Preparation of colloidal chains and studies of their structure and dynamics

Abstract

Micron sized colloidal particles are Brownian, and can be conveniently visualized using optical microscopy. This thesis focuses on the preparation of colloidal assemblies comprising micron-sized colloidal particles linked through flexible linkers. Specifically, colloids linked in a linear fashion to form string-like objects are investigated. In the first part of the thesis, we investigate ice templating of aqueous colloidal dispersions as a route to the preparation of colloidal assemblies. Highly dilute dispersions of polymer coated colloids are ice templated by freezing either isotropically or in a directional manner, and the polymeric shell is cross-linked to form assemblies. The dispersions are at concentrations that are far below that required to form percolated monoliths. There are three different types of assembled structures that form during ice templating: linear chains, two particle-wide tapes and extended sheets. We observe that, with increase in particle concentration from about ~106 to ~108 particles/ml, there is a transition from isolated single particles to increasingly larger clusters. In this concentration range, most of the colloidal clusters are linear or sheet like particle aggregates. Remarkably, the fractional probability for formation of a cluster of n particles, P_n (n>2, up to n ~ 30 particles) scales as n^(-2), independent of particle concentration for isotropic ice templating (over a 100-fold variation in concentration). Results from our experiments compare well with kinetic simulations performed by our collaborator. These simulations do not consider hydrodynamics or, instabilities at the growing ice front due to particle concentration gradients. Thus, clustering of colloidal particles by ice templating dilute dispersions appears to be governed only by particle exclusion by the growing ice crystals that leads to their accumulation at ice crystal boundaries. In contrast to the isotropic freezing experiments, for directional freezing, P_n strongly depends on the concentration of particles. The distribution function (P_n ) obeys a power law P_n~n^(-η), where η varies from 2.27 (for particle concentration ~〖10〗^9 particles/mL) to 3.19 (for〖 10〗^7 particles/mL). To understand the differences between isotropic and directional ice templating, we compare our experimental results with simulations performed by our collaborators. Their lattice simulations ignore hydrodynamic interactions and instabilities at the growing ice front. These simplified simulations capture the experimental results and reveal that the differences in scaling of (P_n ) arise from differences in how the ice crystals close up on themselves as the freezing front propagates. We investigate the dynamics of colloidal chains using the linear colloidal constructs obtained from isotropic ice templating. We study both passive and “active” colloidal chains. Active colloids are those that are not constrained by equilibrium: ballistic propulsion, superdiffusive behavior, or enhanced diffusivities have been reported for active Janus particles. At high concentrations, interactions between active colloids give rise to complex emergent behavior. Their collective dynamics result in the formation of several hundred particle-strong flocks or swarms. We demonstrate significant diffusivity enhancement for colloidal objects that neither have a Janus architecture nor are at high concentrations. We employ uniformly catalyst-coated, viz. chemo-mechanically, isotropic colloids linked into a chain to enforce proximity. Activity arises from hydrodynamic interactions between enchained colloidal beads due to reaction-induced phoretic flows catalyzed by platinum nanoparticles on the colloid surface. This results in diffusivity enhancements of up to 60% for individual chains in dilute solution. Chains with increasing flexibility exhibit higher diffusivities. Simulations (performed by our collaborators), that account for hydrodynamic interactions between enchained colloids due to active phoretic flows accurately capture the experimental diffusivity. These simulations reveal that the enhancement in diffusivity can be attributed to the interplay between chain conformational fluctuations and activity. Our results show that activity can be used to systematically modulate the mobility of soft slender bodies. Finally, we prepare thermos-responsive colloidal chains and investigate thermally induced reversible collapse transitions in such chains. Micron size polystyrene colloidal beads are coated with thermos-responsive copolymer microgels of PNIPAM and allylamine and are assembled into chains by application of an AC field. The combination of long range electrostatic repulsion between colloids and external AC field-induced dipolar interactions result in linear assemblies of the particles. The colloidal monomers that line up are crosslinked through the amine groups in the microgels, so that the particles remain enchained even after removal of the external electric field. We control the flexibility of the colloidal chain by varying crosslinking time. We vary the suspension temperature from 25oC to 55oC, viz. from below to above the LCST of the PNIPAM microgels. We change the temperature slowly and stepwise to ensure that chains are in equilibrium when we characterize them. All the chains show a decrease in size on heating. For rigid chains, the decrease in modest and is not accompanied by a change in shape. Flexible chains form relatively compact structures, resulting in a large increase in the local monomer number density. For chains with intermediate flexibility, the balance between chain rigidity and inter-particle attraction results in the formation of helix-like structures. The fraction of monomers that form helix-like structure increases with temperature and plateaus above the collapse transition temperature of the microgel particles. In the final chapter of the thesis, we present a brief description of interesting problems that could be addressed using the colloidal polymer chains developed in our work.