Shear-induced macroscopic “Siamese” twins in soft colloidal crystals

Abstract

Soft colloidal crystals are inherently highly susceptible to shear. We find by using scanning rheo-SANS and scanning synchrotron-rheo-SAXS experiments that high shear strains cause macroscopic “Siamese” twinning in lyotropic FCC-crystals formed by block copolymer micelles or polymer-coated nanoparticles. The twins extend over the whole gap of the shear cell with one twin always located in the outer region of the gap, and the other twin always located in the inner region of the gap. We could visualize twin boundary surfaces by using transmission electron microscopy. In analogy to the plastic deformation of nanocrystalline metals under high shear stress and high deformation rates we propose a deformation twining mechanism for the formation of such macroscopic twins.