Discovery of liquid directional steering on a bio-inspired surface — ScienceDaily

Maria J. Danford

Inspired by a form of tree leaf, researchers at City College of Hong Kong (CityU) found that the spreading path of distinctive liquids deposited on the exact surface can be steered, solving a obstacle that has remained for more than two hundreds of years. This breakthrough could ignite a new wave of making use of 3D surface structures for intelligent liquid manipulation with profound implications for different scientific and industrial apps, this kind of as fluidics design and style and warmth transfer improvement.

Led by Professor Wang Zuankai, Chair Professor in the Division of Mechanical Engineering (MNE) of CityU, the exploration staff uncovered that the surprising liquid transportation conduct of the Araucaria leaf gives an fascinating prototype for liquid directional steering, pushing the frontiers of liquid transportation. Their findings were posted in the scientific journal Science under the title “A few-dimensional capillary ratchet-induced liquid directional steering”.

Araucaria is a species of tree popular in backyard garden design and style. Its leaf consists of periodically arranged ratchets tilting toward the leaf idea. Each ratchet has a idea, with the two transverse and longitudinal curvature on its higher surface and a reasonably flat, clean base surface. When a person of the exploration staff associates, Dr Feng Shile, visited a concept park in Hong Kong with Araucaria trees, the exclusive surface composition of the leaf caught his awareness.

Exclusive leaf composition allows liquid to spread in distinctive directions

“The typical knowledge is that a liquid deposited on a surface tends to shift in directions that reduce surface power. Its transportation path is decided primarily by the surface composition and has almost nothing to do with the liquid’s properties, this kind of as surface tension,” stated Professor Wang. But the exploration staff uncovered that liquids with distinctive surface tensions exhibit reverse directions of spreading on the Araucaria leaf, in stark distinction to typical knowledge.

By mimicking its normal composition, the staff created an Araucaria leaf-encouraged surface (ALIS), with 3D ratchets of millimetre measurement that permit liquids to be wicked (i.e. moved by capillary motion) the two in and out of the surface airplane. They replicated the leaf’s actual physical properties with 3D printing of polymers. They uncovered that the structures and measurement of the ratchets, in particular the re-entrant composition at the idea of the ratchets, the idea-to-idea spacing of the ratchets, and the tilting angle of the ratchets, are very important to liquid directional steering.

For liquids with substantial surface tension, like water, the exploration staff found that a person frontier of liquid is “pinned” at the idea of the 3D ratchet. Given that the ratchet’s idea-to-idea spacing is equivalent to the capillary size (millimetre) of the liquid, the liquid can go backward towards the ratchet-tilting path. In distinction, for liquids with reduced surface tension, like ethanol, the surface tension acts as a driving drive and allows the liquid to shift ahead together the ratchet-tilting path.

To start with observation of liquid “deciding upon” directional movement

“For the 1st time, we demonstrated directional transportation of distinctive liquids on the exact surface, properly addressing a challenge in the discipline of surface and interface science that has existed because 1804,” stated Professor Wang. “The rational design and style of the novel capillary ratches allows the liquid to ‘decide’ its spreading path primarily based on the interplay amongst its surface tension and surface composition. It was like a miracle observing the distinctive directional flows of different liquids. This was the 1st recorded observation in the scientific environment.”

Even a lot more interesting, their experiments showed that a mixture of water and ethanol can movement in distinctive directions on the ALIS, depending on the concentration of ethanol. A mixture with considerably less than 10% ethanol propagated backwards towards the ratchet-tilting path, when a mixture with a lot more than forty% ethanol propagated toward the ratchet-tilting path. Mixtures of 10% to forty% ethanol moved bidirectionally at the exact time.

“By adjusting the proportion of water and ethanol in the mixture, we can modify the mixture’s surface tension, permitting us to manipulate the liquid movement path,” stated Dr Zhu Pingan, Assistant Professor in the MNE of CityU, a co-creator of the paper.

Managing spreading path by adjusting surface tension

The staff also uncovered out that the 3D capillary ratchets can either advertise or inhibit liquid transportation depending on the tilting path of the ratchets. When the ALIS with ratchets tilting upwards was inserted into a dish with ethanol, the capillary rise of ethanol was greater and more quickly than that of a surface with symmetric ratchets (ratchets perpendicular to the surface). When inserting the ALIS with ratchets tilting downwards, the capillary rise was lower.

Their findings deliver an powerful system for the intelligent guidance of liquid transportation to the goal location, opening a new avenue for composition-induced liquid transportation and emerging apps, this kind of as microfluidics design and style, warmth transfer improvement and sensible liquid sorting.

“Our novel liquid directional steering has quite a few rewards, this kind of as properly-controlled, rapid, prolonged-length transportation with self-propulsion. And the ALIS can be quickly fabricated with out challenging micro/nanostructures,” concluded Professor Wang.

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