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World’s first discovery of liquid directional steering on bio-inspired surfaces

Inspired by the leaves of Araucaria, the team designed the surface of a 3D ratchet structure. This allows liquids with different surface tensions to spread in different directions.Credit: City University of Hong Kong

Inspired by the leaves of certain trees, scientists at City University of Hong Kong have discovered that they can manipulate the direction of diffusion of different liquids deposited on the same surface, solving a challenge that has been going on for more than two centuries. This breakthrough could ignite a new wave of using 3D surface structures for intelligent liquid manipulation, including a variety of scientific and industrial applications such as fluid engineering design and enhanced heat transfer. Has a great effect on.


A research team led by Professor Wang Zuankai of the City U Department of Mechanical Engineering (MNE) discovered that the unexpected liquid transport behavior of Araucaria leaves provides an exciting prototype of liquid directional steering that pushes the frontier of liquid transport. ..Their findings were published in leading scientific journals Chemistry With the title “3D Capillary” RatchetTriggered liquid direction steering. “

Araucariaceae is a popular tree species in garden design. Its leaves are composed of periodically arranged ratchets that tilt toward the tip of the leaf. Each ratchet has a tip, the top surface has both horizontal and vertical curvature, and the bottom surface is relatively flat and smooth. When Dr. Feng Shile, one of the members of the research team, visited a Hong Kong theme park with Araucariaceae trees, the special surface structure of the leaves attracted his attention.

The special leaf structure allows the liquid to spread in different directions

“The conventional understanding is that the liquid deposited on the surface tends to move in a direction that reduces the surface energy. Its transport direction is mainly determined by the surface structure and has nothing to do with the properties of the liquid such as surface tension.” The professor says. king. However, the researchers found that liquids with different surface tensions spread in opposite directions on the leaves of Araucaria, in stark contrast to traditional understanding.

World's first discovery of liquid directional steering on bio-inspired surfaces

The team used a 3D ratchet to design a surface inspired by the leaves of Araucaria by mimicking its natural structure. This allows a mixture of water and ethanol with different surface tensions to spread in three directions: forward, backward and bidirectional.Credit: City University of Hong Kong

By mimicking its natural structure, the team has a leaf-inspired surface (ALIS) of Araucaria with a millimeter-sized 3D ratchet that can suck up liquid inside and outside the surface plane (ie, move it by capillarity). ) Was designed. They reproduced the physical properties of the leaves with 3D printing of polymers. They found that the structure and size of the ratchet, especially the reentrant structure at the tip of the ratchet, the spacing between the tips of the ratchet, and the tilt angle of the ratchet are important for liquid steering.

For liquids with high surface tension, such as water, the researchers found that one frontier of the liquid was “fixed” to the tip of the 3D ratchet. The distance between the tips of the ratchets is comparable to the length (millimeters) of the liquid capillaries, so the liquid can go backwards against the ratchet’s tilt. In contrast, for liquids with low surface tension, such as ethanol, the surface tension acts as a driving force, allowing the liquid to move forward along the ratchet tilt direction.

Liquids with varying surface tensions flow in different directions on the surface inspired by the leaves of Araucaria.Credit: City University of Hong Kong

First observation of liquid “selection” directional flow

“For the first time, we have demonstrated directional transport of different liquids on the same surface and successfully addressed problems in the field of surface and surface science that have existed since 1804,” said Professor Wang. “The rational design of the new capillary latch allows the liquid to” determine “the direction of diffusion based on the interaction of surface tension and surface structure. It was like a miracle of observing the flow of different liquids in different directions. This was recorded first. Observation in the scientific world. “

More interestingly, their experiments showed that a mixture of water and ethanol could flow in different directions on ALIS, depending on the concentration of ethanol. Mixtures with less than 10% ethanol propagated backwards in the ratchet tilt direction, and mixtures with more than 40% ethanol propagated in the ratchet tilt direction. A mixture of 10% to 40% ethanol moved in both directions at the same time.

“By adjusting the ratio of water to ethanol in the mixture, the mixture surface tension, You can manipulate the direction of liquid flow, “said Dr. Zhu Pingan, an assistant professor at MNE in City U, co-author of the paper.

The liquid moves in a circle on a round surface inspired by the leaves of Araucariaceae.Credit: City University of Hong Kong

Adjust the surface tension to control the spreading direction

The team also discovered that a 3D capillary ratchet can facilitate or impede liquid transport depending on the direction of the ratchet’s tilt. When ALIS with the ratchet tilted upward is inserted into a dish containing ethanol, ethanol Higher and faster than a surface with a symmetric ratchet (ratchet perpendicular to the ratchet) Water surface). Inserting ALIS with the ratchet tilted down reduced the increase in capillarity.

World's first discovery of liquid directional steering on bio-inspired surfaces

Members of the research team at City University of Hong Kong: Professor Wang Zuankai (left) and Dr. Zhu Pingan (right).Credit: City University of Hong Kong

Their findings provide effective strategies for intelligent guidance on liquid transport to target destinations, structurally derived liquid transport and microfluidic design, enhanced heat transfer, smart liquid sorting. Open new avenues for new applications such as.

World's first discovery of liquid directional steering on bio-inspired surfaces

Ethanol (left) and water (right) flow in different directions on the surface inspired by the leaves of Araucaria.Credit: City University of Hong Kong

“Our new liquid directional steering has many advantages, including self-propelled, well-controlled, high-speed, long-distance transport, and ALIS is easy to manufacture without complex micro / nanostructures. You can, “concludes Professor Wang.


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For more information:
3D Capillary Ratchet Induced Liquid Direction Steering, Chemistry (2021). DOI: 10.1126 / science.abg7552

Quote: The world’s first discovery of liquid direction steering on a biologically inspired surface (September 16, 2021) is https: //phys.org/news/2021-09-world-discovery-liquid-bio-inspired Obtained from -surface on September 16, 2021 .html

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World’s first discovery of liquid directional steering on bio-inspired surfaces

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