Beetle-inspired discovery creates frost-free zones
23 January 2016
A team of researchers led by Virginia Tech has used chemical micro-patterns to control the growth of frost on a surface caused by condensation.
Writing in the online journal, Scientific Reports, the researchers describe how they used photolithography to pattern chemical arrays that attract water over the top of a surface that repels water, thereby controlling or preventing the spread of frost.
The inspiration for the work came from an unlikely source - the Namib Desert Beetle, which lives in one of the hottest places in the world and is able to collect airborne water.
The insect has a bumpy shell and the tips of the bumps attract moisture to form drops, but the sides are smooth and repel water, creating channels that lead directly to the beetle's mouth.
"I appreciate the irony of how an insect that lives in a hot, dry desert inspired us to make a discovery about frost," saay Jonathan Boreyko, an assistant professor of Biomedical Engineering and Mechanics in the Virginia Tech College of Engineering. "The main takeaway from the Desert Beetle is we can control where dew drops grow."
Working at the Oak Ridge National Laboratory, the researchers developed their beetle-inspired, frost-controlling chemical pattern on a small surface, but they believe the area can be scaled up to large surface areas with thirsty, hydrophilic patterns overlaying a hydrophobic surface.
"We made a single dry zone around a piece of ice," Boreyko says. "Dew drops preferentially grow on the array of hydrophilic dots. When the dots are spaced far enough apart and one of the drops freezes into ice, the ice is no longer able to spread frost to the neighbouring drops because they are too far away. Instead, the drops actually evaporate completely, creating a dry zone around the ice."
The journey of frost across a surface begins with a single, frozen dew drop. "The twist is how ice bridges grow," says Boreyko. "Ice harvests water from dew drops and this causes ice bridges to propagate frost across the droplets on the surface. Only a single droplet has to freeze to get this chain reaction started."
By controlling spacing of the condensation, the researchers were able to control the speed frost grows across surfaces, or completely prevent frost altogether.
"Fluids go from high pressure to low pressure," Boreyko says. "Ice serves as a humidity sink because the vapour pressure of ice is lower than the vapour pressure of water. The pressure difference causes ice to grow, but designed properly with this beetle-inspired pattern, this same effect creates a dry zone rather than frost."