Controlling the Structure, Dynamics, and Rheology of Colloidal Gels with Active Motion

Abstract

In this dissertation, we investigate the connection between the microscopic structure and dynamics and the macroscopic rheology of gels. We study two different types of gels – fractal cluster gels made of polystyrene colloids and psyllium polysaccharide gels. Fractal cluster colloidal gels are a well characterized, model gel system, and they are technologically relevant for industrial applications of colloids. Exploring the effects of embedded active matter in colloidal gels is of scientific interest to improve the understanding of activity on disordered solids and of technological interest for the design of reconfigurable materials with multi-state mechanical properties. In chapter two, we examine the microscopic dynamics of fractal cluster colloidal gels embedded with active matter. We developed a novel experimental set-up that utilizes a hydrogel membrane to controllably deliver hydrogen peroxide – a fuel that drives diffusiophoretic motion of the active Janus colloids – to the gel without disrupting the gel structure. We measure the dynamics of the gel network before and after addition of active motion. We find the addition of active motion to colloidal gels leads to an increase (as large as a factor of 3) in the dynamics of all particles in the gel network. The amount of increase is a function of the ratio of active to passive colloids and the energy of the active colloids. We model the amount of enhancement by accounting for the direct motion of the active colloids and the indirect contribution due to the strain field that the displacement of the active colloids induces on the gel network. In chapter three, we explore how the increase in dynamics is correlated to mechanical properties of the gel network. We designed an experimental approach that combines anti-foam chemicals and oxygen permeable surfaces of rheometer fixtures to suppress the formation of oxygen bubbles (a disruptive by-product of the mechanism for generating active motion) during rheology measurements. We find the incorporation of active matter to colloidal gels leads to a decrease in the viscoelastic moduli of the gels. We measure the moduli as a function of the total active energy, and find the moduli of the gels decrease (by up to a factor of 8.5) with increasing active particle energy. We explore the connections between the microscopic dynamics and macroscopic rheology through microrheology measurements. We also explore potential mechanisms that describe the effects of the activity on the gel structure and the effects of activity acting to change the dynamics of the gel. In chapter four, we characterize the relationship between gel stability, mechanical properties, and hydration kinetics of psyllium polysaccharide gels. Psyllium powders readily absorb water and swell to form a gel, which makes them useful as gelators or thickeners in industrial applications. The kinetic process of hydration and gelation of these gels is not well characterized or understood. We characterize the relationship between hydration kinetics, gel stability, and mechanical rheology of psyllium gels. We present a novel technique to quantify the hydration kinetics by incorporating a fluorescent dye that binds to psyllium and allows us to image the grains during hydration. We find a correlation between gel stability and transient rheological measurements; samples prone to consolidation had lower viscosity at short times, when the hydration process is most active.