An incredibly slippery, self-cleaning “super-glass” can be created from ordinary glass with the application of a simple, transparent coating that was recently invented by researchers from the Wyss Institute for Biologically Inspired Engineering at Harvard University and Harvard School of Engineering and Applied Sciences. The super-glass is so slippery that it even repels oil, and super-sticky materials such as honey — as well as resisting ice formation and the growth of bacterial biofilms. The researchers think that their new super-glass could be used to create “improved” solar panels, super-durable eyeglass lenses, self-cleaning windows, and new medical diagnostic devices.
The new coating was inspired by the ultra-slippery surface of the carnivorous pitcher plant, which feeds on insects by funneling them with their extremely slippery leaves into a “pitcher” filled with digestive fluids. The insects which venture onto the plants can’t grip the super-slippery surface and as a result fall into the digestive liquid and drown.
The new coating was built on previous work done by the researchers — their creation of the award-winning technology known as Slippery Liquid-Infused Porous Surfaces (SLIPS), the slipperiest synthetic surface currently known. This new coating flows in that tradition, being just as slippery but considerably more durable, as well as being completely transparent.
While SLIPS was certainly an important development in the field, this new coating is much closer to what the researchers had in mind when the first began their work on ultra-slippery surfaces. In particular, the original SLIPS needed to be fastened to an existing surface, a labor intensive process. A simple coating process would be much more practical.
“SLIPS repels both oily and aqueous liquids but it’s expensive to make and not transparent,” stated lead author Nicolas Vogel, PhD, a postdoctoral fellow in applied physics at Harvard SEAS. “It would be easier to take the existing surface and treat it in a certain way to make it slippery.”
The researchers sought to develop a coating that accomplishes this and works as SLIPS does. SLIPS’s thin layer of liquid lubricant allows liquids to flow easily over the surface, much as a thin layer of water in an ice rink helps an ice skater glide.
To create a SLIPS-like coating, the researchers corral a collection of tiny spherical particles of polystyrene, the main ingredient of Styrofoam, on a flat glass surface, like a collection of Ping-Pong balls. They pour liquid glass on them until the balls are more than half buried in glass. After the glass solidifies, they burn away the beads, leaving a network of craters that resembles a honeycomb. They then coat that honeycomb with the same liquid lubricant used in SLIPS to create a tough but slippery coating.
By adjusting the width of the honeycomb cells to make them much smaller in diameter than the wavelength of visible light, the researchers kept the coating from reflecting light. This made a glass slide with the coating completely transparent.
These coated glass slides repelled a variety of liquids, just as SLIPS does, including water, octane, wine, olive oil, and ketchup. And, like SLIPS, the coating reduced the adhesion of ice to a glass slide by 99%. Keeping materials frost-free is important because adhered ice can take down power lines, decrease the energy efficiency of cooling systems, delay airplanes, and lead buildings to collapse.
Importantly, the honeycomb structure of the SLIPS coating on the glass slides confers unmatched mechanical robustness. It withstood damage and remained slippery after various treatments that can scratch and compromise ordinary glass surfaces and other popular liquid-repellent materials, including touching, peeling off a piece of tape, and wiping with a tissue.
“We set ourselves a challenging goal: to design a versatile coating that’s as good as SLIPS but much easier to apply, transparent, and much tougher — and that is what we managed,” stated Aizenberg.
The researchers are now working to improve their method so as to allow “better coat curved pieces of glass as well as clear plastics such as Plexiglas, and to adapt the method for the rigors of manufacturing.”
“Joanna’s new SLIPS coating reveals the power of following Nature’s lead in developing new technologies,” stated Don Ingber, MD, PhD, the Wyss Institute’s Founding Director. “We are excited about the range of applications that could use this innovative coating.” Ingber is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and Boston Children’s Hospital, and Professor of Bioengineering at Harvard SEAS.
The new research was just published in the journal Nature Communications.