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Sorting Liquids With a New Leaf-Inspired Surface
A new synthetic surface inspired by the Araucaria leaf can direct different liquids to flow in different directions, potentially leading to further applications in microfluidics and water desalination.

Often used in landscaping, the Araucaria are a group of coniferous trees that are indigenous to the Southern Hemisphere. Its leaves are awl-shaped with sharp-pointed ends that spiral in a scale-like pattern along the stem. Interestingly, water on the Araucaria leaf moves in one direction while ethanol on the same leaf moves in the opposite direction.

Capitalising on this observation, a team of researchers from the Dalian University of Technology, the City University of Hong Kong, and the University of Hong Kong developed a surface that can direct different liquids to flow in different directions.

Directional liquid transport finds applications in microfluidics, chemical reactions, and water harvesting. Such transport on surfaces can be carried out using specialised coatings that attract or repel liquids, or textures with specific curvatures. However, in most cases, fluid transport is restricted to one direction.

In this study, Shile Feng and colleagues looked deeper into the structural characteristics of the Araucaria leaf and observed periodically arranged ratchets tilting towards the tip of the leaf. They found that when continuously infusing water and ethanol on the Araucaria leaf, ethanol would spread along the ratchet-tilting (or forward) direction, while water would spread in the opposite direction.

Knowing this, Feng and colleagues used 3D printing to develop the Araucaria leaf-inspired surface (ALIS) with similar asymmetric profiles and properties as the leaf. Through several experiments, the team demonstrated how different liquids naturally steer in different directions when placed on their novel surface. High-surface-tension liquids like water prefer the backward direction, while low-surface-tension liquids like ethanol and oil prefer the forward direction.

To demonstrate the synthetic surface’s potential for liquid transportation, the team tested liquid transport on a surface with symmetric ratchets and compared it with their developed surface ALIS. They observed that the capillary rise on the ALIS is higher than the surface with symmetric ratchets, suggesting a marked increase in spreading. However, if the ratchet structures on the ALIS was arranged downward, then the capillary rise would be inhibited. This can be used in situations where minimal liquid propagation is preferred.

With this approach, their work demonstrates that 3D capillary ratchets can be utilised in fluid flow, such as well-controlled directional steering, self-propulsion, high-flow velocity, and long-distance transport. This can mean further developments in microfluidics and water desalination as well.

Source: Feng et al. (2021). Three-dimensional capillary ratchet-induced liquid directional steering. Science, 373(6561), 1344-1348.

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