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New adaptive material takes inspiration from tears

10 April 2013

A 'tunable' material that takes it cue from the liquid film that coats your eyes, is easily adaptable for a diverse range of applications from textiles to optics.

Adaptive material with tunable transparency and wettability features (photo: Xi Yao)
Adaptive material with tunable transparency and wettability features (photo: Xi Yao)

Imagine a tent that blocks light on a dry and sunny day, and becomes transparent and water-repellent on a dim, rainy day; or highly precise, self-adjusting contact lenses that also clean themselves; or even pipelines that can optimise the rate of flow depending on the volume of fluid coming through them and the external environmental conditions.
A team of researchers at the Harvard School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering at Harvard have just moved these notions closer to reality by designing a new kind of adaptive material with 'tunable' transparency and 'wettability' features.
“The beauty of this system is that it’s adaptive and multifunctional,” says Professor Joanna Aizenberg of Harvard SEAS and the Wyss Institute.
The new material was inspired by dynamic, self-restoring systems in nature, such as the liquid film that coats your eyes. Individual tears join up to form a dynamic liquid film with an obviously significant optical function that maintains clarity, while keeping the eye moist, protecting it against dust and bacteria, and helping to transport away any wastes.
The bio-inspired material is a continuous liquid film that coats, and is infused in, an elastic porous substrate. It is based on a core concept: any deformation of the substrate, such as stretching, poking, or swelling, changes the size of the pores, which causes the liquid surface to change its shape.
With this design architecture in place, the team has thus far demonstrated the ability to dynamically and precisely control two key functions: transparency and wettability, reports Xi Yao, a postdoctoral fellow at Harvard SEAS and the Wyss Institute.
Sitting at rest, the material is smooth, clear and flat; droplets of water or oil on its surface flow freely off. Stretching the material makes the fluid surface rougher, and this makes it opaque. Significantly, however, it makes every droplet of oil or water that is placed on it to start and stop in a fully reversible manner.

The researchers claim that this capability is far superior to the 'switchable wettability' of other adaptive materials, which simply switch between two states — from hydrophobic to hydrophilic.

“In addition to transparency and wettability, we can fine-tune basically anything that would respond to a change in surface topography, such as adhesive or anti-fouling behaviour,” Yao explains.

They can also design the porous elastic solid such that it responds dynamically to temperature, light, magnetic or electric fields, chemical signals, pressure, or other environmental conditions, he said.
The material is a next generation development of a materials platform that Aizenberg pioneered a few years ago called SLIPS (Slippery Liquid-Infused Porous Surfaces). SLIPS is a coating that repels just about anything with which it comes into contact — from oil to water and blood.
But whereas SLIPS is a liquid-infused rigid porous surface, the new material is a liquid-infused elastic porous surface, which is what allows for the fine control over so many adaptive responses above and beyond its ability to repel a wide range of substances.

According to Aizenberg, a whole range of surface properties can now be tuned, or switched on and off on demand, through stimulus-induced deformation of the elastic material.
The researchers anticipate that the new material could find applications in a diverse range of industries, including oil and gas pipelines, microfluidic and optical systems, building construction, and textiles.

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