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Ice Repellent Airframes?

posted Apr 19, 2011, 1:57 PM by George Finlay   [ updated Oct 8, 2011, 11:17 AM by Nathaniel Cauldwell ]
Building on earlier research, a team led by Dr. Joanna Aizenberg at Harvard has come up with high resolution high speed video lab data that suggest ice-free airframes may be attainable by mimicking a natural design. 
Aizenberg ACSNano November 2010

They developed textured materials that remain ice free when exposed to supercooled liquid water droplets falling from a height of 10 cm. The droplets bounce off quickly while they are still liquid and before they have time to crystallize and adhere. Of course the velocity reached by these droplets is a far cry from aircraft speeds. 

In a phone conference March 9, 2011, Christopher Dumont and James Riley from the icing project team at the FAA Technical Center in Atlantic City, NJ, suggested that Dr. Aizenberg's team explore working with researchers at Cox and Company. Cox has long experience testing anti-icing technology, and a small wind tunnel ideally suited to the team's research. 

This month, the team is beginning to make plans for wind tunnel testing at the Cox LeClerc Icing Research Facility run by Dr. Kamel Al-Khalil in Plainview NY.

Researchers have already figured out how to duplicate some natural non-wetting surfaces like those on lotus leaves and water strider legs. The key concept has been revealed to be microscopic texturing in a surface that is already hydrophobic. Textures that minimize contact with water can often keep the strong affinity even very smooth surfaces have for water from overcoming water’s own surface tension. The water beads up and just rolls off the lotus leaves. Water striders walk on water without wetting their legs and drowning.

So far, Aizenberg’s team has been working at such small scales they were able to use little silicon rectangles on which they etched their textures, using a method long been used in the manufacture of microchips. To apply the textures to the test forms used in the Cox facility they will need to find a more efficient way to manufacture larger samples. Companies like 3M are likely to have the capability to produce the materials in the volumes and at the costs that will make them practical for aircraft, should the wind tunnel testing prove successful.

In earlier work, a team led by Dr. Di Gao at the University of Pittsburg used polymer coatings containing microscale particles of silicon to obtain similar ice repellant results. It is conceivable that this method may prove a more efficient way to manufacture and apply the textured coating to airframes. 
Gao Langmuir October 2009

The microscale patterns are truly tiny, and seem unlikely to cause increased drag. The brick and honeycomb patterns that are most promising for example are less than 10 microns wide and less than 3 microns raised off the surface. For comparison, a red blood cell is about 8 microns in diameter.

The team is concerned that their results may not be repeatable with higher velocities and smaller droplet sizes typical of aircraft icing scenarios. They saw what might be an indication of this when droplets at lower temperatures below -25 dC were pinned to the textured surface and froze before they had time to bounce clear. A similar result may be found at higher velocities. Smaller droplets typically encountered in clouds seem likely to require less time to freeze, which may lead them to pin to the surface.

What a leap ahead for aviation if durable coatings could be applied to airframes that were so repulsive to supercooled water that icing were to be entirely prevented. Even if not entirely prevented, if ice were made to slide off easily without elaborate counter measures, flying would be more efficient and safer.  The ice that accumulated at the colder conditions in the Harvard experiments proved to be much less securely attached to the surface, probably thanks to the texture.