Mechanical Engineers At MIT Develop Naturally Waterproof Surface

Details -- Scientists working in New England are well aware that they have chosen to pursue their careers in a region of the country that is used to wet weather. With that in mind, it is probably of little surprise that engineering research into increasing the efficiency of hydrophobic materials has discovered by a team of mechanical engineers from the Massachusetts Institute of Technology.

The research, which has been published in the journal Nature, has been instrumental in creating what some believe to be the most waterproof material ever, with the scientists involved admitting that they took inspiration from the way butterfly wings stay dry, whatever the weather. If this is the case, it could have significant applications in terms of making certain metals, fabrics and ceramics less susceptible to moisture, and potentially be used to prevent ice build-up in a number of engineering resources such as wind turbines or even aircraft wings.

Reducing water contact:
According to the World Science Report, the key factor was finding a means to surpass the minimum time it takes for a bouncing droplet of water - in other words, rain - to stay in contact with a surface, with the notion being that minimizing the interaction between that surface and the water would, potentially, improve the waterproofing potential of a chosen material. This theoretical length - known as the Rayleigh time - is extremely important in nature, with previous studies identifying the lotus leaf as the one that stays dry the most. "The time that the drop stays in contact with a surface is important because it controls the exchange of mass, momentum, and energy between the drop and the surface," said Kripa Varanasi, the Doherty Associate Professor of Mechanical Engineering at the university. "If you can get the drops to bounce faster, that can have many advantages." While the aforementioned lotus was deemed to be the gold standard, the MIT researchers now think that ridges found on the wings of the Morpho butterfly, as well as nasturtium leaves, may actually hold the answer. According to MIT News, the team was able to break through the Rayleigh limit and demonstrate contact times that were 40 percent shorter than previously thought possible, with the eventual aim being to reduce that time by up to 80 percent. To achieve this, they added macroscopic ridges to a number of hydrophobic surfaces, allowing the droplets to split when coming into contact with the ridged material. While the amount of time that the water stayed in contact with the chosen surface was nanoseconds, the fact remains that any reduction in duration that water stays in contact with a surface or material is of great interest to engineers or companies who manufacturer products that are affected by wet weather.

"We've demonstrated that we can use surface texture to reshape a drop as it recoils, in such a way that the overall contact time is significantly reduced," said James Bird, an assistant professor of mechanical engineering at Boston University and the paper's lead author. "The upshot is that the surface stays drier longer if this contact time is reduced, which has the potential to be useful for a variety of applications."

Surpassing the lotus effect:
While the MIT research remains in its early stages, it continues to receive support from the Defense Advanced Research Projects Agency and the National Science Foundation. One of the key elements in terms of hydrophobic material implementation is the fact that the are often found to be quite brittle, even when taking into account the mimicking of the "lotus effect" that has been applied to industrial materials such as paints and roof tiles.

"The key challenge is durability ," said Varanasi. "Most super-hydrophobic materials are fragile polymers - they don't stand up to abrasion, or high temperatures. But combining our textures with stronger materials - such as metals and ceramics - we can overcome these durability challenges." This desire to create long-lasting effects could be where the nature-inspired ridges become most useful. Limiting ongoing contact with materials that corrode or rust in areas of heavy moisture could create, in the opinion of the MIT engineers, an increase in efficiency, an essential component in heavy machinery and the aviation industry. Creating the ridges themselves is fairly simple - even at a macroscopic level - and the researchers believe that there is a market for industrial applications, such as for existing waterproof coatings that may not always do what they say on the tin. But the sector that could see the most benefit is fabrics, especially in the leisure industry.

"Sportswear, lab coats, military clothing, tents - there are a whole range of situations where you want to stay dry," said Varanasi, in an interview with the BBC. "Now we need to bring in the designers - how can you make a fabric that has these new features?"