News & Seminars | HKUST Department of Physics

Abstract

Dense suspensions, composed of solid grains suspended in a liquid medium, com[1]monly exhibit shear-thickening behavior, where viscosity increases in response to higher shear rates or shear stresses. In this thesis, we experimentally investigated the control of suspension rheology using boundary effects on multiple length scales. Specifically, we explored two distinct boundary effects: (1) the macroscopic boundary confinement imposed by shear plates at suspension interfaces, and (2) the microscopic phase boundaries created by introducing an immiscible liquid into the suspensions.

First, we systematically investigated the effect of boundary confinements on the shear thickening rheology of dense granular suspensions. Under highly confined condi[1]tions, different dense suspensions were found to consistently exhibit size-dependent flow curves, or even the rarely reported non-monotonic (S-shaped) flow curves in steady states. By performing in-situ boundary stress microscopy (BSM) measure[1]ments, we observed enhanced stress heterogeneities in confined suspensions, where concentrated high-stress domains propagated stably either along or against the shear direction. Combining rheological characterizations on different length scales, we un[1]covered an underlying relationship between local stress concentrations and the emer[1]gence of non-monotonic flow curves for highly confined suspensions. By purposefully designing the imposed boundary confinements, we were able to exert precise control over the rheology of granular suspensions.

Second, we further investigated the impact of water inclusions on the rheological prop[1]erties of oil-based shear thickening suspensions with varying particle concentrations. With the addition of water droplets, the suspensions were found to consistently ex[1]hibit enhanced shear thinning behavior at low stresses and reduced shear thickening behavior at high stresses. This could possibly be attributed to the interplay between shear thinning induced by droplet deformation and shear thickening induced by par[1]ticle contact networks. As the particle concentration increased, we found a signifi[1]cant decrease in the viscosity of multi-phase suspensions compared with single-liquid phase suspensions within the shear thickening regime. By performing in-situ BSM measurements, we observed a notable reduction of the stress heterogeneities induced by concentrated high-stress clusters in multi-phase suspensions. We hypothesized that these water droplets acted as “soft particles”, effectively dissipating local stress con[1]centrations and diminishing shear thickening strength. Extension experiments further demonstrated the effective suppression of dynamic jamming in multi-phase suspen[1]sions.

Taken together, our findings introduce two novel approaches for dynamically con[1]trolling suspension rheology by designing boundary conditions across multiple length scales. These approaches lay a solid foundation for the development of intelligent transport systems capable of handling shear thickening fluids.