Liquid Crystal Orientation Transition on Microtextured Substrates

Abstract

A uniform alignment of liquid crystal (LC) with finite pretilt was observed on microtextured substrates that were lithographically fabricated with alternating horizontal and vertical corrugations. As the period of alternation was decreased toward 0:8 m, the nematic LC alignment on these substrates changed from inhomogeneous in plane, copying the substrate corrugations, to a uniform configuration with a large pretilt of 40 . This transition is pertinent to a frustrated boundary wherein a lowering in the LC elastic energy due to spatial variation in the LC orientation compromises an increase in the surface anchoring energy. A model based on this idea demonstrates good agreement with the experiment. This result may open up a new arena for tailoring substrate characteristics for LC alignment. DOI: 10.1103/PhysRevLett.91.215501 PACS numbers: 61.30.Hn, 61.30.Dk, 61.30.Eb Substrate conditioning for the alignment of liquid crystal (LC) molecules is essential to the operation of LC displays. Most existing techniques used to produce the alignment effect involve uniform treatment of the substrate surface. An unconventional approach to uniform LC alignment, based on inhomogeneous patterning of the substrate, was proposed some time ago [1,2]. The idea is to create a situation wherein the LC director, in coping with the inhomogeneous substrate pattern, acquires so large an elastic energy that the LC has to compromise with a uniform configuration. In general, this frustrated boundary condition can be achieved by making the pattern period small enough. A few versions of this idea, including the use of inhomogeneous surfactant treatment [1,3] and inhomogeneous rubbing [4], have been reported successful. More recently, an interesting application of these substrates in multistable display, which may potentially lead to lower energy consumption, was demonstrated [5,6]. In these demonstrations, the LC director makes zero pretilt with the substrate, and the switching between different stable LC states was achieved by varying an in-plane electric field. However, out-of-plane switching is preferred since the electric field can be more uniform. But that requires the LC pretilt to be finite [7], which has not been found in any reported embodiments [1,3,4]. In yet some designs, a large LC pretilt is crucial as it widens the range of LC elastic constants usable, which would otherwise be too narrow to be practical [8]. Here we show that on lithographically made substrates with alternating horizontal and vertical corrugations, the nematic alignment undergoes an orientational transition that results in a large pretilt angle (40 ). A simple model used to simulate the transition not only showed good agreement with the experiment but also revealed that the polar surface energies of the frustrated boundary are only 1=10 of typical values. Weak surface anchoring energies are virtuous for lowering device energy consumption and response time [9]. Figure 1 shows a subset of the microtextured substrates employed in this study. They consist of arra...

Ordered interfaces for dual easy axes in liquid crystals

“…On the other hand, if one easy axis only is defined, W must be positive to insure one energy minimum only. It is known that microstructured substrates formed by alternating domains of different homogeneous planar anchoring, induce an average planar orientation if the domains are small enough (smaller than 1µm) [33]. This suggests that microscopical structure of the easy axes monitors the nature of the anchoring potential.…”

Detection of graphene domains and defects using liquid crystals

“…1b An LC is a self-assembled, birefringent, dielectric, elastic continuum, and its anisotropic direction of alignment is determined and confined by an anisotropic boundary domain. If the anchoring energy and the boundary domain are large enough to accommodate the elastic energy of the LC, the LC molecules preferentially align according to the anisotropy of each domain, thereby permitting visualization of the exact shape of each domain 25 ( Supplementary Fig. 11).…”

Phase behavior of a nematic liquid crystal in contact with a chemically and geometrically structured substrate

“…Adjacent regions of planar and homeotropic anchoring within a single substrate have been created, for example, by microcontact printed polar and apolar thiols, respectively, on an obliquely evaporated ultra-thin gold layer. Whereas many experimental and theoretical studies have focussed on the understanding of the interactions and phase behavior of NLCs in contact with either geometrically structured substrates (see e.g., [1,2,3,4,5,6,7,8,9,10,11,12,13]) or chemically patterned substrates (see e.g., [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]), NLCs near geometrically structured and chemically patterned substrates have not been investigated yet. In view of the rich behavior of NLCs even on geometrically structured or chemically patterned substrates, a combination of both surface treatments may open new possibilities for an improved performance of NLC cells.…”