Experimental Investigation of Spatially Heterogeneous Dynamics in Dense Colloidal Liquids

Despite decades of research, the origins of spatially heterogeneous dynamics in proximity to the glass transition as well as the length scales associated with these heterogeneities, remains hotly debated as it is rather difficult extract them from experiments [1-4]. However, in recent years direct visualization of model colloidal systems via confocal microscopy has become popular and provided key insights into the glass transition in bulk domains [3, 4]. In this thesis, we use confinement to probe how spatial cooperativity arises when a colloidal liquid is constrained to progressively smaller dimensions and measure the length scales associated with such motions. Additionally, we examine how sensitivity to dynamic length scales is dependent on the sizes of tracers embedded in matrix suspensions that are close to the colloidal glass transition. These studies are not only of fundamental interest, but they have wide ranging applications that crosses disciplines ranging from the instability of sub-100 nm polymer nanostructures used in nanolithography and data storage applications to protein folding [1-5]. In Chapter 3, we find impeded structural relaxation of hard-sphere colloidal suspensions in confinement. Interestingly, we find similar behavior in a suspension of soft, deformable spheres in Chapter 4 where we examine colloidal gelation in confinement as well as the emergence of spatially heterogeneous dynamics that has a distinct structural origin. In the second part of this work, we focus exclusively on heterogeneous dynamics and methods to extract dynamic length scales associated with spatially heterogeneous dynamics. In Chapter 5, we study the tracer size sensitivity to spatially heterogeneous dynamics in matrix suspensions that are close to the colloidal glass transition. In Chapter 6, we confine hard-sphere suspensions to progressively smaller dimensions to probe the confinement effect on the growth of dynamic heterogeneities via confocal microscopy. Heterogeneous dynamics abruptly increases as the gap is reduced below a critical spacing, defining the confinement length scale. In chapter 7 we examine spatially heterogeneous dynamics in confined colloidal liquids under oscillatory shear.