Inspired by a kind of tree leaf, scientists discovered that the spreading direction of different liquids deposited on the same surface can be steered, solving a challenge that has remained for over two centuries. This breakthrough could ignite a new wave of using 3D surface structures for intelligent liquid manipulation with profound implications for various scientific and industrial applications, such as fluidics design and heat transfer enhancement.

Led by Professor Wang Zuankai, Chair Professor in the Department of Mechanical Engineering (MNE) of CityU, the research team found that the unexpected liquid transport behaviour of the Araucaria leaf provides an exciting prototype for liquid directional steering, pushing the frontiers of liquid transport. Their findings were published in the scientific journal Science under the title «Three-dimensional capillary ratchet-induced liquid directional steering».

Araucaria is a species of tree popular in garden design. Its leaf consists of periodically arranged ratchets tilting towards the leaf tip. Each ratchet has a tip, with both transverse and longitudinal curvature on its upper surface and a relatively flat, smooth bottom surface. When one of the research team members, Dr Feng Shile, visited a theme park in Hong Kong with Araucaria trees, the special surface structure of the leaf caught his attention.

Special leaf structure enables liquid to spread in different directions

«The conventional understanding is that a liquid deposited on a surface tends to move in directions that reduce surface energy. Its transport direction is determined mainly by the surface structure and has nothing to do with the liquid’s properties, such as surface tension,» said Professor Wang. But the research team found that liquids with different surface tensions exhibit opposite directions of spreading on the Araucaria leaf, in stark contrast to conventional understanding.

By mimicking its natural structure, the team designed an Araucaria leaf-inspired surface (ALIS), with 3D ratchets of millimetre size that enable liquids to be wicked (i.e. moved by capillary action) both in and out of the surface plane. They replicated the leaf’s physical properties with 3D printing of polymers. They found that the structures and size of the ratchets, especially the re-entrant structure at the tip of the ratchets, the tip-to-tip spacing of the ratchets, and the tilting angle of the ratchets, are crucial to liquid directional steering.

Story Source:
Materials provided by City University of Hong Kong. Note: Content may be edited for style and length.