June 14, 2006 — A film surface that mimics a desert beetle's back to both attract and repel water could greatly improve efforts to harvest drinking water from the air in dry regions.
The surface, developed by scientists at the Massachusetts Institute of Technology in Cambridge, does what nature already figured out for the Stenocara beetle of the Namib Desert in southern Africa.
The insect's back is patterned with water-repelling spaces interspersed with water-attracting bumps.
In the early morning fog that frequents the desert, the beetle tilts its body into the wind.
Tiny water droplets accumulate on the bumps until the drops grow too heavy to balance. As they roll off, the drops slide down a path of water-repelling spaces right into the insect's mouth.
Materials chemist Lei Zhai, chemical engineer Robert Cohen, materials scientist Michael Rubner and their colleagues have created an intricately designed surface that behaves in a similar way.
"We can move water around on the surface in any pattern we like," said Cohen.
To create the surface, the researchers dipped a piece of film containing microscopic pores into alternating solutions of positively charged polymers and negatively charged nanoparticles of silica.
The opposites attracted, with the polymer serving as a glue to bind the silica to the porous film.
Next, the scientists coated the film with a thin layer of waxy molecules.
The film repelled water like a freshly waxed car. But the scientists were only halfway there.
To intersperse the surface with water-loving bumps, Zhai used a tiny pipette to precisely place drops of water and alcohol containing positively charged molecules onto the film.
The molecules burrowed into the pores of the film to link up with the oppositely charged silica. In doing so, they became anchored in the layers of the film, with a portion of the molecule sticking above the surface.
Zhai used the exposed bit as a platform to build up the water-attracting bumps, thereby completing the successful biomimicry of the beetle wing.
Cohen thinks the technique could be used not only to build more effective fog harvesting screens than those currently in use, but also to isolate drops of drug-related chemicals on a surface for testing.
Such a system could make so-called lab-on-a-chip technology cheaper and a lot simpler, said Julian F. V. Vincent, professor of Biomimetics at the University of Bath in London.
But certain challenges would have to be addressed first.
"If you're playing around with very small amounts of water it seems to me that you're in danger of having the volume of water changing quite quickly, depending on how much water is in the air around it," he said.
For now Cohen and his team are not concerned with developing the film's potential applications, but are motivated to more closely mimic the beetle.
"Students find it inspirational to know that there is some analog in nature to the work they are doing," said Cohen.
