Fluid superscreening and polarization following in confined ferroelectric nematics

Published in Physics
Fluid superscreening and polarization following in confined ferroelectric nematics
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The recent discovery of ferroelectric nematic (NF) fluids [1-3] has opened up a new frontier of soft matter science, thanks to their unique physical properties and potential applications. Ferroelectric nematics are liquid crystalline phases composed of highly polar, rod-shaped molecules that spontaneously align with their dipoles in a common direction, resulting in a large electric polarization comparable to that of ferroelectric crystals, while retaining the fluidity typical of conventional liquids. Due to this unique combination of properties, the NF phase has an ultra-sensitive response to external fields and an electrostatic phenomenology that is not yet fully understood but holds promise for future technologies.

In our article titled "Fluid superscreening and polarization following in confined ferroelectric nematics", we investigated the electro-optic response of a ferroelectric nematic fluid, RM734, confined to glass microchannels of various shapes. These microchannels were connected to gold electrodes to which we applied small electric fields (see Figure 1). Surprisingly, we discovered that when a potential difference (∆V) is applied between the ends of the channel, the NF phase orders itself with its polar director – and thus its polarization – continuously following the winding paths of the channels, regardless of their shapes, even in sections that are aligned against the "naive" electrode-to-electrode electric field direction. This intriguing response suggests a similar behavior of the local electric field, which is guided within the channel.

Figure 1)  a: schematics of a microchannel. Yellow cylinders represent the gold wire electrodes. b: Polarized optical microscope images between crossed polarizers of the Z, L and S channels, respectively, filled with RM734 in the NF phase at T = 130° C and oriented so that the lateral sections are at 45 degrees to (top panels) or along (bottom panels) the analyzer while ∆V = 1 V is applied to the electrodes.

We interpret this behavior as a consequence of what we term “fluid superscreening”, the prompt elimination of any electric field component perpendicular to the channel walls, due to polarization charges deposited on the surfaces through a minute reorientation of the polarization field. This phenomenon can be likened to soft magnetic systems, where magnetic fields are guided by canceling out their components that are normal to surfaces when they exceed the coercive field. In soft magnetic systems, this behavior is typically explained by the mismatch between the large magnetic permeability of ferromagnetic materials and the small magnetic permeability of the surrounding non-magnetic materials. The similarity between these physically distinct systems suggests that the easy optical access to polar switching in NF materials could be of significant interest in studying soft ferromagnets and high-permittivity dielectrics.

To better understand the behavior of confined NF fluids we also studied the response of the material after reversal of the sign of ∆V. When the potential is switched, a complex multi-step rearrangement of the liquid crystal occurs (Figure 2) starting with a sudden loss of order and the formation of irregular textures composed of micron-sized domains. Over time, these domains coarsen and merge, reducing the number of defect lines. Eventually, polar order is reestablished through the nucleation of a uniform region, which expands via the motion of two interfaces towards the electrodes. We discovered that the kinetics of this process is proportional to the applied field, indicating that the switching behavior indirectly provides information about the local electric field inside the channel. Moreover, we demonstrated that the local field remains constant in every position of the same channel, even in differently shaped channels, provided that the width, length, and applied potential remain constant. These findings reinforce the idea of a guided field within NF-filled microchannels.

Figure 2) Series of polarized optical microscopy images showing polarization inversion in the central portion of a Z channel after potential reversal. This series shows the formations of domains (t < 50ms), their coarsening and merging (50ms < t < 100ms) and the motion of the defected-uniform interfaces, represented as blue lines in the last panel (t > 100ms).

Finally, to better describe the experimentally observed phenomena, we conducted numerical simulations of the equilibrium structure and switching dynamics of NF fluids confined to microchannels. The equilibrium structure was obtained by minimizing the total free energy, including elastic and electrostatic contributions, using a relaxation method. The simulation results confirmed our experimental findings, with polarization continuously following the channels from electrode to electrode, even in bent-shaped geometries at equilibrium, and exhibiting a multistage switching process under potential reversal. These computational results underscored the crucial role of fluid superscreening, where the deposition of polarization charges screens any field component perpendicular to the channel surfaces, directing the electric field along the channel with roughly constant and uniform magnitude during switching.

Our findings suggest the possibility of geometrical control of the optical axis in multiple positions within electro-optic devices by leveraging the propagation of order within channels. More broadly, this study establishes a conceptual framework for understanding the behavior of NF materials in various settings in which geometrical confinement effects are relevant, such as composite structures, porous and/or disordered media with quenched or annealed disorder, and in the presence of topographically patterned substrates.

References:
1. Mandle, R. J., Cowling, S. J. & Goodby, J. W. A nematic to nematic transformation exhibited by a rod-like liquid crystal. Phys. Chem. Chem. Phys. 19, 11429–11435 (2017)
2. Nishikawa, H. et al. A fluid liquid-crystal material with highly polar order. Adv. Mater29, 1702354 (2017)
3. Chen, X. et al. First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: polar domains and striking electro-optics. Proc. Natl Acad. Sci. USA 117, 14021–14031 (2020)

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