The more we discover about the natural world, the more we discover that nature is the greatest engineer. Previous research considered that fluids can only be transported in a fixed direction on species with specific fluid communication properties and that they cannot change the direction of transport. Recently, researchers from Hong Kong Polytechnic University (PolyU) demonstrated that an African plant controls water movement in a previously unknown way – and this could lead to breakthroughs in a range of technologies in fluid dynamics and nature-inspired materials, including applications requiring multi-step and repeated reactions, such as microassays, medical diagnosis and solar desalination etc. The study was recently published in the international academic journal Science.
Fluid transport is an unsung wonder of nature. For example, tall trees must transport enormous amounts of water from their roots to their highest leaves every day, and they do so in perfect silence. Some lizards and plants conduct water through capillaries. In the desert, where it is crucial to make the most of scarce moisture, some beetles can capture fog-borne water and channel it down their backs using a chemical gradient.
Scientists have long tried to hone humanity’s ability to move fluids in one direction. Applications as diverse as microfluidics, water extraction and heat transfer depend on the efficient targeted transport of water or other fluids, on small or large scales. While the above types provide nature-based inspiration, they are limited to moving fluids in one direction. A research team led by Prof. WANG Liqiu, Otto Poon Charitable Foundation Professor of Smart and Sustainable Energy, Chair Professor of Thermal-Fluid and Energy Engineering, Department of Mechanical Engineering at PolyU, has discovered that the succulent plant Crassula muscosa, native to Namibia and South Africa, can transport liquids in selected directions.
Together with colleagues from the University of Hong Kong and Shandong University, the PolyU researchers observed that when two separate shoots of the plant were infused with the same fluids, the fluids were transported in opposite directions. In one case, the liquid flowed exclusively to the tip, while the other shoot directed the flow directly to the plant root. Given the arid but foggy conditions in which C. muscosa lives, the ability to capture water and transport it in selected directions is a lifeline for the plant.
Because the shoots were kept horizontal, gravity can be excluded as a cause of the selective transport direction. Instead, the plant’s special properties come from the small leaves found on the shoots. Also called ‘fins’, they have a unique profile, with a swept-back body (resembling a shark’s fin) that tapers to a narrow end that points towards the tip of the plant. The asymmetry of this shape is the secret of the selective directional fluid transport of C. muscosa. It all has to do with manipulating the meniscus: the curved surface on top of a fluid.
Specifically, the key lies in subtle differences between the fin shapes on different shoots. When the rows of fins bend sharply towards the tip, the liquid on the shoot also flows in that direction. However, on a shoot whose fins – even though they still point towards the tip – have a more upward profile, the direction of movement is towards the root. The direction of flow depends on the angles between the shoot body and the two sides of the fin, as these control the forces exerted on the droplets by the meniscus – blocking the flow in one direction and steering it in the other.
Armed with this insight into how the plant directs fluid flow, the team created an artificial mimic. Called CMIAs, for ‘C. muscosa-inspired arrays’, these 3D printed fins act like the tilted leaves of C. muscosa and control the orientation of fluid flow. Although the fins of a natural plant shoot are immobile, cleverly, the use of magnetic material for artificial CMIAs allows them to be reoriented at will. By simply applying a magnetic field, the fluid flow through a CMIA can be reversed. This opens the possibility for fluid transport along dynamically changing paths in industrial and laboratory environments. Alternatively, the flow could be redirected by changing the distance between the fins.
Numerous areas of technology can benefit from CMIAs. Prof. Wang said: “Real-time directional fluid flow control applications are envisioned in microfluidics, chemical synthesis and biomedical diagnostics. The biology-mimicking CMIA design could also be used not only for transporting fluids, but also for mixing them, for example in a T-shaped valve. The method is suitable for a range of chemicals and overcomes the heating problem associated with some other microfluidic technologies.”
