(232i) Analysis of Structures Formed by Application of Step Strain to Dense Colloidal Suspensions

Under shear flow dense colloidal suspensions have demonstrated shear thinning and shear thickening rheological behavior and have been studied extensively due to both fundamental interest of the complex system and crystal formation under shear flow (Krishnamurthy, 2005, J. Rheology. Behavior of colloidal suspensions has been studied in both hard sphere and charged stabilized systems in both simple and oscillatory shear by means of dynamic light scattering (Ackerson et al, 1988, PRL). Ackerson et al (Ackerson 1991, Physica A) established stress and strain criteria for colloidal crystal formation under shear flow, however, in such methods information on the disorder to order transition is difficult to acquire. (Cerda 2008). In order to understand the formation of colloidal crystals under shear flow we studied structural changes in dense colloidal suspensions by direct visualization using confocal microscopy. A parallel plate assembly with a stationary bottom plate and a moveable top plate is loaded with suspensions of charge stabilized, refractive index matched poly (12-hydroxystearic acid) stabilized poly(methyl methacrylate) colloids of size about one micron suspended in the viscous solvent dioctyl phthalate (colloid volume fraction ~ 0.35) with small quantities of photopolymer. Samples were imaged, a step strain was applied, and samples were immediately quenched by exposure to UV light and imaged again. Strain amplitudes were of order of amplitude 1. Particle centroids were located in 3D by means of quantitative image processing. Local crystallinity was quantified by application of local bond order parameter criteria developed by ten Wolde et al. (ten Wolde et al., 1996, Journal of Chemical Physics). We determined the effect of an application of a precise amount of step strain at controlled shear rate in a parallel plate device and compare the results to previous shear studies as well as our previous work with spin coating (Shereda 2008, PRL).