forces vs. short-range repulsions [35] and of normal stress differences to particle migration [36] in sheared or flowing hard-sphere colloids. The design of practical suspensions, however, often requires the interactions between particles to be tailored to control microstructure as well as rheological properties [37], such as viscoelasticity or normal stress differences.…”