The nanotextured glass showing its anti-fogging abilities, and regular foggy glass
Clear and foggy

Researchers in the US have developed a new type of textured glass that they claim is glare free and could either be self-cleaning or highly anti-fogging. The surface of the material – described as "multifunctional" glass – has a nanotextured array of conical features that is coated in a surfactant, giving the glass its desirable properties. The researchers hope that in the future the glass can be cheaply manufactured so that it could be used in optical devices such as smartphone screens and televisions, solar panels, car windshields and even windows in buildings.

Rolling drops

Scientists already know that a superhydrophobic surface – a surface that repels water, such as a lotus leaf – arises from a combination of surface "roughness" and certain intrinsic chemical properties. Water drops bounce or roll easily off such surfaces, taking with them any dust and dirt, so cleaning the surface. It is known that such enhanced superhydrophobicity can be achieved by patterning a surface with a dense array of conical pillars with rounded caps. These shapes make the surface water-repellent, while the conical shapes give a greater resistance to a loss of superhydrophobicity.

Conical nano-forest

In this latest material, the surface pattern consists of tiny cones that are five times as tall as their base width of 200 nm. The researchers, led by George Barbastathis and colleagues from the Massachusetts Institute of Technology (MIT) in the US, fabricated the glass using coating and etching techniques adapted from the semiconductor industry. The process involves coating a silicon-oxide substrate with several thin layers of a surfactant, including a photoresist layer, which is then illuminated with a grid pattern and etched away to finally produce the dense conical array. Using one type of surfactant makes the surface highly superhydrophobic.

Interestingly, if the same conical structures are coated with a different surfactant, it promotes superhydrophilicity – it allows a continuous water film to grow over it. This makes it highly anti-fogging. Barbastathis tells physicsworld.com that "it is the nanocones that determine the optical properties, while the hydrophilic and hydrophobic nature of the glass is determined by the surfactant used, as they determine the wetting properties of the surface". Although the capped nanocones look rather frail under a microscopic, the researchers say their calculations show that the cones should be resistant to a range of forces, from the impact of raindrops in a strong downpour, wind-driven pollen and grit or even human handling. However, further testing is necessary to see how well the surfaces survive over time in practical applications.

Image showing the densely packed nanocones
Forest of nanocones

Inspired by nature

According to the researchers, the inspiration for their multifunctional glass came from nature – where many biological surfaces perform multiple specific tasks. For their work, they looked at everything from water-repellent lotus leaves to the Namib Desert beetle, which is capable of collecting water from fog on its hardened wings, and to moth eyes that helped develop anti-reflective coating.

The researchers point out that solar panels – which often lose efficiency becuase of dirt or dust layers – could be protected by a layer of the "self-cleaning glass". Furthermore, the anti-reflective properties of the glass would be better at transmitting incident light, especially when the Sun's rays are inclined at a sharp angle to the panel. Other applications could include using the glass in microscopes and cameras that are taken into humid environments, where both the anti-reflective and anti-fogging capabilities could come in handy.

An interesting application of using the glass with a different surfactant could be in building windows. "You could have superhydrophobic glass on the outside of the windows so that rainwater is washed off, and superhydrophyllic glass on the inside of the building so the windows do not fog up," explains Barbasththis.

In a brief on the MIT news site, the researchers say that in the future, textured glass could be produced "simply by passing it through a pair of textured rollers while still partially molten; such a process would add minimally to the cost of manufacture".

The research is published in ACS Nano 10.1021/nn301112t.