Much to the chagrin of those of us in the Northern Hemisphere, winter is once again on its way. For many of us, this means a return to icy roads, sidewalks, power lines and even airplane wings. Traditionally, the main methods of getting rid of this ice – or at least, keeping it under control – involve the use of salt and/or de-icing chemicals. Both of these are labor-intensive, environmentally-unfriendly, plus the salt kills grass and causes cars to rust. Now, however, researchers from Harvard University are developing nanostructured materials that could keep ice from ever forming on surfaces in the first place.
Like the superhydrophobic coating developed by a University of Pittsburgh-led team that mimics the rutted surface of lotus leaves to reduce the surface area to which water can adhere, the Harvard team got their inspiration from natural models. Although the principle is the same, instead of the lotus leaf the Harvard team turned to the eyes of mosquitoes and the legs of water striders for their inspiration. In both cases, the insects are able to keep these body parts dry due to an array of tiny bristles that repel droplets of water by minimizing the available surface area.
The researchers proceeded to create silicon surfaces incorporating various nanoscale shapes, patterns and geometries, such as bristles, blades, honeycombs and bricks. When they watched slow-motion videos of supercooled droplets hitting some of these surfaces, they saw that that the droplets would initially spread out, but then retract back into a sphere and bounce off before they could freeze. The surfaces with interconnected patterns were particularly effective. On regular smooth surfaces, by contrast, the droplets would simply spread out and freeze.
The nanostructured materials were shown to prevent the formation of ice down to a temperature of -30C (-22F). Even below that, what ice did form wasn’t able to adhere well, so would be relatively easy to remove.
“We see this approach as a radical and much needed shift in anti-ice technologies,” said team leader Prof. Joanna Aizenberg. “The concept of friction-free surfaces that deflect supercooled water droplets before ice nucleation can even occur is more than just a theory or a proof-of-principle experiments. We have begun to test this promising technology in real-world settings to provide a comprehensive framework for optimizing these robust ice-free surfaces for a wide range of applications, each of which may have a specific set of performance requirements.”
The research was recently published in the journal ACS Nano.