Soliton walls paired by polar surface interactions in a ferroelectric nematic liquid crystal

Abstract

Surface interactions are responsible for many properties of condensed matter, ranging from crystal faceting to the kinetics of phase transitions. Usually, these interactions are polar along the normal to the interface and apolar within the interface. Here we demonstrate that polar in-plane surface interactions of a ferroelectric nematic NF produce polar monodomains in micron-thin planar cells and stripes of an alternating electric polarization, separated by $${180}^{{{{{{\rm{o}}}}}}}$$ 180 o domain walls, in thicker slabs. The surface polarity binds together pairs of these walls, yielding a total polarization rotation by $${360}^{{{{{{\rm{o}}}}}}}$$ 360 o . The polar contribution to the total surface anchoring strength is on the order of 10%. The domain walls involve splay, bend, and twist of the polarization. The structure suggests that the splay elastic constant is larger than the bend modulus. The $${360}^{{{{{{\rm{o}}}}}}}$$ 360 o pairs resemble domain walls in cosmology models with biased vacuums and ferromagnets in an external magnetic field.

Ferroelectric nematic liquids with conics

“…Since 8 , K 1 in N F should be much larger than K 1,0 in N and larger than the twist K 2 and bend K 3 constants in N F . Experiments on planar DIO cells 18 suggest , in line with the observed predominance of bend in the textures of conics. Expulsion of splay is not absolute, however, since some splay develops at the border of vortices with opposite sense of polarization circulation, Fig.…”

“…Some variation of the DW and +1 disclinations along the z direction is expected, as in-plane deformations might trigger out-of-plane distortions; known examples are the so-called splay canceling 30 , structural twist in confined achiral nematics 31 , 32 , and twist relaxation of bend 29 mentioned above. The submicron details of the 3D structure of DWs should be explored by means such as electron microscopy since 3D optical imaging by fluorescence confocal polarizing microscopy (FCPM) 33 and coherent anti-Stokes Raman Scattering (CARS) 34 , 35 are not reliable because of the high birefringence of DIO 18 and RM734 (Supplementary Fig. 2b ), which defocuses the probing light beam 33 .…”

“…1d , or estimated from interference colors of the textures according to the Michel–Levy chart; is the birefringence; n o and n e are the ordinary and extraordinary refractive indices, respectively. The temperature dependence of DIO birefringence ∆ n was measured previously at the wavelength 535 nm 18 . At the temperatures of interest in our study, ∆ n = 0.20 at 45 °C and ∆ n = 0.19 at 65 °C.…”

“…The recently discovered ferroelectric nematic liquid crystal (NF) [6][7][8] is a liquid with a macroscopic spontaneous polarization 𝐏, locally parallel to the director 𝐧 ̂≡ −𝐧 ̂, which specifies the average quadrupolar molecular orientation 9 . The polarization-director 𝐏, 𝐧 ̂ couple could be aligned by an approach widely used in the studies and applications of liquid crystals 10 , namely, by confining the material between two glass plates with rubbed polymer coatings 7,8,[11][12][13][14][15][16][17][18][19] . In these samples, the DW shape is defined by the anisotropic surface interactions with the "easy axis" of the substrate and by the orientational elasticity of NF.…”

Ferroelectric nematic droplets in their isotropic melt

“…In these materials (called DIO and RM734, respectively) the ferroelectric phase did not form directly below the isotropic phase, but below an intervening non-ferroelectric nematic phase. These publications inspired many groups worldwide to study the nature of the ferroelectric nematic (N F ) phase of liquid crystals as summarized in a perspective article by Sebastia ´n et al 8 The N F phase with polar order 9,10 has unique viscoelasticity, 11 topology 12,13 and electrooptical properties [14][15][16] that may play a key role in many future technological advances like data storage, sensors, mechanical actuators, displays with sub-millisecond switching, and other optoelectronic applications. [15][16][17] In the past five years, many N F materials have been synthesized but only a few were found to exhibit a direct isotropic to ferronematic transition.…”