🔬 condensed matter

Effect of a Cone Shape on the Motion of Active Janus Colloids

By combining 3D printing with AC electric field experiments, this study reveals that the cone shape of Janus colloids significantly influences their propulsion velocity and direction, with spherical particles moving fastest under induced-charge electrophoresis and large cone-protrusions accelerating under self-dielectrophoresis.

Original authors: Zhiyuan Zhao, Teun W. J. Verouden, Dillip K. Mohapatra, Evert M. E. Simons, Janne-Mieke Meijer

Published 2026-06-16

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CC BY 4.0

Original authors: Zhiyuan Zhao, Teun W. J. Verouden, Dillip K. Mohapatra, Evert M. E. Simons, Janne-Mieke Meijer

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 tiny, self-driving car that doesn't need a battery or gasoline. Instead, it runs on electricity from the air around it. This is what scientists call an "active colloid." In this study, researchers wanted to figure out how the shape of these tiny cars affects how fast they can drive.

Here is the story of their experiment, broken down into simple concepts:

1. The "Car" Design: Building with Light

The researchers needed to build these tiny cars in very specific shapes. Instead of using a mold, they used a high-tech 3D printer that uses two beams of light to "draw" plastic structures in mid-air, layer by layer. This is like using a laser pen to build a house out of invisible ink that instantly turns into solid plastic.

They built four types of "cars":

The Perfect Sphere: A smooth ball.
The Printed Sphere: A ball that looks round on top but has a flat bottom (because it was printed on a glass surface).
The Small Cone: A ball with a short, pointy tail sticking out.
The Big Cone: A ball with a long, pointy tail sticking out.

To make them move, they painted exactly half of each shape with a shiny layer of gold (like a half-moon sticker). This gold coating is the "engine" that reacts to electricity.

2. The "Road": The Electric Field

They dropped these tiny cars into a drop of water sandwiched between two glass plates. Then, they turned on an alternating current (AC) electric field. Think of this electric field as a giant, invisible wind blowing back and forth very quickly.

When the wind hits the gold-coated half of the car, it creates a tiny flow of water around the particle, pushing it forward. This is the "engine" kicking in.

3. The Race: Who is Fastest?

The researchers turned on the electric wind and filmed the cars racing. Here is what they found:

The Winner: The Perfect Sphere was the fastest. It zipped along smoothly.
The Runners-Up: The Small Cone was the second fastest, and the Big Cone was third.
The Loser: Surprisingly, the Printed Sphere (the one with the flat bottom) was the slowest by far.

The Analogy: Imagine trying to run. The Perfect Sphere is like a runner with perfect form. The Cones are like runners wearing backpacks; the bigger the backpack (the longer the cone), the harder it is to run fast. But the Printed Sphere is like a runner trying to run while dragging a heavy, flat board attached to their feet; it just drags them down.

4. The "U-Turn": Changing Direction

The most fascinating part of the experiment was what happened when they changed the speed of the electric wind (the frequency).

Low Frequency: When the wind blew slowly, all the cars drove forward with their pointy or flat sides leading (the non-gold side).
High Frequency: When they sped up the wind, every single car suddenly did a U-turn and started driving backward, now with their gold-coated side leading.

It's like a dance floor where the music changes tempo. At a slow beat, everyone dances facing one way. At a fast beat, everyone spins around and faces the opposite direction. The researchers found that the "magic speed" where this flip happens was about the same for all shapes, but the speed at which they danced changed depending on their shape.

5. The Big Surprise

The researchers expected that adding a cone (making the shape more pointy) would just make the cars slower because they are heavier and drag more water. That was true for the low-speed "wind."

However, when the cars flipped direction (the high-speed "wind"), the Big Cone actually got faster than the Small Cone! It seems that in this specific high-speed mode, having a long pointy tail actually helps the car cut through the water more efficiently, almost like a fish's tail helping it swim faster in a specific current.

Summary

This paper is a race report for microscopic 3D-printed robots. The main takeaways are:

Shape matters: Being a perfect ball makes you the fastest. Adding pointy bits usually slows you down, unless you are driving in a specific "high-speed" mode where the pointy bits might actually help.
Flaws matter: Even a tiny flat spot on a round ball (from the printing process) can make it the slowest of all.
Control: By simply changing the speed of the electric field, you can make these tiny robots drive forward or backward, regardless of their shape.

