Natural structures are extremely diverse having evolved over millions of years into many forms with a mind-boggling array of functions. The success with which these natural structures provide functional solutions for plants and animals has inspired a new generation of chemists and engineers to look to nature for innovative technological solutions to modern problems. Drs Stuart Thickett and Chiara Neto, and Prof. Andrew Harris, all from the University of Sydney, are looking to a beetle from the Namib Desert for a means to make surfaces that are capable of efficiently collecting water from the atmosphere. Most of the moisture in the Namib Desert comes in the form of fog and the back of the Stenocara beetle is structured to capture it.
Tiny bumps on the beetle’s back attract water (hydrophilic). Between the bumps is a waxy material that repels water (hydrophobic). This differential arrangement of chemicals allows water droplets to nucleate on the bumps and grow until they are too big to stay there any longer. They then fall off and run along channels, between the bumps, into the insect’s mouth.
A material with these properties could be used to collect water in coastal settings where humidity can be high, or to provide a source of clean water in emergency situations.
In an attempt to create such a surface in the lab, Dr Thickett is using layered hydrophilic and hydrophobic polymers. A layer of hydrophilic polymer is placed on a hydrophobic polymer sheet and heated until only the hydrophilic layer melts. This liquid layer breaks apart in a process called dewetting, forming an array of small droplets, just like oil in a non-stick frying pan. When the temperature is lowered the polymer solidifies and the pattern of hydrophilic bumps is fixed in place. The two light micrographs on the left were taken in the AMMRF at the University of Sydney. They show the hydrophilic pattern that is created and the corresponding preferential formation of water droplets, just as occurs on the back of Stenocara.
The team has shown that their biomimetic surface is significantly more effective than a wholly hydrophilic surface. It appears that the small area of the bumps provides a focus for droplet formation, allowing them to grow rapidly. Hydrophobic channels would facilitate the efficient movement of the droplets to the collection point.
The team’s recent paper in Advanced Materials is just one example of progress in the burgeoning field of biomimetics.