Surface ordering and capillary phenomena of confined hard cut-sphere particles.
Surface ordering and capillary phenomena of confined hard cut-sphere particles.
复制标题
受限硬切球颗粒的表面排序和毛细管现象。
作者：
M. M. Piñeiro;A. Galindo;A. O. Parry
为了研究禁闭对各向同性(I)-向列型(N)相变的影响，分别对=1500和=3000的长径比为0.1的硬切球体进行了等温-等压和Gibbs系综蒙特卡罗(GEMC)计算机模拟。我们首先考虑了自由系统，并证实了各向同性(I)态、向列态(N)和柱状态(COL)的稳定化。我们详细地研究了I-N相变，发现了=0.355和=0.368的共存密度。然后考虑两种类型的墙的平板几何形状：完全排除颗粒的硬壁和排除颗粒质心的‘吸附’墙。发现吸附体壁有利于平面(边向)取向，这导致第一层吸附分子的形成，然后作为后续粒子的粗糙硬壁，并促进无序状态。利用Gibbs系综模拟，我们确定了体系的毛细管相图，并确定了吸附作为孔宽度的函数。从Gibbs系综模拟得到的毛细管相图对应于具有一级毛细各向同性转变的相图，以及与∼3的壁分离相关的毛细临界点。硬壁被认为促进了切割球体的向同向同性(面)取向，并促进了向列相的稳定。在这种情况下，由GEMC模拟得到的毛细管相图显示出一级毛细管网状化转变，以及∼4壁面分离的毛细管临界点。
Isothermal-isobaric and Gibbs ensemble Monte Carlo (GEMC) computer simulations of = 1500 and = 3000 hard cut spheres of aspect ratio / = 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 =0.355 and =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 ∼3. 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 ∼4.