Liquid Crystal Orientation Transition on Microtextured Substrates

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 $0.8\mu \mathrm{m}$, 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 $\sim 40°$. 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.

Figure 1

AFM topographical images of several microtextured substrates we fabricated. The scale bars are $3\mu \mathrm{m}$. The arrow in the $\lambda =2.2\mu \mathrm{m}$ sample indicates from where the cross-sectional profile in the lower panel was drawn.

Figure 2

Optical reflection micrographs taken with crossed polarizers of LC cells made from a series of microtextured substrates. The upper plates of the cells are $\mathrm{I}\mathrm{T}\mathrm{O}/\mathrm{\text{glass}}$ coated with PI uniaxially rubbed along $y$. The microtextures in this layout have the same orientation as those in Fig. 1.

Figure 3

Inset: reflection images of a LC film freshly deposited on the $\lambda =0.8\mu \mathrm{m}$ microtexture rotated at different angles as indicated, sandwiched between crossed polarizers. Main panel: measured (symbols) and simulated (continuous lines) ${\mathrm{I}}_{\mathrm{c}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{s}\mathrm{e}\mathrm{d}}\mathrm{/}{\mathrm{I}}_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{e}\mathrm{l}}$ of the $\lambda =0.4$, 0.8, and $1.4\mu \mathrm{m}$ LC cells.

Figure 4

Results from the free energy minimization calculations displayed as average pretilt angle vs $\lambda$, for two sets of $W_{\theta }$ and $W_{\varphi }$ that give the two bounds to the observed transition. Dotted lines are guides to the eye.