Flow Assisted Assembly of Multilayer Colloidal Crystals Studied using Confocal Laser Scanning Microscopy.

Abstract

Colloidal crystals are highly ordered particle arrays with potential applications including sensors, optical switches, and photonic materials. For production on an industrially viable scale, processes must be developed to form crystals with low defect densities, good long range order, and favorable kinetics. Application of a field to a concentrated colloidal suspension accelerates crystal formation. Ackerson et al.(Ackerson, 1991) established that systems with stress-based Peclet numbers above one resulted in crystal formation. We investigate formation of colloidal crystals by studying structural changes that occur upon shearing using confocal microscopy. Charge-stabilized poly(methylmethacrylate) particles (Φ = 0.35) suspended in dioctyl phthalate were used for experiments. After application of shear, assembled structures were immobilized by UV exposure. The full sample thickness was imaged using confocal microscopy. Particle centroids were located in 3D by means of image processing and local crystallinity was quantified by application of local bond order parameter criteria (tenWolde, 1996). We present microstructural analysis of structures formed by both spin coating and uniform shear flow. Spin coating produces spatiotemporal variation in the ordering of concentrated colloidal dispersions that is a universal function of the local reduced critical stress and macroscopic strain. Samples produced at Peclet numbers greater than one and macroscopic strains above two resulted in crystal formation. A plot of the cryrstalline fraction versus Peclet number yielded a sharp order to disorder transition at Peclet number of order unity. The effect of volume fraction on the Peclet number theory was studied. Results indicated that the theory applied to volume fractions within the crystalline regime. Strain requirements for crystal formation of samples undergoing step strain deformation in a parallel plate geometry were investigated by applying stains of 1-300 to samples with fixed gaps of 150μm. We found the velocity profile to be non-linear across the gap. This inhomogeneity was strongly correlated with the movement of the crystalline boundary. A strain of 160 was required for full sample crystallization. The movement of the crystalline boundary was modeled as a 1-D crystallization and fitted as two linear regions. Fundamental knowledge gained from these studies will allow shear processes to be evaluated for industrial use.