Congratulations to Ziwei Guo, 5th year PhD student, whose paper “Partitioning of Size-mismatched Impurities to Grain Boundaries in 2-d Solid Hard Sphere Monolayers” has been accepted for publication in the ACS journal Langmuir.  In this work, Ziwei used the Solvent Repacking Monte Carlo algorithm to model how, in a two-dimensional array of perfectly spherical colloids, impurities of different sizes are distributed between ordered regions and the less-ordered interfaces (grain boundaries) between them.  Solvent Repacking Monte Carlo provides a unique approach to this problem, as it allows efficient sampling of how larger particles can be distributed within a background of smaller particles, occupying space that would be taken up by multiple smaller particles.  Part of the motivation for this work was to better understand experiments reported by Lavergne et al. from the Colloids group led by Profs. Aarts and Dullens at the University of Oxford.

Ziwei’s results highlight the special behavior of impurities of diameter 4 times that of the smaller particles.  We expected that these should pack nicely in the ordered lattice, fitting neatly into a hexagonal region with little disruption to the surroundings, but were interested to see that at the grain boundaries they tend to occupy regions with pentagonal symmetry.   This influences their preference for the type (misorientation) of grain boundary they prefer to occupy, giving a distinctive pattern relative to impurities of different sizes.  Stay tuned for a link to the article when it appears, and for more Monte Carlo simulation of quasi-2d sphere packings by Ziwei and undergraduate researcher Peiyao Wu.