Charged Three-Dimensional Solitons in a Ferroelectric Nematic Liquid Crystal

Netra Prasad Dhakal; Zakaria Siddiquee; Hiroya Nishikawa; Fumito Araoka; James Gleeson; Samuel Sprunt; Antal Jakli

AI summary · arXiv: 2611.06884v1 · cond-mat.soft

Charged Three-Dimensional Solitons in a Ferroelectric Nematic Liquid Crystal

Netra Prasad Dhakal, Zakaria Siddiquee, Hiroya Nishikawa, Fumito Araoka, James Gleeson, Samuel Sprunt, Antal Jakli

Original paper Submitted to arXiv on 04 Nov. 2026 30 pages, 8 figures License: CC BY-NC-ND 4.0 Also in: fr

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The essentials

The authors report spontaneously propagating, oppositely charged solitons in ferroelectric nematic DIO after rapid cooling from its antiferroelectric phase.

Question

Can three-dimensional charged solitons form and propagate in a ferroelectric nematic without an applied electric field? The paper investigates their structure, motion, and a proposed mechanism for their formation.

Method

The authors observed DIO films 1.7–8.2 µm thick by polarized optical microscopy while cooling from the SmZA phase into the ferroelectric nematic phase at rates of 0.2–10 °C/min. They also examined DIO doped with EMIM-TFSI, tested RM734 and FNLC 919, and simulated director, polarization, and electrostatic fields.

Findings

Result V-shaped structures in DIO propagated along the rubbing direction without an external field, and their speed increased with cooling rate.
See the passage in the paper
They are always observed to propagate parallel to the rubbing direction, but opposite to the direction of the Vee.
Authors’ interpretation The observed director splay and the annihilation of counterpropagating solitons support the interpretation that opposite-moving structures carry opposite charges.
See the passage in the paper
The observations that the oppositely moving solitons annihilate each other and they have opposite director splay, i.e., opposite sign bound charge densities, support the idea that the solitons are charged and the oppositely moving solitons have different charges.
Authors’ interpretation The authors propose that soliton splay screens remnant mobile charges left by the antiferroelectric-to-ferroelectric transition.
See the passage in the paper
This is the charge which we propose is compensated with local, induced bound charges associated with non-zero splay.
Result The simulations produced textures resembling experimental observations and reproduced the motion under the stated condition of strongly enhanced splay elasticity.
Result Solitons were not observed in the tested RM734 or FNLC 919 samples.
See the passage in the paper
No solitons were observed on cooling to NF phase in either of these materials.

Limitations

The experimental observations are confined to the tested materials and conditions; solitons were found in DIO but not in the two other tested ferroelectric nematic materials.
See the passage in the paper
No solitons were observed on cooling to NF phase in either of these materials.
The proposed explanation involving remnant mobile charge is presented as a mechanism inferred from observations and simulations, rather than as a direct measurement of the charge distribution.
See the passage in the paper
We propose this is possible owing to the remnant distribution of mobile ionic species after rapid cooling through transition from the antiferroelectric SmZA to the NF.

Why it matters

The authors argue that ferroelectric nematic liquid crystals can host spontaneously propagating charged three-dimensional solitons without external-field driving. They propose that transient charges carried across a sharp phase transition enable polarization splay to form and propagate as a screening structure.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: Ferroelectric nematic; propagating solitons; polarization splay; ionic screening; antiferroelectric transition

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Comprehensive Summary
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Original abstract

The paper investigates whether stable, propagating three-dimensional solitons can form spontaneously in a ferroelectric nematic liquid crystal without an applied electric field, and what physical mechanism might produce them. The authors report observations in DIO after rapidly cooling it from its antiferroelectric SmZA phase into the ferroelectric nematic phase. They propose that mobile ionic charge left out of equilibrium by the transition is screened by localized splay of the ferroelectric polarization, producing charged structures that travel through the liquid crystal. The experiments used DIO films 1.7–8.2 micrometers thick, with planar alignment at the substrates. Samples were cooled through the SmZA-to-ferroelectric-nematic transition at rates of 0.2–10 degrees Celsius per minute. The researchers observed the textures using polarized optical microscopy, examined director orientation with different optical configurations, and measured the structures’ motion. They also added small concentrations of the ionic liquid EMIM-TFSI to test the effect of mobile ions. Finally, they compared results from other ferroelectric nematic materials and performed numerical simulations of the director, polarization, and electrostatic fields. In DIO, the team observed bright, elongated, V-shaped structures shortly after the transition. Their arms correspond to opposite out-of-plane director tilts of about 12 degrees, indicating a splay distortion. The structures move along the rubbing direction, opposite to the V’s point, and occur in roughly equal numbers in the two orientations. They are transient: in a film about 2 micrometers thick, they are visible at around 65.5 degrees Celsius, but none remain at 64.5 degrees Celsius. Their speed is about 3 micrometers per second at constant temperature and reaches roughly 6 micrometers per second during cooling at 10 degrees per minute. The authors report that speed increases with cooling rate, while noting that faster cooling also means the structures are observed at a higher average temperature, where viscosity is lower. The structures also depend on film thickness. Their director tilt increases with thickness, with an estimated threshold near 1.5 micrometers and a tilt-angle measure of about 45 degrees above 5 micrometers. Oppositely moving structures collide and disappear rather than passing through one another, consistent with the proposed model of oppositely charged solitons. Adding 0.016 percent ionic liquid increases their number and changes their close encounters: some structures join to form lens-shaped objects instead of annihilating. At 0.08 percent, no solitons are observed. No comparable solitons were found in RM734, which has little or no antiferroelectric range, or in the FNLC 919 mixture. The authors propose that the antiferroelectric phase has a periodic charge distribution associated with its polarization or density modulation. A sufficiently rapid transition may leave mobile charges temporarily out of equilibrium. Local polarization splay then creates bound charge that screens these ionic excesses, producing charged distortions that propagate in opposite directions according to their charge. Simulations produce optical patterns resembling those observed and reproduce the motion under the model’s assumptions, including a splay elastic constant about two orders of magnitude larger than the twist or bend constants. The authors interpret the findings as evidence for spontaneously formed, charged, propagating three-dimensional solitons. However, the charge distribution and screening mechanism are proposed rather than directly measured, and the interpretation relies partly on simulations. The observations are also limited to particular cooling conditions and materials, so the paper does not establish how broadly the effect occurs or fully characterize the structures’ three-dimensional dynamics.

