The dynamics and phase behavior of suspensions of stimuli-responsive colloids.

摘要

The role of colloids in science has changed dramatically over the past few decades as they are being used as novel advanced materials for various applications e.g. paint/coating, electronics, cosmetics, and pharmaceutics. Direct observation and manipulation of colloids are now very common and colloidal dispersions are being used as model systems for studying equilibrium/non-equilibrium thermodynamics and kinetics of molecular and atomic systems. While most original pioneering works in colloidal science has been performed with a focus on hard sphere systems, researchers now devote considerable attention to more complex soft colloids. Aqueous microgels form one of the most extensively studied classes of soft materials due to their unique tunability and responsiveness. However, the fundamental knowledge about microgels is still rather poor, in particular for multi-responsive systems, and there are a variety of open questions that need to be answered. As a contribution to this field, this thesis describes studies of the dynamics, phase behavior, interparticle interactions, and hydrodynamics of stimuli-responsive pNIPAm-co-AAc microgels. Since these microgels are multi-responsive to the external stimuli temperature, pH, and ionic strength, one can tune their interparticle interactions and size. These colloidal particles serve as excellent model systems to probe the relationship between colloidal interactions and phase behavior. As a first step, we established our core experimental methodology, by demonstrating that particle tracking video microscopy is an effective technique to quantify various parameters in colloidal systems, such as the hydrodynamic particle radius, local temperature of a dilute suspension, and rheological/micro-structural properties of the embedding medium. Then we used the technique in combination with a microfluidic device that provides in situ control over sample pH to probe the phase behavior of pNIPAm-co-AAc microgel suspensions. In essence, the experimental set-up enables changes in effective particle volume fractions by changing pH, which can be used to construct the phase diagram. In order to explain the unique features of the microgel phase diagram, we measured the underlying pairwise interparticle potential of pNIPAm- co-AAc microgels directly in quasi-2D suspension and proved that the interactions are pH dependent and can range from weakly attractive to soft repulsive. Finally, the hindered Brownian diffusion due of colloidal particles confined by hard walls was investigated systematically and striking differences between hard sphere and soft sphere were found, with soft pNIPAm- co-AAc microgels showing surprising mobility even under strong confinement.