Abstract
A molecular simulation study of binary mixtures of hard spherocylinders (HSCs) and hard spheres (HSs) confined between two structureless hard walls is presented. The principal aim of the work is to understand the effect of the presence of hard spheres on the entropically driven surface nematization of hard rod-like particles at surfaces. The mixtures are studied using a constant normal-pressure Monte Carlo algorithm. The surface adsorption at different compositions is examined in detail. At moderate hard-sphere concentrations, preferential adsorption of the spheres at the wall is found. However, at moderate to high pressure (density), we observe a crossover in the adsorption behavior with nematic layers of the rods forming at the walls leading to local demixing of the system. The presence of the spherical particles is seen to destabilize the surface nematization of the rods, and the degree of demixing increases on increasing the hard-sphere concentration.
Highlights
IntroductionParticle shape is one of the most important features governing the collective behavior of colloidal suspensions.[1,2] As the shape of particles deviates from spherical geometry, a rich phase behavior emerges due to the additional orientational degrees of freedom giving rise to a variety of complex crystal, plastic crystal, and liquid crystal (LC) structures.[3,4,5,6,7] Colloidal particles are appealing as model systems to study various physical phenomena due to the possibility of controlling the range, strength, and form of the interparticle interactions, allowing for the design of interactions that are purely repulsive at short range, i.e., approaching the hard-core interaction limit.[8] In his pioneering work of 1949, Onsager[9] offered a successful explanation for the isotropic-nematic phase transition observed in uniaxial anisotropic particles such as thin rods modeled as hard spherocylinders: cylinders of diameter D and length L capped at each end by hemispheres of diameter D
We first discuss our findings for the phase behavior of pure hard spherocylinders with an aspect ration of L/D = 5 in planar confinement between parallel structureless hard walls over a wide range of equilibrium pressures up to conditions where the bulk isotropicnematic phase transition is found
The appearance of this peak is explained by the fact that for these low-density states, hard spherocylinders with an aspect ratio of L/D = 5 maintain their orientational freedom at z ∼ 3D, which corresponds to half of the total length L + D of the particles.[31,32]
Summary
Particle shape is one of the most important features governing the collective behavior of colloidal suspensions.[1,2] As the shape of particles deviates from spherical geometry, a rich phase behavior emerges due to the additional orientational degrees of freedom giving rise to a variety of complex crystal, plastic crystal, and liquid crystal (LC) structures.[3,4,5,6,7] Colloidal particles are appealing as model systems to study various physical phenomena due to the possibility of controlling the range, strength, and form of the interparticle interactions, allowing for the design of interactions that are purely repulsive at short range, i.e., approaching the hard-core interaction limit.[8] In his pioneering work of 1949, Onsager[9] offered a successful explanation for the isotropic-nematic phase transition observed in uniaxial anisotropic particles such as thin rods modeled as hard spherocylinders: cylinders of diameter D and length L capped at each end by hemispheres of diameter D. As the aspect ratio L/D of the particles is decreased, the theory becomes less reliable, and more accurate methods such as higher-order density functional theories (DFTs)[11,12] and computer simulations[10,13] are required to accurately describe the ordering transitions
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