Surface topography and rotational symmetry breaking

Abstract

The surface electroclinic effect, which is a rotation of the molecular director in the substrate plane proportional to an electric field E applied normal to the substrate, requires both a chiral environment and C(2) (or lower) rotational symmetry about E. The two symmetries typically are created in tandem by manipulating the surface topography, a process that conflates their effects. Here we use a pair of rubbed polymer-coated substrates in a twist geometry to obtain our main result, viz., that the strengths of two symmetries, in this case the rub-induced breaking of C(∞) rotational symmetry and chiral symmetry, can be separated and quantified. Experimentally we observe that the strength of the reduced rotational symmetry arising from the rub-induced scratches, which is proportional to the electroclinic response, scales linearly with the induced topographical rms roughness and increases with increasing rubbing strength of the polymer. Our results also suggest that the azimuthal anchoring strength coefficient is relatively insensitive to the strength of the rubbing.

Electroclinic effect in a chiral paranematic liquid-crystal layer above the bulk nematic-to-isotropic transition temperature

“…2 clearly shows a deviation opposite from those found elsewhere [42,43,44]. Although the induced order parameter S at the surface has been shown to increase in a mean-field-like manner [24], the chiral coupling with the substrate and the anchoring strength coefficient both play a role in the surface ECE response [34].…”

“…This easy axis typically is created by rubbing the substrate or polarized UV light exposure of the polymer alignment layer. Thus a surface ECE has been observed for chiral surfaces and an achiral liquid crystal [34,35,36], and for an achiral surface with an easy axis and a chiral liquid crystal [33].…”

“…As noted above, a surface ECE has been observed at temperatures in the nematic phase [32,35,36]. Importantly, smectic fluctuations near the substrate are not required for this effect to occur: The reduced symmetry at the surface, along with chirality at the surface, is sufficient to generate a surface ECE in the nematic phase [34]. The purpose of this work is to address the question of what occurs above the bulk isotropic -chiral nematic phase transition temperature on cooling the liquid crystal toward T IN .…”

“…At a liquid crystal / substrate interface, one may find an analogous rotation proportional to the applied field; this is the so-called surface ECE [30,31,32,33]. There are two conditions: i) The required chiral environment can be due to either liquid crystal chirality or surface chirality -or both; and ii) the required reduced rotational symmetry is due to the existence of an easy axis for planar alignment and the subsequent reduction of symmetry from a C ∞ to a C 2 rotation axis perpendicular to the surface [34]. This easy axis typically is created by rubbing the substrate or polarized UV light exposure of the polymer alignment layer.…”

“…In quasi-equilibrium the bulk liquid crystal director (away from the surface) is rotated elastically by the liquid crystal surface region [35,36]. The magnitude of this surface electroclinic coefficient e c = dθ/dE depends on the chemistry of the liquid crystal, its dipole moment, and the coupling between the liquid crystal and the substrate's easy axis [34]. But a necessary ingredient for the existence of a surface ECE is chirality at the interface.…”