(13b) Assembly of Anisotropic Colloidal Particles in Slit Confinement

Phases with diverse structural order can be realized by coupling the anisotropy associated with particle shape with the restriction of the entropic degrees of freedom of these particles by external potentials like in a slit confinement. In this work, we used Monte Carlo simulations to study the entropic-driven assembly behavior of hexagonal prisms and cylinder-shaped particles under quasi-2D confinement separations, using a parallel hard-wall slit confinement model. These particles are chosen to allow us to explore how confinement can tilt the balance between their competing tendencies to form structures with hexagonal or tetragonal 2D ordering by a single-component system. Our results revealed two types of first order phase transition upon increasing the concentration: 1) solid-solid transition (6-fold→4-fold symmetry solid) occurring through lattice symmetry breaking, and 2) solid to dense-liquid (disorder) to solid (6-fold symmetry→disorder→4-fold symmetry). The dense-liquid phase is rather peculiar as it occurs sandwiched between two solid phases and lacks both translational and orientational order. Similar phase behavior can be expected for n-gonal prism shaped particles where n ≥ 6. We conducted complementary experiments to probe the phase behavior of microfabricated cylinder-shaped colloidal particles confined in a wedge cell. For this purpose, we used fabricated particles with precise shape and size monodispersity using photolithography in a polymeric SU-8 photoresist [Hernandez, C. J., & Mason, T. G. (2007). J. Phys. Chem. C, 111, 4477-4480; Badaire, S. et al. (2007). JACS, 129, 40-41]. Fast confocal microscopy imaging of the cylinder-shaped particles assembled in a wedge confinement cell revealed the intermediate dense liquid (disorder) phase along with the 6-fold hexagonal and 4-fold square solid phases at different densities and confinement height. The unusual phase transitions reported in this work are attributed to particles having two distinct and dynamically disconnected rotational states: unflipped (with cylinder/hexagonal prism face parallel to the wall plane) and flipped (with cylinder/hexagonal prism side parallel to the wall plane), which have dissimilar translational entropy across the slit gap and cast distinct projection areas over the wall plane that favor distinct 2D lattice packing. We have also investigated the phase behavior of hexagonal prisms in a soft-repulsive wall model to show that it can provide a basis for designing an alternative experimentally viable strategy to dynamically bridge those rotational states observed for hard-slit phase behavior.