Preventing the havoc wrought when freezing rain collects on roads, power lines,
and aircrafts could be only a few nanometers away. A University
of Pittsburgh-led team demonstrates in the Nov. 3 edition of Langmuir a
nanoparticle-based coating developed in the lab of Di Gao, a chemical and petroleum
engineering professor in Pitt's Swanson School of Engineering, that thwarts
the buildup of ice on solid surfaces and can be easily applied.
The paper, by lead author and Pitt doctoral student Liangliang Cao, presents
the first evidence of anti-icing properties for a burgeoning class of water
repellants—including the Pitt coating—known as superhydrophobic
coatings. These thin films mimic the rutted surface of lotus leaves by creating
microscopic ridges that reduce the surface area to which water can adhere. But
the authors note that because ice behaves differently than water, the ability
to repulse water cannot be readily applied to ice inhibition. Cao's coauthors
include Gao, Jianzhong Wu, a chemical engineering professor at the University
of California at Riverside, and Andrew Jones and Vinod Sikka of Ross Technology
Corporation of Leola, Pa.
The team found that superhydrophobic coatings must be specifically formulated
to ward off ice buildup. Gao and his team created different batches made of
a silicone resin-solution combined with nanoparticles of silica ranging in size
from 20 nanometers to 20 micrometers, at the largest. They applied each variant
to aluminum plates then exposed the plates to supercooled water (-20 degrees
Celsius) to simulate freezing rain.
Cao writes in Langmuir that while each compound containing silica bits of 10-or-fewer
micrometers deflected water, only those with silica pieces less than 50 nanometers
in size completely prevented icing. The minute surface area of the smaller fragments
means they make minimal contact with the water. Instead, the water mostly touches
the air pockets between the particles and falls away without freezing. Though
not all superhydrophobic coatings follow the Pitt recipe, the researchers conclude
that every type will have a different particle-scale for repelling ice than
for repelling water.
Gao tested the coating with 50-nanometer particles outdoors in freezing rain
to determine its real-world potential. He painted one side of an aluminum plate
and left the other side untreated. The treated side had very little ice, while
the untreated side was completely covered. He produced similar results on a
commercial satellite dish where the glossed half of the dish had no ice, but
the other half was encrusted.