🔬 condensed matter

Electrically controlled propulsion of skyrmions in chiral nematic

Jiahao Chen et al. demonstrate that an electric field can drive skyrmions in chiral nematic liquid crystals along preprogrammed trajectories with variable speeds via flexoelectric polarization, offering a new approach to controllable microscale dynamics in soft matter.

Original authors: Oleg D. Lavrentovich

Published 2026-03-27

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Oleg D. Lavrentovich

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where tiny, invisible particles can swim through a thick, sticky fluid, not by flapping fins or spinning tails, but by simply being told where to go with a gentle push of electricity. This is exactly what scientists have achieved with a new discovery involving "skyrmions."

Here is the story of how they did it, explained simply:

1. The Problem: The "Thick Honey" Problem

For centuries, scientists have been fascinated by how tiny things move in fluids. But at the microscopic scale, physics is tricky. Imagine trying to swim in a pool filled with thick honey. If you stop moving, you stop instantly because the fluid is so sticky (viscous) that it overpowers your momentum. Also, the fluid is constantly jiggling due to heat, which makes it hard to keep a straight line.

Usually, scientists try to move tiny particles in water (which is isotropic, meaning it's the same in all directions). But water is a bit boring for control.

2. The Solution: A "Twisted" Fluid

The researchers decided to use a special fluid called a chiral nematic liquid crystal. Think of this not as water, but as a crowd of people holding hands in a line, all facing the same direction.

The Director: In this crowd, everyone is aligned. Let's say they are all facing East.
The Twist: Because these molecules are "chiral" (handed), they naturally want to twist as you move through the crowd, like a spiral staircase.

In this twisted crowd, you can create a special "bubble" or "soliton" called a skyrmion.

What is a Skyrmion? Imagine a tiny, self-contained tornado in the middle of the crowd. The people in the very center are facing North, while the people on the outside are facing South. They are connected by a smooth, twisting transition.
Why doesn't it disappear? In normal fluids, a bubble like this would shrink and pop because it costs too much energy to maintain the twist. But in this special twisted fluid, the "spiral staircase" nature of the crowd holds the bubble together, keeping it stable like a solid marble.

3. The Magic Trick: The "Electric Steering Wheel"

The big breakthrough is how they make these skyrmions move.

The AC Field (The Engine): The scientists apply an alternating current (AC) electricity. This makes the crowd of molecules wiggle up and down rapidly. This wiggling creates tiny currents in the fluid that push the skyrmion forward. Think of this as the engine running.
The DC Field (The Steering Wheel): Here is the clever part. The AC field alone just makes the skyrmion wiggle in place. To make it go somewhere, they add a tiny bit of direct current (DC) electricity.
- Normally, electricity just pushes things. But because of a quirk in physics called the flexoelectric effect, the shape of the twisted crowd changes slightly depending on the direction of the DC field.
- Imagine the skyrmion is a boat. The AC field is the wind filling the sails. The DC field is the rudder. By flipping the polarity (positive or negative) of the DC field, they can turn the rudder left or right. By changing the strength, they can speed up or slow down.

4. The "Flexoelectric" Secret

Why does the electricity act like a steering wheel?
Think of the molecules in the liquid crystal as tiny cones.

In a flat, calm crowd, the cones point randomly left and right. No net movement.
But when the crowd is twisted or bent (like in our skyrmion), the cones are forced to lean in a specific direction. This leaning creates a tiny electric charge (polarization).
When the scientists apply the DC electric field, it grabs onto this leaning charge and pulls it, creating a torque (a twisting force) that pushes the whole skyrmion in a specific direction.

5. Why This Matters

This is a huge step forward for "soft matter" science.

Micro-Robots: Imagine using these skyrmions as tiny delivery trucks. Because they can be steered precisely with electricity, they could carry microscopic cargo (like medicine) to a specific spot inside the body or a micro-chip.
Data Transport: They could also carry light or information, acting as tiny messengers in future optical computers.

In a nutshell:
The scientists found a way to turn a stable, particle-like "tornado" in a twisted fluid into a controllable vehicle. They use a fast-wiggling electric field to make it move, and a simple on/off electric switch to steer it anywhere they want, all without any moving mechanical parts. It's like teaching a ghost to drive a car.

Based on the provided text, here is a detailed technical summary of the paper regarding the electrically controlled propulsion of skyrmions in chiral nematic liquid crystals.

