Mean-field density functional theory of ananoconfined classical, three-dimensional Heisenberg fluid. I. The role of molecularanchoring

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 144, 194704 (2016) and may be found at https://doi.org/10.1063/1.4949330.In this work, we employ classical density functional theory (DFT) to investigate for the first time equilibrium properties of a Heisenberg fluid confined to nanoscopic slit pores of variable width. Within DFT pair correlations are treated at modified mean-field level. We consider three types of walls: hard ones, where the fluid-wall potential becomes infinite upon molecular contact but vanishes otherwise, and hard walls with superimposed short-range attraction with and without explicit orientation dependence. To model the distance dependence of the attractions, we employ a Yukawa potential. The orientation dependence is realized through anchoring of molecules at the substrates, i.e., an energetic discrimination of specific molecular orientations. If the walls are hard or attractive without specific anchoring, the results are “quasi-bulk”-like in that they can be linked to a confinement-induced reduction of the bulk mean field. In these cases, the precise nature of the walls is completely irrelevant at coexistence. Only for specific anchoring nontrivial features arise, because then the fluid-wall interaction potential affects the orientation distribution function in a nontrivial way and thus appears explicitly in the Euler-Lagrange equations to be solved for minima of the grand potential of coexisting phases.DFG, 65143814, GRK 1524: Self-Assembled Soft-Matter Nanostructures at Interface

A perspective on the microscopic pressure (stress) tensor: history, current understanding, and future challenges

The pressure tensor (equivalent to the negative stress tensor) at both microscopic and macroscopic levels is fundamental to many aspects of engineering and science, including fluid dynamics, solid mechanics, biophysics, and thermodynamics. In this perspective paper, we review methods to calculate the microscopic pressure tensor. Connections between different pressure forms for equilibrium and non-equilibrium systems are established. We also point out several challenges in the field, including the historical controversies over the definition of the microscopic pressure tensor; the difficulties with many-body and long-range potentials; the insufficiency of software and computational tools; and the lack of experimental routes to probe the pressure tensor at the nanoscale. Possible future directions are suggested.Comment: This is the final version accepted by The Journal of Chemical Physics (in press). The link to the article is https://aip.scitation.org/doi/abs/10.1063/5.013248

Monte Carlo study of quadrupolar Kihara fluids

Monte Carlo simulations of linear Kihara fluids with embedded point quadrupoles are reported for several elongations and values of the quadrupole. The quadrupolar contribution to the Helmholtz free energy is obtained and the effect of quadrupole on the structure is studied. The data obtained should be useful to check perturbation theories of convex quadrupolar Kihara fluids. An attempt to model carbon dioxide with the quadrupolar Kihara model has also been carried out. It is shown that accounting for the quadrupole moment in the model improves the agreement with measured properties. Overall this agreement is found to be good. These results encourage further theoretical work with this potential model

Imprinting substrate structures onto a nematic liquid crystal

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 137, 144703 (2012) and may be found at https://doi.org/10.1063/1.4757391.By means of Monte Carlo simulations in the grand canonical ensemble we study the morphology of the nematic phase of a simple model liquid crystal interacting with an alternating sequence of chemically different stripes. The stripes anchor molecules such that their orientation is either parallel or perpendicular with the substrate plane. The different molecular orientations are realized through anchoring functions that cause an energetic penalty for molecules oriented in an undesired fashion. We consider combinations of monostable and degenerate anchoring fields. The nature of the nematic phase is characterized through both the local nematic order parameter and the associated local director field. We observe states of uniaxial or biaxial symmetry depending on the ratio of stripe widths and the range of fluid-substrate attraction. In some cases the specific substrate pattern causes regions of biaxial symmetry to coexist with a bulk-like regime sufficiently far away from the substrates in which the local director field indicates a (homogeneous) bent state of the nematic liquid crystal.DFG, 65143814, GRK 1524: Self-Assembled Soft-Matter Nanostructures at Interface

Crossover from single-file to fickian diffusion in carbon nanotubes and nanotube bundles: pure components and mixtures

The diffusion mechanism of pure component Ar and binary mixtures of Ar/Kr and Ar/Ne confined in single-walled carbon nanotubes (SWNTs) and bundles was investigated by a combined Grand Canonical Monte Carlo and molecular dynamics study. For Ar confined in SWNTs, a crossover from single-file to Fickian diffusion existed when the density of Ar was a minimum as a function of the SWNT diameter. Argon diffused by a single-file mechanism in SWNTs smaller than an accessible diameter of 1.76σAr, corresponding to (7,7), (12,0) and (8,6) SWNTs but by a Fickian mechanism for SWNTs larger in diameter. Both components in Ar/Kr mixtures had a single-file diffusional mechanism in (6,6) and (7,7) SWNTs and a Fickain mechanism for SWNTs larger in diameter. Likewise, both components in a Ar/Ne mixtures had a single-file diffusional mechanism in a (6,6) CNT, and Ar had a single-file diffusional mechanism in a (7,7) SWCNT. However, Ne in the Ar/Ne mixture exhibited Fickian diffusion in the (7,7) SWNT , which indicated bi-modal diffusion. Larger diameters of SWNTs provided Fickian diffusion for both components in an Ar/Ne mixture. Argon diffused in a (25,0) SWNT bundle (with a bimodal pore size distribution) in a bimodal mechanism, with Ar diffusing in single-file in interstitial sites and in a Fickian mechanism in inner nanotube channels. In all cases of single-file diffusion the mean-squared displacement (MSD) of the fluid molecules had a square root of time dependence, while molecules diffusing by a Fickian mechanism had a MSD with a linear time dependence