Surface ordering and capillary phenomena of confined hard cut-sphere particles

Abstract

Isothermal–isobaric NPT and Gibbs ensemble Monte Carlo (GEMC) computer simulations of N = 1500 and N = 3000 hard cut spheres of aspect ratio L/D = 0.1, respectively, are carried out in order to investigate the effects of confinement on the isotropic (I)–nematic (N) phase transition. We first consider the free system, and confirm the stabilisation of isotropic (I), nematic (N) and columnar (Col) states. We examine in detail the I–N transition and find coexistence densities of ρ_I^*=0.355 and ρ_N^*=0.368. A slab geometry is then considered for two types of walls: a hard wall, which excludes the particles entirely, and an ‘adsorbent’ wall which excludes the centre of mass of the particles. The adsorbent wall is found to favour planar (edge-on) alignment, which results in the formation of a first layer of adsorbed molecules, which then acts as a rough hard wall for subsequent particles, and promotes disordered states. Using Gibbs ensemble simulations we determine the capillary phase diagram of the system, and the adsorption as a function of pore width. The capillary phase diagram obtained from Gibbs ensemble simulations corresponds to one with a first-order capillary isotropisation transition, with an associated capillary critical point for a wall separation of ∼3D. The hard walls are seen to promote homeotropic (face-on) alignment of the cut spheres, and promote the stabilisation of the nematic phase. In this case the capillary phase diagram obtained from the GEMC simulations exhibits a first-order capillary nematisation transition, and a capillary critical point for a wall separation of ∼4D.