Jun 26, 2006
Nanostructured surface mimics Namib desert beetle
Researchers at the Massachusetts Institute of Technology (MIT), US, have copied the structure of a beetle's wings to make surfaces with hydrophilic/superhydrophobic patterning. The artificial surfaces could have applications in water harvesting, controlled drug release, open-air microchannel devices and labs-on-a-chip.
The researchers read about the beetle's wings in a paper in Nature in 2001. "If you sat at your desk and tried to just think of ways to do things, it would take a very long time," said Robert Cohen of MIT. "Once you see these things in action, it's obvious what you have to do." Other scientists have used bio-mimicry to recreate the superhydrophobic properties of the Lotus leaf.
The Stenocara beetle, which lives in Africa's Namib desert, uses its wings to capture moisture from the morning fogs that are the most reliable source of water in the region. The fog is so light that normal condensation can't take place.
The insect angles its wings forwards and upwards into the wind. Water droplets in the fog coalesce onto hydrophilic bumps about 100 µm in diameter on the wing surfaces. Eventually the droplets become so heavy that they pull away and roll down the surrounding hydrophobic surface areas of the wing to the beetle's mouth.
To create their artificial structures, Robert Cohen, Michael Rubner and colleagues first made superhydrophic coatings by decorating microporous poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) microstructures with PAH/silica nanoparticles. They coated the resulting rough surface with a hydrophobic network of semi-fluorosilane molecules.
Next the researchers created hydrophilic regions by adding droplets of polyelectrolytes such as poly(acrylic acid) or poly(fluorescein isothiocyanate allylamine hydrochloride) (FITC-PAH) in a water/2-propanol solution. The team believes that these charged polymer chains formed electrostatic bonds with the PAH or nanoparticles of the substrate while parts of the chain remained on the surface, changing its wetting properties.
When such a structure incorporating 750 µm diameter hydrophilic spots of PAA was sprayed with a fine mist of water, it caused the droplets to aggregate together at the hydrophilic regions. According to the scientists, the hydrophilic patterns could be created using techniques such as inkjet printing, micropipetting and microcontact printing.
The team also modified the superhydrophobic surfaces with charged small dye molecules such as methylene blue and 2-propanol rose bengal in a solution of water/2-propanol. Adding the dye molecules made regions of the surface hydrophilic but, unlike for the larger polyelectrolyte modifiers, washing the structure with water removed dye molecules from its surface and restored the original superhydrophobic state.
But some dye molecules remained trapped inside the microstructure where water was unable to reach them due to the superhydrophobic surface. The only way to remove this dye was by rendering the surface hydrophilic with a PAA water/2-propanol solution before washing with water. This technique could have applications in drug storage and controlled release.
The team was able to make superhydrophilic canals by applying superhydrophilic multilayers onto hydrophilic stripes on the superhydrophobic surface. Such structures could have applications in microfluidic devices.
The researchers reported their work in Nano Letters.