(102a) Colloidal Crystallization in Start-up of Steady Shear Flow Studied by Confocal Microscopy

Concentrated colloidal suspensions exhibit complex rheology and dynamics that is relevant to their ability to crystallize into ordered arrays upon the application of fields such as sedimentation and shear. Classic work by Russel and coworkers established the existence of a critical field strength for crystallization in sedimentation. Likewise, in shear flow, Ackerson and coworkers discovered stress criteria for colloidal crystal formation under shear flow. Today, new methods for direct visualization of colloids offer the possibility of observing the microstructural changes that result in the disorder to order transition. These techniques can thus help clarify how such field-induced transitions occur. Here we seek such understanding by studying the structural changes of colloidal suspensions in start-up of steady-shear flow. Our method uses 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. To quench assembled structures for interrogation by 3D confocal microscopy, the suspension composition (colloid volume fraction ~ 0.35) included a small fraction of photopolymer that was subsequently polymerized by UV exposure. A parallel plate shear cell was loaded with the colloidal suspension and samples were imaged both before and after a step strain was imposed. The colloidal crystal array and particle centroids were located in 3D by means of quantitative image processing. The effect of strain and shear rate on local crystallinity along an axis perpendicular to the substrate was quantified. The results are compared to the mechanisms for shear-induced colloidal crystallization discussed in the literature. The direct visualization results provide a test that yields new insights relative to previous comparisons to results of rheology and scattering.