Surfaces fighting frost

Materials World magazine
,
29 Oct 2018

A new passive technology to prevent frosting could do away with chemical anti-freeze solutions. Ellis Davies reports.

The build-up of ice and frost on infrastructure is estimated to cost the economy several billion dollars per year in the USA alone. As a solution to the problem, researchers at Virginia Tech, USA, have developed a chemical-free anti-frost surface structure using a microscopic array of elevated grooves that can keep surfaces 90% dry and frost-free, indefinitely. The team envisions this type of surface for use in aerospace, automotive and the heating, ventilation, and air conditioning sectors.

Up to 4,000l of anti-freeze is currently used to de-ice a single aircraft, which is both expensive and environmentally harmful. Frosting causes flight delays, power issues and can cause major mechanical problems. Besides vehicles, frost is also well known to adversely impact the efficiency and durability of the outdoor components of heat pumps. Salting, while effective, needs to be actively pursued, and the use of heating is energy intensive.

‘In our previous research, we deposited a water drop on a surface, froze it, and then exposed it to humid conditions,’ said Jonathan Boreyko, one of the authors of the paper, Passive antifrosting surfaces using microscopic ice patterns, published in ACS Applied Materials & Interfaces. ‘We found that a dry zone forms around the icy droplet, where no dew or frost can grow. However, the dry zone we observed was only a millimetre or so in size, so was not very practical for most real-life surfaces. This inspired the present research, where we had the idea to pattern an array of ice features on a surface, in order to obtain overlapping dry zones that could scale up to large surface areas.’

Researchers used laser cutting to build wicking patterns into an aluminium surface, which can arrange water into microscopic stripe arrays, using laser cutting. ‘The vapour pressure of ice is quite low, meaning that it attracts nearby humidity – like salts. So when these water stripes freeze into ice, they siphon all nearby moisture from the air to keep the intermediate surface areas completely dry. You have heard the expression “fight fire with fire”. Well here, we are fighting ice with ice,’ said Boreyko. The grooves take up around 10% of the materials surface, meaning that the remaining 90% stays dry.

Although the team used aluminium, Boreyko pointed out to Materials World that the attractive feature of the technique is that it is not limited to aluminium, but can be applied to any material, as long as it’s properly patterned.

Other tricks up the sleeve

‘Our technique can be used for any sort of application where you want to keep most of the surface dry, not only from frost but also from moisture,’ says Boreyko. ‘However, the use of ice as the humidity sink is only appropriate in winter environments. Something else like saltwater or anti-freeze should be used in the patterning if we want to harness the same concept in warmer climates. The appeal of using ice is that it never gets watered down as it absorbs moisture, whereas using salts or anti-freeze would not work for nearly as long as it becomes diluted by harvested water.’

Until now the team has not focused on the manufacturing and scale-up aspects of the technique, but Boreyko says this is something they will be taking into account going forward. ‘We are in talks with several aerospace and heating, ventilation, and air conditioning companies. Our goal is to enter a second phase of research focused on commercial translation’.