The mechanism, which mechanical engineers from the City University of Hong Kong (CityU) describe in Nature Materials, can transport droplets of liquid against gravity and even for the first time along a vertical surface. Prof Wang Zuankai of CityU collaborated with Prof Xu Deng of the University of Electronic Science and Technology of China and Prof Hans-Jürgen Butt of the Max Planck Institute for Polymer Research in Germany on the work, key to which is manipulation of surface charge via liquid contact.
The researchers developed a superamphiphobic (water- and oil-repellent) surface and dropped a chain of water droplets onto it. On impact, the droplets immediately spread out, retracted, and rebounded from the surface. This resulted in electron separating from the droplets and creating a negative charge on the surface.
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By varying the height from which the droplets fell, the density of the charge on the surface gradually changed and formed a gradient. When I droplet was subsequently placed on the surface, this charge gradient acted as a driving force, making the droplet effectively self-propelled and moving towards the direction of higher charge density.
The team explains that previous methods for forcing droplets to move on the surface involves a trade-off: they can only be made to move at high speed over a short distance, and the further they moved, the slower they go. These techniques depend on altering the wetting gradient of the surface: that is, the extent to which surface tension keeps the droplets in their shape. But the charge density gradient technique does not have this drawback, they state. Moreover, the gradient can be easily changed once this created, enabling the programming of droplet motion pathways along the surface. The surface need not be flat or rigid, and liquids with low surface tension and low dielectric constant can be transported, including blood and salt solutions.
"We envision that our innovation in using surface charge density gradient to program droplet transport, which was not explored before, will open up a new research direction and potential in applications. For example, in bio-medicine, the design of surfaces with preferential charge density gradient may influence cell migration and other behaviours," said Prof Wang. The strategy could be applied to microfluidic lab-on-a-chip devices such as those used for biological analysis, and for engineering applications involving fluid dynamics, Prof Deng added.
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