Rheological study of granular suspensions and polymer glasses

Complex fluids cover a large range of materials. This thesis focuses on two types of such systems: granular suspensions that show shear thickening and polymer glasses, viscoelastic solid materials. Suspensions of granular particles have sparked great interest in recent years, not only due to interest from fundamental research but also because of their potential in industrial applications in areas like food production, pesticides, cosmetics, construction and oil well engineering. Depending on the suspending-liquid properties, particle sizes and volume fractions, granular suspensions show very rich rheological behaviours such as shear thickening, elevated normal stresses and an intriguing elongational rheology. Precisely these three rheological behaviours are systematically studied in three chapters (Chapters 3-5) of this thesis. The final chapter (Chapter 6) deals with the issue of the polymer glass transition. Traditionally, for thermoplastics, the so-called glass transition temperature (Tg) determines the transition between solid-like and liquid-like material. Accordingly, when the temperature is decreased below Tg, the initially liquid polymer melt becomes hard and brittle, and behaves like a glass. Our experiments show that besides increasing the temperature, a glass to liquid transition can also be achieved by imposing shear; thus, a critical glass-transition strain amplitude is defined. Knowledge of the polymer glass rheology is important for understanding and optimizing their performance in industrial as well as in everyday applications.