Grain boundary pinning in doped hard sphere crystals

Direct visual observations on how grain boundaries are pinned between multiple large spherical impurities during colloidal hard sphere crystallisation are presented. The fluid is stabilized between impurities and acts as a precursor for grain boundary formation. The range of fluid stabilisation by a single impurity is characterized by the frustration length, which goes through a maximum as a function of the impurity-to-particle size ratio. Grain boundaries are more strongly confined to the area between two impurities as the ratio between the impurity-to-impurity spacing and the combined frustration lengths decreases. Our results identify the key parameters in grain boundary formation in doped systems, which may lead to a better control of the grain boundary density in materials.