Directed Self-Assembly of Spheres into a Two-Dimensional Colloidal Crystal by Viscoelastic Stresses

Ordering induced by shear flow can be used to create specific microstructures in particle suspensions. They are determined by the balance between a range of forces, such as direct interparticle, Brownian and hydrodynamic forces. The latter are modified when dealing with viscoelastic rather than Newtonian matrices. In particular, one dimensional string-like structures of spherical particles have been observed to form along the flow direction in shear thinning viscoelastic fluids, a phenomenon never observed in Newtonian fluids at similar particle volume fractions. Here we report for the first time on the formation of a Directed Self-Assembly (DSA), flow-induced two-dimensional planar crystals of spherical particles. The novel microstructure is formed when particles are suspended in viscoelastic, wormlike micellar solutions and only when the applied shear rate exceeds a critical value. In spite of the very low volume fraction (less than 0.01), particles arrange themselves in 2-D crystalline patches along the flow direction. This is a bulk phenomenon, as 2-D crystals form throughout the whole gap between plates, the gap thickness being much larger than the particle size. Patches lay in planes parallel to the shearing surfaces