2 November 2021

Team solve liquid spreading conundrum with leaf-inspired surface

Hong Kong researchers solve two-hundred-year-old mystery using Araucaria leaf-type design.

© Shutterstock/Subrata Nath Photography

Scientists at City University of Hong Kong have discovered that the spreading direction of different liquids deposited on the same surface can be steered – a theory that has puzzled researchers for over two centuries.

Led by Professor Wang Zuankai, Chair Professor in the Department of Mechanical Engineering at the University, the team found that the unexpected liquid transport behaviour of the Araucaria leaf provides a prototype for liquid directional steering.

The findings, published in the journal Science, show that by mimicking the natural structure of the Araucaria leaf, the team designed an Araucaria leaf-inspired surface with 3D ratchets that enable different water-ethanol mixtures, which are subject to varying levels of surface tensions, to spread in three directions - forward, backward and even bi-directional.

The team found that the structures and size of the ratchets, especially the re-entrant structure at the tip of the ratchets and the tip-to-tip spacing of the ratchets, are crucial for liquid directional steering.

“The conventional perception is that a liquid deposited on a surface spread in a direction dictated by the surface structure but independent of the liquid properties, such as surface tension”, said Professor Wang. “We demonstrated the first-ever achievement in the directional selection of liquid transport, which successfully addresses a problem in the field of surface and interface science that has existed since 1804.” 

For a liquid with high surface tension, like water, one frontier of the liquid is pinned at the tip of the 3D ratchets, whose tip-to-tip spacing is comparable to the capillary length (millimetre), so the liquid goes backward relative to the ratchet arrangement. In contrast, for a low surface tension liquid like ethanol, the surface tension acts as a driving force, and the liquid moves forward along the ratchet-tilting direction. The rational design of the novel capillary ratchets creates enough room for the liquid to “decide” its spreading direction

For the water-ethanol mixture with proper water content, the liquid moves bi-directionally at the same time. “With different proportions of water and ethanol in the mixture, the surface tension is varied, so the liquid flow direction can be altered arbitrarily,” said Professor Wang. This provides an effective strategy for intelligent guidance of liquid transport to the target destination, opening a new avenue for structure-induced liquid transport and emerging applications.