The study shows that if we want to build better microscopic robots for the future, we have to be very careful about their exact shape, because even small changes can drastically change how fast they move.

Technical Summary: Effect of a Cone Shape on the Motion of Active Janus Colloids

Problem Statement
While the propulsion of active colloids is known to be governed by symmetry and shape, there is a lack of systematic investigation into how small, gradual geometric effects influence the locomotion of anisotropic active colloids. Existing studies often compare disparate shapes where changes in geometry coincide with changes in the size or shape of the symmetry-breaking component (e.g., the metal coating), making it difficult to isolate the specific influence of particle morphology on propulsion velocity.

Methodology
The authors employed a combination of high-resolution two-photon polymerization (2PP) 3D printing and AC electric field experiments to study anisotropic Janus microswimmers.

Fabrication: Using Nanoscribe 3D printing, the team fabricated a series of colloidal particles with identical metal-coated hemispheres (30 nm Au on 5 nm Cr) but varying non-coated geometries. The shapes included:
1. A standard sphere (chemically synthesized).
2. A printed sphere (3D printed, resulting in a flat base due to substrate adhesion).
3. A hemisphere with a small truncated cone (HS-Cone2, 2 µm height).
4. A hemisphere with a long truncated cone (HS-Cone4, 4 µm height).
Experimental Setup: Particles were suspended in an aqueous medium between ITO-coated glass coverslips. An alternating current (AC) electric field (6 Vpp) was applied vertically.
Characterization: Particle motion was tracked using bright-field video microscopy (10 fps) in a quasi-two-dimensional plane. Mean-squared displacement (MSD) analysis was used to extract translational velocities and confirm active, ballistic motion. Experiments were conducted across a frequency range of 1 kHz to 1 MHz to probe different electrokinetic regimes.

Key Results

Shape-Dependent Velocity: Under identical propulsion conditions (10 kHz), the particles exhibited distinct velocities based on their geometry:
- Fastest: Chemically synthesized Janus spheres ( $v \approx 8.9$ µm/s).
- Intermediate: HS-Cone2 ( $v \approx 7.4$ µm/s) and HS-Cone4 ( $v \approx 4.0$ µm/s).
- Slowest: The 3D-printed sphere with a flat base ( $v \approx 2.6$ µm/s).
  The data indicates that increasing conical deformation generally reduces propulsion speed, likely due to increased particle volume/weight. However, the presence of a flat side on the printed sphere caused a drastic reduction in velocity, attributed to out-of-plane rotations and altered hydrodynamic slip layers.
Direction Reversal Mechanism: All particle shapes exhibited a reversal in the direction of motion as the driving frequency increased.
- Low Frequency (< ~50 kHz): Motion is driven by Induced-Charge Electrophoresis (ICEP), where particles move with their dielectric (non-coated) side forward.
- High Frequency (> ~50 kHz): Motion is driven by Self-Dielectrophoresis (sDEP), where particles reverse direction and move with their metallic (gold-coated) side forward.
  The critical frequency ( $\omega_{cr}$ ) for this reversal remained consistent across shapes (approximately 50 kHz), indicating that the fundamental electrokinetic mechanism is robust to the geometric variations studied.
Regime-Specific Behavior: While the critical frequency was shape-independent, the propulsion velocity in the sDEP regime was shape-dependent. Notably, the HS-Cone4 (large cone) exhibited a higher velocity in the sDEP regime compared to the HS-Cone2, suggesting that larger conical protrusions can enhance speed in the high-frequency regime.

Significance and Claims
The paper claims to provide a systematic insight into the link between active particle geometry and propulsion velocity by isolating shape variables while keeping the symmetry-breaking metal coating constant. The authors assert that:

Geometric Tuning: Propulsion speed can be tuned not only by external field parameters (frequency, voltage) but also by morphological design.
Mechanism Robustness: The interplay between ICEP and sDEP provides a reliable, remote-control mechanism for fuel-free anisotropic microswimmers, regardless of the specific cone shape.
Design Implications: The findings offer guidelines for engineering active colloids. Specifically, the observation that a flat side drastically reduces speed and that large cones can enhance speed in the sDEP regime are critical for designing optimized microrobots for applications such as targeted delivery and complex micromanipulation.

The study concludes that 3D printing combined with AC electric fields offers a robust framework for fabricating and controlling these systems, paving the way for sophisticated microrobots with tailored shapes for enhanced functionality.