Self-Assembly and Self-Propulsion of Colloidal Particles Using Shape and Janus Anisotropy.

Abstract

Anisotropic colloidal particles can serve as the building block for a variety of complex materials with a range of different functional properties. We demonstrate the properties of anisotropic particles that are complex in shape and surface properties in three different systems. Shape anisotropy is achieved by increasing particle eccentricity from spheres to ellipsoids. Surface anisotropy introduces distinct physical and/or chemical halves to colloidal particles, called Janus particles. In this dissertation, we utilize shape and biphasic properties to generate interesting kinetic and thermodynamic properties of materials. In particular, experimental and theoretical studies allow us control over the self-assembly and self-propulsion of colloids by influencing anisotropic features.

Working with metallodieletric Janus spheres – polystyrene colloids with a hemispheric gold cap – the role of hydrophobicity and gold thickness in salt-induced self-assembly is studied. We quantify experimentally and with modeling, the distinct surface interactions. Modeling these biphasic spheres as multilayered systems predicted the interactions at short and long-range. Metallodieletric colloids can also be ellipsoidal and patterned with other metals, thereby introducing shape anisotropy with biphasic interactions. We study the active motion of dilute dispersions of prolate spheroidal polystyrene particles half-coated with platinum along its major axis. Hydrogen peroxide is catalytically decomposed to water and oxygen at the platinum surface and the chemical energy is converted into autonomous motion. We discuss the physical origins of the observed trajectories of motion and extract the forces and torques generated from active motion as a function of particle eccentricity. Lastly, shape anisotropy can be useful for self-assembly. We examine how oblate spheroids (discoids) densify during field-assisted assembly. We report the application of direct current (dc) electric fields and visible light to suspensions of discoids to achieve localized dense three-dimensional packings that are both disordered and ordered.The application of the dc electric field enables particle assembly and the use of light promotes packing to higher densities. Taken together, we find that particle anisotropy plays a decisive role in these three different colloidal systems. By systematically exploring its impact, we further identify the utility of anisotropy in designing colloidal systems with functional interactions as well as dynamic properties.