Textures of Nematic Liquid Crystal Cylindric-Section Droplets Confined by Chemically Patterned Surfaces

Abstract

The director fields adopted by nematic liquid crystals (LCs) that are confined by the surface to form long, thin droplets are investigated using polarising optical microscopy. Samples are produced by de-wetting of the LC on a surface patterned with alternating high-surface energy and low-surface energy stripes of 10–30 μm width. The droplets obtained are expected to adopt a profile which is that of a longitudinal section of a cylinder and, as this suggests, the director fields observed are variants in the case where the LC is constrained in a cylindrical capillary or fibre. Hence, when there is normal anchoring at the air interface, the textures observed are related to the well-known escaped radial texture (for the nematic LC mixture E7) or plane polar texture (for the LC mixture MLC6609). More surprising is the observation that the nematic LC mixture MLC7023, which is anchored in a planar or tilted manner at the air interface, also gives what appears to be an escaped radial director field. As an exploration of the possibility of using these systems in creating sensors, the effects of adding a chiral dopant and of adding water to the substrates are also investigated.

Alternating defects and egg and dart textures in de-wetted stripes of discotic liquid crystal

“…It may simply be that (within the confines of the droplet) this is the thermodynamically favoured way for these domains to be matched up with each other. Arguments of this kind have been used to explain another example of alternating defects; the alternation of radial and hyperbolic defects in escaped radial director fields of nematic LCs [21,43]. An alternative approach to the problem is to treat these features as paramorphotic; to argue that they are produced because the development of strain at the isotropic to columnar transition results in buckling of the columns.…”

“…In the case of a previous study of columnar discotic LCs using stripes ~10 μm wide we found droplet profiles that approached those of a hemi-cylinder (Figure 4(a), contact angles approaching 90°) [22] but in more recent studies of calamitic nematic LCs on stripes ~20 μm wide we have found very low contact angles (~10°); much more shallow longitudinal sections of a cylinder. (Figure 4(b)) [21].…”

“…For the transmission mode images (Figures 6, 7, 9-11) the patterned substrates were made using the positive photoresist S1813 as described in Crystals [21]. The substrates were glass, the high-energy stripes were glass and the low-energy stripes were a silane-on-glass self-assembled monolayer, which was made from 1 H, 1 H, 2 H, 2 H-perfluorodecyltrichlorosilane.…”

“…Production of the thin de-wetted stripes of DLCs, which is the main theme of this paper, relies on isotropic phase de-wetting of a thin film of the LC on a surface patterned with narrow alternating stripes of high energy (hydrophilic) and low energy (hydrophobic) surface [21,22]. On such a surface there is spontaneous de-wetting of the LC onto the high energy stripes.…”

“…On such a surface there is spontaneous de-wetting of the LC onto the high energy stripes. On these small scales, the profile of the droplets formed is determined by surface tension leading to even curvature at the air to LC interface; Figure 4 [21]. In the case of a previous study of columnar discotic LCs using stripes ~10 μm wide we found droplet profiles that approached those of a hemi-cylinder (Figure 4(a), contact angles approaching 90°) [22] but in more recent studies of calamitic nematic LCs on stripes ~20 μm wide we have found very low contact angles (~10°); much more shallow longitudinal sections of a cylinder.…”