Microstructure and rheology of concentrated colloidal suspensions with varying nanotribological interactions
Date
2022
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
The shear thickening of dense colloidal suspensions is an active area of research to understand the non-linear flow response relevant to various processing conditions, such as high-speed coating, spraying, printing, pumping, and other industrial applications. Efforts in theoretical models and simulations seek to examine the underlying physical forces acting between particles in the suspension, including nanotribological forces such as lubrication hydrodynamics and frictional contact forces, to predict suspension shear rheology. However, few experimental investigations directly measure the nanotribological forces acting between particles or measure the associated suspension microstructure under flow, despite these being essential for connecting colloidal forces to the measured bulk rheology. Thus, there is a scientific need to perform direct nanotribological and microstructural measurements to resolve the origins of this complex rheological behavior. Such research has technological value for improving the processing of high solid dense suspensions, which is often limited by shear thickening, and commercial products that benefit from the shear thickening behavior, such as door stops and speed bumps and armor movement reactive fabrics. Thus, the overarching goal of this thesis is to systematically investigate how the nanotribological interactions affect the microstructure and rheology of concentrated colloidal suspensions. ☐ This thesis is divided into two research aims: The first aim, presented in Chapters 3 and 4, is to test the theoretical framework of friction contact models with rheological and nanotribological measurements on model suspensions with controlled surface properties. In Chapter 3, we first present comprehensive experimental tests of a friction contact model based on correlating simulation results against rheological measurements for both model and industrial colloidal dispersions complemented by independent estimates of the particle-scale friction coefficients from literature surveys. The comparisons emphasize the sensitivity of the first normal stress difference can distinguish between states of shear thickening dominated by hydrodynamic friction or contact friction. Based on the findings in Chapter 3, a systematic exploration of nanotribological measurements using lateral force microscopy (LFM) is presented in Chapter 4. Our systematic studies qualitatively agree with the Stribeck relationship regardless of the solvent environment. It is also confirmed that the friction coefficient obtained from the bulk rheology lies in the high Sommerfeld number regime, suggesting that directly applying the friction coefficient obtained from the nanotribological measurements for predicting the rheology using the friction contact model is not quantitative. ☐ The second aim of this thesis presented in the remaining chapters is to investigate the underlying relationship between suspension microstructure and shear rheology relationship with the aid of small angle neutron scattering (SANS) to further explore consequences of these different nanotribological forces on suspension rheology. In Chapter 5, these effects of nanotribological interparticle interactions are explored via Flow-SANS to identify the microstructural differences in the nearest neighbor distribution under flow between two model colloidal suspensions with very different levels of surface contact friction. We find quantitative differences in the non-equilibrium microstructure resulting from differences in the nanotribological interactions operative in the shear thickened state, demonstrating that microstructural measurements can distinguish between micromechanical mechanisms and that simulations are not accurate enough to statistically predict this. The extent of structure formation in shear thickening is measured in Chapter 6 for showing that hydroclusters are very localized for continuous shear thickening suspensions, with qualitative differences with simulation predictions but in agreement with measurements using optical and rheo-optical methods. Finally, novel rheo-SANS measurements of the transient microstructure, documented in Chapter 7, exploit the differences in how these nanotribological forces act under flow cessation and reversal to further test their influence on macroscale rheology. These findings provide quantitative information valuable for those modeling suspension processing as well as suggestions for further model improvement to resolve the underlying mechanisms of shear thickening in colloidal suspension rheology.
Description
Keywords
Colloidal suspension, Microstructure, Nanotribology, Rheology, Shear thickening