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
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