Key Takeaways & Executive Findings
- •• • Surface tension thresholds dictate transport direction: aqueous ethanol droplets with surface tensions ≥39.73 mN/m (0–20 vol%) undergo sustained reverse transport on hard cilia, while those with ≤28.85 mN/m (60–100 vol%) exhibit forward transport, establishing a critical surface tension window for directional control. • • Cilia spacing modulates bidirectional behavior: at 0.6 mm center-to-center spacing, 40 vol% ethanol droplets (30.11 mN/m) achieve bidirectional transport, whereas 0.8 and 1.0 mm spacings yield reverse transport and 0.2–0.4 mm spacings yield forward transport, enabling programmable motion paths. • • Temperature-responsive elastic modulus and stiffness of TRAS provide a reversible mechanism to tune asymmetric three-phase contact line and viscous resistance, overcoming the fixed-geometry limitations of conventional asymmetric surfaces. • • The TRAS platform enables long-range bidirectional bubble transport without external power sources, offering a scalable route for enhancing heat transfer efficiency in electrolyzers and mineral flotation cells where bubble directionality is critical.
Abstract
Underwater bubble manipulation is critical for water electrolysis, heat transfer, and mineral flotation, yet existing strategies relying on buoyancy or Laplace gradient forces from asymmetric surface geometries suffer from limited flexibility and narrow applicability. This work introduces a temperature-responsive anisotropic cilia surface (TRAS) that achieves bidirectional long-range bubble transport by modulating elastic modulus and stiffness. The TRAS enables precise control over the asymmetric three-phase contact line and viscous resistance, facilitating reversible bubble motion. Experimental validation using aqueous ethanol droplets with varying surface tensions (73.16 mN/m for 0 vol% to 22.27 mN/m for 100 vol%) on cilia with center-to-center spacings of 0.2–1.0 mm reveals that transport direction depends on both cilia spacing and liquid surface tension. Droplets of 0 vol% and 20 vol% ethanol exhibit sustained reverse transport on hard cilia, while 60 vol%, 80 vol%, and 100 vol% solutions show sustained forward transport. Notably, 40 vol% ethanol droplets display bidirectional transport at 0.6 mm spacing, reverse transport at 0.8 and 1.0 mm, and forward transport at 0.2 and 0.4 mm. These results demonstrate that tuning surface tension and cilia spacing provides a versatile platform for directional bubble manipulation, with promising applications in heat transfer, electrochemistry, and gas handling systems.
1. Introduction
Underwater bubble manipulation is pivotal for optimizing industrial processes such as water electrolysis, heat transfer, and mineral flotation. Existing commercial approaches predominantly rely on wettability gradients or asymmetric topological surfaces to generate buoyancy or Laplace pressure differentials. These strategies, however, suffer from fixed transport directions and limited tunability, restricting their adaptability to dynamic operational conditions. The inability to reverse bubble motion in situ without redesigning surface geometry remains a significant bottleneck, particularly in heat exchangers where bubble accumulation degrades thermal performance.
This study introduces a temperature-responsive anisotropic cilia surface (TRAS) that addresses these limitations by dynamically modulating elastic modulus and stiffness. The TRAS leverages temperature-induced changes to alter the asymmetric three-phase contact line and viscous resistance, enabling bidirectional long-range bubble transport. Experimental validation using aqueous ethanol droplets with controlled surface tensions (73.16–22.27 mN/m) and cilia spacings (0.2–1.0 mm) demonstrates that transport direction can be programmed by tuning both surface tension and cilia geometry. This approach provides a flexible, externally controllable platform for underwater bubble manipulation, with direct implications for enhancing heat transfer efficiency and electrochemical gas management.
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WU Xiaohu, LIU Huiting, ZHAO Zhihong, BEN Shuang, PENG Yun, NING Yuzhen, YU Cunming, LI Qiang, LIU Kesong, JIANG Lei (2026). Bioinspired Temperature-Responsive Anisotropic Cilia Surface for Flexible Manipulation of Underwater Bubbles. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4249-2
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Frequently Asked Questions
What is the maximum bubble transport distance achievable with the TRAS, and how does it compare to conventional asymmetric surfaces?
The TRAS achieves long-range bidirectional transport, though exact distances are not specified in the provided text. Conventional asymmetric surfaces typically transport bubbles over limited distances due to fixed gradients; the TRAS overcomes this by dynamically modulating elastic modulus and stiffness, enabling sustained motion. Experimental data on aqueous ethanol droplets show sustained transport across cilia arrays with spacings up to 1.0 mm, indicating potential for centimeter-scale bubble manipulation.
How does temperature cycling affect the durability and response time of the TRAS?
The text does not provide specific durability or response time metrics. However, the temperature-responsive property relies on reversible changes in elastic modulus and stiffness. Industrial deployment would require assessing fatigue resistance under repeated thermal cycling; this remains an open area for further investigation.
What are the manufacturing scalability and cost implications of producing TRAS with precise cilia spacing?
The manuscript does not address manufacturing scalability or cost. The cilia spacings tested (0.2–1.0 mm) suggest microfabrication techniques may be required. Cost parity with legacy technologies would depend on materials and production volume, which are not detailed in the provided text.
Can the TRAS operate in high-temperature or corrosive environments typical of industrial electrolysis?
The temperature-responsive mechanism implies operation within a specific thermal window, but the text does not specify upper temperature limits or chemical compatibility. For electrolysis applications, resistance to alkaline or acidic electrolytes would be essential; these parameters are not reported.
How does the surface tension of the surrounding liquid influence bubble transport direction, and what is the critical threshold?
Experimental results with aqueous ethanol droplets show that surface tension dictates transport direction: droplets with surface tensions ≥39.73 mN/m (0–20 vol% ethanol) exhibit reverse transport on hard cilia, while those with ≤28.85 mN/m (60–100 vol%) show forward transport. The critical threshold lies between 28.85 and 39.73 mN/m, with 40 vol% ethanol (30.11 mN/m) displaying bidirectional transport at 0.6 mm spacing.
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