Frost-Free Planes: A MechE Team Refines a Flawed Solution to a Dangerous Problem

Professor Kripa Varanasi and a student
MIT Assistant Professor Kripa Varanasi (left) and graduate student J. David Smith work with a cold stage, a system for cooling materials in a precisely controlled way, to study frost formation. Photo credit: Patrick Gilooly, MIT News Office

 

Ice is one of the most treacherous substances on earth. It can seize up the engine of an aircraft, snap high voltage power lines, and jettison workers off an oilrig into a frigid ocean. But solutions to icy build-up can be as problematic as the hazard itself: Many de-icing chemicals are toxic and require continual reapplication, and heating coils embedded in surfaces can prove logistically challenging and use an inordinate amount of energy.

Kripa Varanasi, the d’Arbeloff Assistant Professor of Mechanical Engineering, and his student J. David Smith, along with Tao Deng and colleagues at General Electric, recently discovered a critical flaw in what many considered the best solution on the horizon. Until now, researchers have assumed that if a surface repelled water, it repelled ice. Varanasi and his colleagues, however, discovered that a surface must also repel frost.

Their research, published recently in Applied Physics Letters, indicates that most super-hydrophobic coatings don’t work consistently because of frost. Technically speaking, frost is ice that forms on a surface directly from a vapor state or by the freezing of condensed droplets. The formation of frost can completely defeat the water-repelling properties of a surface designed to inhibit ice buildup—and, in fact, could actually promote ice formation.

But the team’s findings weren’t all bad news. Their research also suggests that a more complicated, patterned surface might work.

Water-repellant is not necessarily frost-repellant

The idea behind super-hydrophobic coatings is that they cause water to bead up into droplets instead of spreading out across a surface. Researchers have assumed that such coatings would also prevent ice from forming or sticking to the surface. But using an environmental scanning electron microscope to study the process, Varanasi and his team found there’s a limit to the extent these coatings can prevent ice from adhering to a smooth surface.

“If frost forms, it actually aggravates the problem,” Varanasi says, because it provides a kind of foundation on which ice quickly can build up to form a thick layer. “We need to be able to control this first phase when ice nucleation occurs,” he says.

Electron Microscope Images of frost

Snapshot images of frost formation on textured hydrophobic surfaces imaged for the first time in a scanning electron microscope.

But in understanding the limitations of this solution, Varanasi and his team discovered a few promising answers. The key was looking at what happens when water droplets are in rapid motion before striking a surface. It turns out that certain kinds of complex nanoscale texturing of the surface can drastically improve the hydrophobic qualities, even on a moving surface, by preventing the forming droplets from finding a suitable flat surface to stick to. “We have to go another way; we have to do textures,” Varanasi says, adding that getting the size and configuration of these textures exactly right will be the subject of future research.

He also notes that in order to be a practical solution for the many applications where ice buildup is a problem, it must be possible to manufacture the patterning on large surfaces at reasonable cost. The true breakthrough, Varanasi says, will take place when someone finds a way to produce “scalable manufacturing techniques with material that can survive in these kinds of applications.”

The first step toward a new set of challenges—and discoveries

Neelesh Patankar, Associate Professor of Mechanical Engineering at Northwestern University, believes Varanasi’s work is significant, because “it shows clearly that frost formation imposes severe constraints on the development of textured icephobic surfaces. This was not previously known.” Patankar notes that the fact that Varanasi and his team showed that textures of a certain scale (micron-sized patterns) are not viable is an important result in the field “since it possibly tells us where not to look.”

The next challenge on the de-icing frontier as Patankar defines it: “What are the right parameters, specifically, the roughness scale, that could lead to icephobic surfaces even in the presence of frost formation?” Varanasi and other researchers have a head start toward getting the answers to those essential questions.meche logo

 

Excerpted from an MIT News article by David Chandler.