Colloidal Crystallization in Confinement: Icosahedral Symmetry {\&} Plastic-Crystal Transitions

The crystallization of spherical nanoparticles in liquid droplets results under certain conditions in crystals with an icosahedral shape. Experiments with larger spherical colloids and computers simulations of hard particles demonstrate that such crystal shapes do not rely on energetic arguments, but already result from confinement and entropy alone. Experiments on rod shaped fluorescent colloidal particles that are monodisperse enough to form nematic and smectic liquid crystal phases under conditions where the double layer thickness is small compared to the diameter of the rods show that if the double layer thickness is significantly larger than the rod length plastic crystals with a body centered crystal structure are formed. In such crystals there is three dimensional positional order, but no orientational order of the rods. These plastic crystals under strong planar confinement show intriguing phase behavior where plastic crystal and full crystal phases alternate as a function of the separation between the confining plates.