Novel re-entrant transition as a bridge of broken ergodicity in confined monolayers of hexagonal prisms and cylinders

Monte Carlo simulations were used to study the entropy-driven monolayer assembly of hexagonal prisms and cylinders under hard slit confinement. At the conditions investigated, the particles have two distinct dynamically disconnected rotational states: unflipped and flipped, that cast distinct projected areas over the wall plane that favors either hexagonal or tetragonal packing. Our simulations revealed a re-entrant melting transition where a disorder Flipped-Unflipped (FUN) phase is sandwiched between a fourfold tetratic phase at high concentrations and a sixfold triangular solid at intermediate concentrations. The FUN phase contains a mixture of flipped and unflipped particles with high translational and rotational mobility. Complementary experiments with fabricated cylindrical microparticles confined in a wedge cell validated the formation of the simulated phases with a comparable fraction of flipped particles and structures, i.e., the FUN phase, triangular solid, and tetratic phase, indicating that both experiments and simulations approach sample analogous basins of particle-orientation phase-space. We also investigated the role of entropic forces on the re-entrant phase behavior by varying particle aspect ratio, (i.e., the ratio of circular face diameter to height of the particle).