Colloidal Crystallization via Simultaneous Depletion and Electric Field Mediated Interactions

Johns Hopkins University, Baltimore MD

Investigators

Abstract

1234981 PI: Bevan The ability to assemble nano- and micro- colloidal particles into ordered materials and reconfigurable devices provides a basis for emerging technologies (e.g. photonic crystals, meta-materials, cloaking devices, solar cells, etc.) and for improving traditional particle based materials (e.g. ceramics, coatings, minerals, foods, drugs). Despite the range of applications employing colloidal particles, current capabilities for manipulating microstructures in such systems are limited in two ways: the degree of order than can be obtained, and the time required to generate ordered structures. Both of these limitations are due to fundamental problems with designing, controlling, and optimizing the thermodynamics and kinetics of colloidal assembly processes. The limiting factor in colloidal assembly is generally not the unavailability of methods to manipulate colloids or create sufficiently complex components, but rather the inability to reliably assemble structures without producing kinetically trapped, jammed, or dynamically arrested configurations that are defect ridden or even entirely amorphous. To address this problem, this project will use experimental and modeling approaches to constructively combine the best aspects of complementary colloidal assembly mechanisms. To control colloidal assembly, this project will employ tunable depletion attraction between colloidal particles (mediated by unadsorbing micelles, nanoparticles, or macromolecules) and electric field mediated colloidal interactions. The goal is to use such tunable interactions in conjunction with recently developed dynamic models of colloidal assembly to formally engineer the defect density and kinetics in a colloidal crystallization process. The conceptual strategy is to exploit the strengths, and avoid the weaknesses, of each independent approach by considering that: (1) depletion mediated assembly is primarily a thermodynamic approach in that interactions between particles determine how they assemble themselves; the resulting structures are thermodynamically stable but are prone to kinetic problems (e.g. point defects, polycrystals, gels, glasses), and (2) electric field mediated assembly is more amenable to dynamically altering colloidal assembly kinetics, which is appealing because it provides more control over stochastic processes, but particles simply disassemble once external intervention ceases. As a result, this research will investigate viable kinetic pathways for the rapid assembly of defect free colloidal crystals using various serial and parallel combinations of electric field mediated mechanisms (to control transport, structural evolution, and annealing in growing or pre-existing crystals) and depletion mediated assembly (to produce mechanically stable equilibrium crystals). This research will significantly advance our fundamental understanding of the design rules and control parameters for tuning colloidal interactions to assemble nano-/micro- particles into perfectly ordered periodic structures. The ability to robustly assemble and reconfigure ordered materials on length scales comparable to the wavelengths of electromagnetic radiation will provides a basis for producing multi-scale "meta-materials" with unique electric, magnetic, and optical properties. Although the exotic properties of such materials have been theoretically predicted and demonstrated in proof-of-concept materials obtained via laborious microfabrication, no existing process is sufficiently controllable, scalable, and robust to enable the use of such materials in advanced commercial applications. The novelty of this research effort lies in the use of quantitative measurement and modeling tools to enable the constructive integration of complementary colloidal assembly methods to achieve defect free microscopic structures (where trial-and-error discovery alone might fail). Ultimately, the proposed research will provide broad fundamental understanding of how thermal motion, interparticle interactions, and external fields determine the chemical physics of microscopic ordering mechanisms. Beyond the technical outcomes, broader impacts of this research project will include the generation of rich visual data from experiments (e.g. images, videos), simulations (e.g. renderings, animations), and analyses (e.g. multi-dimensional plots) for use in various classroom, laboratory, outreach, and dissemination activities.