Ordering, clustering, and wetting of hard rods in extreme confinement

Abstract

The effect of out-of-plane orientational and positional fluctuations is examined in the phase behavior of hard spherocylinders confined between two parallel walls. The stability of isotropic, nematic, and solid phases is studied for aspect ratios (κ=1+L/σ, where σ and L are the diameter and length of the cylinder) of 8, 10, and 16, while the width of the slitlike pore, H, is set in the quasi-two-dimensional regime, σ<H≤2σ. Using replica exchange Monte Carlo (REMC) simulations and Onsager theory we provide evidence for the stabilization of the nematic order with increasing pore width. The minimum surface density necessary to exhibit nematic order remains nearly the same with increasing H, while the upper bound of nematic order is postponed to higher densities in detriment of the single-layer-solid phase. We prove that a drying-out effect takes place in the pore as the density increases linearly from the wall to the middle of the pore in the ideal gas limit. This behavior is kept for the isotropic phase and partially in the nematic one, while a wetting behavior is observed at the walls mostly for the solid phases. For κ=8, we have also observed a compressibility signal of a transition between an isotropic fluid and a cluster-like fluid. This cluster phase is destabilized by increasing H. Finally, we build a master diagram, where the drying-wetting, solid-solid, and close packing density curves are independent from κ.