Using roughness-controlled depletion attractions, we form dense monolayers of hard regular polygonal microplatelets that diffuse in a plane. We study emergent structures, as well as translational and rotational dynamics, as a function of particle area fraction by optical video microscopy. We encounter a striking variety of transitions, including disorder-order, order-order, and order-disorder, as well as frustrated glassy behavior. Although these particle shapes are relatively simple and the interactions are effectively hard, our observations reveal the rich complexity that can arise from entropy when nontrivial colloidal structures are crowded in a plane.
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