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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.

ABSTRACT. Radiocarbon analysis of the carbonaceous aerosol allows an apportionment of fossil and non-fossil sources of airborne particulate matter (PM). A chemical separation of total carbon (TC) into its subfractions organic carbon (OC) and elemental carbon (EC) refines this powerful technique, as OC and EC originate from different sources and undergo different processes in the atmosphere. Although 14 C analysis of TC, EC, and OC has recently gained increasing attention, interlaboratory quality assurance measures have largely been missing, especially for the isolation of EC and OC. In this work, we present results from an intercomparison of 9 laboratories for 14 C analysis of carbonaceous aerosol samples on quartz fiber filters. Two ambient PM samples and 1 reference material (RM 8785) were provided with representative filter blanks. All laboratories performed 14 C determinations of TC and a subset of isolated EC and OC for isotopic measurement. In general, 14 C measurements of TC and OC agreed acceptably well between the laboratories, i.e. for TC within 0.015-0.025 F 14 C for the ambient filters and within 0.041 F 14 C for RM 8785. Due to inhomogeneous filter loading, RM 8785 demonstrated only limited applicability as a reference material for 14 C analysis of carbonaceous aerosols. 14 C analysis of EC revealed a large deviation between the laboratories of 28-79% as a consequence of different separation techniques. This result indicates a need for further discussion on optimal methods of EC isolation for 14 C analysis and a second stage of this intercomparison.