Local structural ordering in surface-confined liquid crystals

Abstract

The effect of the interplay between attractive nonlocal surface interactions and attractive pair long-range intermolecular couplings on molecular structures of liquid crystals confined in thin cells with flat solid surfaces has been studied. Extending the McMillan mean field theory to include finite systems, it has been shown that confining surfaces can induce complex orientational and translational ordering of molecules. Typically, local smectic A, nematic, and isotropic phases have been shown to coexist in certain temperature ranges, provided that confining cells are sufficiently thick, albeit finite. Due to the nonlocality of surface interactions, the spatial arrangement of these local phases can display, in general, an unexpected complexity along the surface normal direction. In particular, molecules located in the vicinity of surfaces can still be organized in smectic layers, even though nematic and/or isotropic order can simultaneously appear in the interior of cells. The resulting surface freezing of smectic layers has been confirmed to occur even for rather weak surface interactions. The surface interactions cannot, however, prevent smectic layers from melting relatively close to system boundaries, even when molecules are still arranged in layers within the central region of the system. The internal interfaces, separating individual liquid-crystal phases, are demonstrated here to form fronts of local finite-size transitions that move across cells under temperature changes. Although the complex molecular ordering in surface confined liquid-crystal systems can essentially be controlled by temperature variations, specific thermal properties of these systems, especially the nature of the local transitions, are argued to be strongly conditioned to the degree of molecular packing.

Structural, Optical and Dynamic Properties of Thin Smectic Films

“…There are several methods to investigate the coupling between structural and dynamic properties of thin smectic films, both suspended in air or deposited on the solid surface, and molecular structure. A usual route in these studies is provided by a combination of statistical-mechanical theories [7,8,[11][12][13][16][17][18][19][20][21]23,25,26] and optical and calorimetric techniques [1,2,6,9,10,14,15,22,24,[29][30][31][32][33].…”

“…In this Section it will be considered the LC system confined between two parallel surfaces. Both the structural and optical properties of such LC system will be considered in the framework of the mean-field model [25,26]. The smectic phase, consisting of N smectic layers (each of the thickness d) oriented parallel to the bounding surfaces was considered to be the initial state of the LC system.…”

“…The geometry of the LC system used for theoretical analysis is shown in Figure 1. Two types of interactions will be considered, first, the short-range, rapidly decaying with distance, interactions of the smectic layers with confining surfaces (V i (i = 0, s)) and, second, the long-range van der Waals interactions between smectic layers (V ij (i = j)) [25,26]. Taking into account that the length of the smectic layers is much bigger than the thickness Nd, we can suppose that all the physical quantities depend only on the z-coordinate counted from the lower bounding surface.…”

“…These have been studied in different compounds [1,22,23] and consist of stepwise reductions of the value of C p (T) as the temperature T is raised above T AI(N) (bulk). Both by the calorimetric studies [1,2] and the theoretical investigations [7,23] it has been shown that the reduction of C p (T) is associated with the reduction of the number of layers in the film.In turn, according to experimental results obtained by applying the technique of fluorescence scanning laser confocal microscopy [24], orientational and translational order of LCs across confining films can be much more complex than that predicted theoretically by mean-field modeling [7,25,26]. This may suggest that complex molecular orderings in thin LC films, both deposited on the solid surfaces or stretched over an opening, can appear as the result of the interplay between short-range forces and long-range intermolecular interactions.…”

“…In turn, according to experimental results obtained by applying the technique of fluorescence scanning laser confocal microscopy [24], orientational and translational order of LCs across confining films can be much more complex than that predicted theoretically by mean-field modeling [7,25,26]. This may suggest that complex molecular orderings in thin LC films, both deposited on the solid surfaces or stretched over an opening, can appear as the result of the interplay between short-range forces and long-range intermolecular interactions.…”