Using optical microscopy, we observe entropy-driven chiral symmetry breaking in 2D dense phases of hard Brownian polygonal platelets that have been lithographically fabricated and dispersed in water.
For equilateral triangles, we identify a new type of liquid crystalline phase, which we call a triatic liquid crystal. The triatic phase is similar in some ways to a hexatic liquid crystal of disks, but it is distinguishably different because the three nearest neighbors around a given triangle have local anticorrelations in their molecular orientational order.
Moreover, rotational entropic effects cause a form of local chiral symmetry breaking in the triatic phase that can be distinguished by a new lateral shift order parameter.
In a different system of 72-degree rhombs, the rhombs form rhombic colloidal crystals when concentrated in 2D, and these crystals exhibit a propensity to form twinned structures. We have observed simple contact twins, polysynthetic twins, and cyclic twins. Interestingly, we find a longer-range form of chiral symmetry breaking in the 2D rhombic crystals, related to the pointing direction of the rhombs relative to the crystal axes. Moreover, the chirality changes sign across the twin boundary of a simple contact twin.
Thus, entropy maximization alone is adequate for driving chiral symmetry breaking in monodisperse systems of simple achiral shapes.