Uncovering flow instabilities and the role of inter-particle surface friction in shear-thickening suspensions of colloidal rods

Abstract

Shear thickening (ST) is a well-known phenomenon observed in suspensions, characterized by an increase in viscosity with applied shear stress. Recent advancement in our understanding indicates the role of inter-particle surface contact friction in ST of dense suspensions. In such suspensions with an increase in volume fraction of particles, ST also increases and transits from continuous shear thickening (CST) to discontinuous shear thickening(DST). In the DST regime, the presence of a negative slope in shear-stress versus shear-rate plot indicates the presence of flow instability. No such instability is expected or observed in the CST regime. In this thesis, we study whether surface friction of the particles plays any role in ST, and are flow instabilities present in suspensions of anisotropic particles in the CST regime. This thesis consists of four chapters. Chapter 1 provides a brief introduction to two different models which try to explain ST in suspensions. The first model is the hydrodynamic model and the second one is the inter-particle frictional model. In the latter part of this chapter, various strategies for tuning ST are discussed. Chapter 2 provides details of the experimental setup and synthesis protocols followed to synthesize silica colloidal rods and coat them with temperature-sensitive polymers. Chapter 3 discusses the findings from our studies which can be subdivided into two different sections. In the first section, temperature-sensitive polymer-coated colloidal rods are used to tune the surface friction and their flow behavior as a function of temperature is studied by suspending them in aqueous medium. In the second section, flow instabilities are explored in the CST regime in the suspensions of silica colloidal rods and water-glycerol mixture. To do so, stress relaxation measurements are performed at different volume fractions for two different measuring system geometries. Chapter 4 concludes our findings and provides insight into our understanding of the subject so far.