Surface energetics of freely suspended fluid molecular monolayer and multilayer smectic liquid crystal films

Abstract

A study of the surface energetics of the thinnest substrate-free liquid films, fluid molecular monolayer and multilayer smectic liquid crystal films suspended in air, is reported. In films having monolayer and multilayer domains, the monolayer areas contract, contrary to predictions from the van der Waals disjoining pressure of thin uniform slabs. This discrepancy is accounted for by modeling the environmental asymmetry of the surface layers in multilayer films, leading to the possibility that preferential end-for-end polar ordering of the rod shaped molecules can reduce the surface energy of multilayers relative to that of the monolayer, which is inherently symmetric.fluid film | liquid crystals T he attractive Van der Waals interaction makes it possible for the existence of the liquid phase, which, at a temperature lower than the critical temperature, can coexist and share an interface with a gas phase that has different structure and properties (1). At this interface, the density changes across a distance on the order of the intermolecular distance of the liquid, resulting in a difference between the tangential stress and the normal pressure which generates the surface tension, equal to the excess surface energy (2, 3). It is thus interesting to study the surface energetics and tension in ultra-thin films in which the density and the normal pressure inside the liquid are not those of the bulk. These differences become larger as film thickness becomes smaller, so ultrathin films are of interest. However, liquid films in air thinner than h ≈ 2.5 nm rupture via spontaneous pore formation due to surface tension (4). Thus, in order to be stable against rupture, films must be internally structured to suppress thinning and maintain h ≥ 2.5 nm.Fluid smectic liquid crystals (LCs) are three dimensional (3D) phases of rod shaped molecules organized into periodic stackings of molecular layers, where each layer is a 2D liquid. At the air/LC interface, the smectic layering exhibits a strong tendency for the layers to be parallel to the interface, and thus for the molecules at the interface to be in the same smectic layer, typically of thickness d ≈ 3 nm. This organization enables the formation and stabilization of freely suspended smectic films consisting of an integral number, N, of smectic layers, where, for some materials, N can be as small as N ¼ 1, that is, fluid monolayers stable for extended periods have been made in several different materials (5,6,7,8). These fluid films are unique among monolayer liquid systems in that they are in a symmetric environment with vapor on both sides. Such substrate-free monolayer fluid molecular films are of fundamental interest in soft matter nanoscience, serving as fluid systems of reduced dimensionality for the study of fluctuation, symmetry, and interface effects (9, 10). Here we show that single and few layer smectic films can be used to measure the surface energy of a substrate-free fluid molecular monolayer.In smectic layers, the molecular long axes are narrowly distributed in...

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Acoustic streaming in two-dimensional freely suspended smectic liquid crystal films

“…Based on experimental measurements, we can also estimate the variation of horizontal velocity δu with a film thickness. For highly inhomogeneous acoustic excitation we obtain ηδu/h ∼ ∆p, where η = 5 • 10 −2 Pa•s is smectic dynamic viscosity [29] and ∆p ∼ 0.5 Pa is acoustic pressure, and thus δu ∼ 2 • 10 −3 mm/s. At the same parameters, the measured horizontal velocity was u ∼ 25 mm/s, and therefore the velocity field in thin smectic films is two-dimensional with a high accuracy.…”

Rapid thickness mapping of free-standing smectic films using colour information of reflected light II: real-time areal mapping using lookup table scheme

“…Free-standing smectic films are a quasi-2D condensed matter system, whose thicknesses can span from two to several hundred molecular layers [1,2]. As a critical physical variable, measurement of the film thickness is always a significant part of the film characterisation [1,[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. The existing techniques for thickness measurement are mostly optical, based on the multiple interference of incident light.…”

“…The existing techniques for thickness measurement are mostly optical, based on the multiple interference of incident light. For thinner films of up to 50 nm, the quadratic dependence of reflection intensity provides an accurate estimate of thickness [3][4][5][6][7][8][9]. For thicker films, the observation of the interference spectrum or colour under white light illumination plays a complementary role with a varying degree of precision [1,7,[10][11][12][13].…”