1. Problem Statement

Controlling microscale dynamics in fluids is a significant challenge in soft matter physics due to the dominance of viscous forces over inertia and the disruptive nature of thermal fluctuations. While external electric fields are commonly used to drive motion, traditional approaches in isotropic fluids (like water) or standard nematic liquid crystals face limitations:

Lack of Directionality: Standard nematics are paraelectric and possess head-tail symmetry ( $\hat{n} \equiv -\hat{n}$ ), meaning dielectric torques are insensitive to field polarity and cannot inherently provide a "sense of direction."
Instability of Solitons: In standard nematics, particle-like solitons (localized distortions of the director field) are thermodynamically unstable; shrinking them reduces total elastic energy, causing them to disappear.
Dimensionality Constraints: Most nematic topological defects are point- or line-like, whereas 3D particle-like solitons are required for complex microscale transport.

2. Methodology and Physical Mechanism

The study investigates the behavior of skyrmions (3D particle-like solitons) within a chiral nematic liquid crystal confined between glass plates with transparent electrodes. The experimental and theoretical approach relies on the interplay between specific material properties and electric field configurations:

System Setup: The liquid crystal is doped with chiral molecules to stabilize skyrmions via the Dzyaloshinskii-Moriya mechanism, preventing their collapse. The director ( $\hat{n}$ ) is initially aligned parallel to the plates (e.g., along the $x$ -axis).
Field Configuration:
- AC Field: An alternating current (AC) field is applied perpendicular to the plates ( $z$ -axis). Due to positive dielectric anisotropy ( $\Delta\varepsilon > 0$ ), this field reorients the director, creating oscillations.
- DC Field: A direct current (DC) field is superimposed on the AC field.
Core Mechanism (Flexoelectricity): The propulsion is driven by the flexoelectric effect, predicted by Robert B. Meyer.
- In deformed regions (splay or bend), the conical shape of liquid crystal molecules breaks parity, generating a macroscopic flexoelectric polarization ( $\mathbf{P}_{fl}$ ).
- The coupling between this polarization and the electric field creates a torque ( $\mathbf{\Gamma}_{fl} = \mathbf{P}_{fl} \times \mathbf{E}$ ).
- AC Component: Causes oscillation of $\mathbf{P}_{fl}$ and the director, generating hydrodynamic flows.
- DC Component: Breaks the symmetry of the director deformations, determining the amplitude and direction of the time-averaged $\mathbf{P}_{fl}$ . This acts as a "steering wheel," allowing the skyrmion to move in a specific direction based on the DC field's polarity and amplitude.

3. Key Contributions

Demonstration of Steerable Skyrmions: The authors (Li, Zhang, de Pablo, Lu, et al.) successfully demonstrated that skyrmions in chiral nematics can be driven along preprogrammed 2D trajectories with variable speeds.
Polarity-Dependent Control: They resolved the paradox of how an axially symmetric soliton acquires directionality. By utilizing the flexoelectric effect, they showed that the system can distinguish field polarity, unlike standard dielectric effects.
Theoretical Framework: The paper provides a physical explanation linking flexoelectric polarization to hydrodynamic flow, illustrating how the time-averaged interaction between the oscillating field and the static DC bias results in unidirectional propulsion.
Analogy to Directrons: The authors clarify the complex skyrmion dynamics by drawing parallels to simpler "directrons" (dissipative solitons in uniform nematics), showing that splay and bend deformations propel particles parallel or perpendicular to the director, respectively.

4. Results

Omnidirectional Propulsion: By adjusting the amplitude and polarity of the DC field, the skyrmions can be steered in any direction within the 2D plane of the cell.
Variable Speed: The speed of the skyrmion is tunable by modulating the electric field parameters.
Stability: The chiral nature of the medium successfully stabilizes the skyrmions against the elastic energy minimization that would otherwise cause them to shrink and vanish.
Hydrodynamic Coupling: The motion is confirmed to be driven by hydrodynamic flows generated by the oscillating flexoelectric polarization, rather than simple dielectric migration.

5. Significance and Applications

New Dimension in Microscale Hydrodynamics: This work adds a layer of control to soft matter dynamics, moving beyond simple isotropic transport to anisotropic, field-steered particle motion.
Targeted Delivery: The deformed director regions of skyrmions naturally attract colloidal inclusions. This suggests potential applications in the targeted delivery of optical information or micro-cargo (e.g., drugs or chemical agents) within microfluidic systems.
Future Directions: The findings open avenues for exploring collective skyrmion dynamics, where particles interact via hydrodynamic, elastic, and electrostatic forces. It also points toward the use of "flexoelectric nematics" (where polarization exists in the ground state) for even more efficient field coupling.
Fundamental Physics: The study bridges the gap between topological soliton physics, liquid crystal electro-optics, and low-Reynolds-number hydrodynamics, offering a robust platform for designing active soft matter systems.