Charged Solitons on the Move in a Ferroelectric Liquid Crystal

Can a liquid crystal spontaneously produce moving, three-dimensional structures without an applied electric field? In “Charged Three-Dimensional Solitons in a Ferroelectric Nematic Liquid Crystal,” Netra Prasad Dhakal, Zakaria Siddiquee, Hiroya Nishikawa, Fumito Araoka, James Gleeson, Samuel Sprunt et al. investigate striking, short-lived patterns that appear as the material DIO transforms into a ferroelectric nematic phase.

The researchers propose that the structures are charged solitons: localized distortions that travel through the material. Their results suggest a possible link between the material’s cooling history, mobile ions, and bends in its electric polarization. But while the observed motion and shapes support this explanation, the proposed charge-screening mechanism has not been measured directly.

Watching structures emerge during cooling

The team studied thin DIO films, between 1.7 and 8.2 micrometers thick, held in planar alignment at their substrates. They cooled the material from an antiferroelectric phase called SmZA into its ferroelectric nematic phase at rates ranging from 0.2 to 10 degrees Celsius per minute. Polarized optical microscopy revealed bright, elongated shapes soon after the transition.

Viewed from above, many of these structures look like a V. Their two arms correspond to opposite out-of-plane tilts of the director—the direction that describes the local alignment of the liquid crystal molecules. The tilt is about 12 degrees, and its pattern indicates a splay distortion: the director, and associated polarization, spreads apart or converges locally.

The V-shaped structures move along the direction used to rub and align the substrate, traveling opposite to the V’s point. Both orientations appear in roughly equal numbers. Their motion is measurable: at constant temperature, their speed is about 3 micrometers per second. During cooling at 10 degrees per minute, it reaches roughly 6 micrometers per second. Faster cooling is associated with faster movement, although the structures are also observed at higher average temperatures under those conditions, where the liquid crystal is less viscous.

Short-lived, thickness-dependent, and interactive

The structures do not persist throughout the ferroelectric phase. In a film about 2 micrometers thick, they can be seen around 65.5 degrees Celsius, but none remain at 64.5 degrees. Their form also depends on film thickness. The director tilt increases in thicker samples, with an estimated thickness threshold near 1.5 micrometers. Above 5 micrometers, a measure of the tilt angle is about 45 degrees.

Interactions between the structures offer another clue. When oppositely moving examples collide, they disappear rather than passing through each other. This is consistent with the idea that they carry opposite charges. The researchers also tested the role of mobile ions by adding small amounts of the ionic liquid EMIM-TFSI:

At 0.016 percent, the number of observed structures increases. Some close encounters produce lens-shaped objects rather than the disappearance seen in collisions between oppositely moving structures.
At 0.08 percent, no solitons are observed.

The effect was not found in the other materials tested. Neither RM734, which has little or no antiferroelectric range, nor the FNLC 919 mixture showed comparable solitons. This contrast points to a possible role for the transition from the SmZA phase, though it does not establish that the same behavior is impossible in other materials or conditions.

A proposed route from ions to moving distortions

The authors’ explanation begins with the antiferroelectric phase, which they propose has a periodic charge distribution associated with its polarization or density modulation. If cooling drives a sufficiently rapid transition, mobile ionic charge may be left temporarily out of equilibrium. A localized splay of the ferroelectric polarization creates bound charge, which could screen the ionic excesses. The resulting charged distortions may then travel in opposite directions according to their charge.

Numerical simulations help test whether this picture can reproduce the observations. They generate optical patterns resembling the experimental images and reproduce motion under the model’s assumptions. One notable requirement is a splay elastic constant about two orders of magnitude larger than the twist or bend constants.

Promising evidence, open questions

Together, the transient shapes, their directed motion, their collisions, and their response to added ions make a persuasive case for spontaneously formed, charged, propagating three-dimensional solitons in DIO under the reported conditions. The experiments also show how strongly their appearance depends on temperature, film thickness, cooling rate, and material composition.

Important uncertainties remain. The proposed ionic charge distribution and its screening by polarization splay were not directly measured, and simulations support the interpretation rather than independently confirming it. The study also does not fully characterize the structures’ three-dimensional dynamics or establish how broadly they occur beyond the materials and cooling conditions tested. The work therefore offers a compelling mechanism to investigate, as well as a vivid example of how a liquid crystal’s transition history may shape the structures that emerge.

Created on 09 Nov. 2026

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Charged Three-Dimensional Solitons in a Ferroelectric Nematic Liquid Crystal

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Charged Solitons on the Move in a Ferroelectric Liquid Crystal

Watching structures emerge during cooling

Short-lived, thickness-dependent, and interactive

A proposed route from ions to moving distortions

Promising evidence, open questions

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