The unique microscopic structure of the feathers of the eastern bluebird, a type of North American thrush, has inspired the creation of a simple-to-produce, scalable and robust novel synthetic material that could be used in batteries and water filters.
The striking brilliant blue of the eastern bluebird’s wings is due to a network of channels in its feathers with a diameter of only a few hundred nanometers. Seeing the potential for this networked structure as a usable material inspired a team of researchers from ETH Zürich to attempt to replicate it in the lab.
Using transparent silicone rubber as the starting material, the researchers placed it in an oily solution and left it to swell for several days in an oven heated to 140 °F (60 °C). It was then cooled to reduce the solubility of the liquid and the rubber extracted from the oily solution.
Analyzing the material under the microscope to see how its nanostructure had changed during the procedure, the researchers identified network structures similar to those in the bluebird’s feathers. The only real difference was the thickness of the channels formed; in the feather, they are approximately 200 nanometers, and in the synthetical material, they’re 800.
The key to forming the novel network structure in the material is phase separation. You may have encountered the phenomenon in the kitchen when trying to mix oil and vinegar to create a salad dressing. The liquids do mix when shaken but separate when the shaking stops. However, an alternative method can be used to mix the oil and vinegar: heating and then cooling. It’s the principle the researchers applied here, and interrupting the process is what creates the channels required.
“We are able to control and select the conditions in such a way that channels are formed during phase separation,” said Carla Fernández-Rico, the study’s lead author. “We have succeeded in halting the procedure before the two phases merge with each other completely again.”
The method used by the researchers produced synthetic material that’s several centimeters in size and is scalable.
“In principle, you could use a piece of rubbery plastic of any size,” Fernández-Rico said. “However, you’d then also need correspondingly large containers and ovens.”
The researchers say their novel material has drawn interest from the physics community.
“We have a simple system made of only two ingredients, but the final structure obtained is very complex and controlled by the properties of the ingredients,” said Fernández-Rico. “We have been approached by several theoretical groups that are proposing the use of physical models in order to understand the key physical principles of this new process and to predict its outcome.”
Practically speaking, they say the material could potentially be used in batteries and water filters. For water filters, the ratio of surface to volume is enormous when channel-like structures are used, making for more efficient removal of contaminants. If the surface area is insufficient, the solids will impact the media at a high velocity, causing premature degradation of the filter’s surface membrane or the underlying substrate media. An insufficient filter area will also increase the pressure drop through the system and result in higher energy consumption.
The battery electrolyte is a liquid or paste-like solution inside batteries and transports positively charged ions between the cathode and anode terminals. One of the reasons batteries lose their charging capacity over time – or fail – is because the ions react with the electrolyte, which causes the electrodes to establish physical contact and damage the battery. A solid electrolyte made from the material developed here would avoid physical contact between the electrodes while maintaining good ion transport through the battery.
“However, the product is still a long way from being ready for market,” Fernández-Rico said. “While the rubbery material is cheap and easy to obtain, the oily phase is quite expensive. A less expensive pair of materials would be required here.”
The researchers plan to improve the material, focusing on sustainability.
“Many natural polymers, such as cellulose or chitin, have a structure similar to the rubber used in our work,” said Fernández-Rico.
The study was published in the journal Nature Materials.
Source: ETH Zürich