Phase Behavior of Bowl-Shaped Colloids: Order and Dynamics in Plastic Crystals and Glasses

Charged fluorescent bowl-shaped colloids consisting of a polystyrene core surrounded by a poly(N-isopropylmethacrylamide) shell are obtained by nanoengineering spherical composite microgels. The phase diagram of these soft bowl-shaped colloids interacting through long-range Yukawa-type interactions is investigated using confocal laser scanning microscopy. The bowl-shaped structure leads to marked differences in phase-behavior compared to their spherical counterpart. With increasing number density, a transition from a fluid to a plastic crystal phase, with freely rotating particles, followed by a glass-like state is observed. It is found that the anisotropic bowl shape frustrates crystallization and slows down crystallization kinetics and causes the glass-like transition to shift to a significantly lower volume fraction than for the spheres. Quantitative analysis of the positional and orientational order demonstrates that the plastic crystal phase exhibits quasi-long range translational order and orientational disorder, while in the disordered glass-like phase the long-range translational order vanishes and short-range rotational order appears, dictated by the specific bowl shape. It is further shown that the different structural transitions are characterized by decoupling of the translational and orientational dynamics.