Nanonails enable electrically tunable surfaces

  • 30-Jun-2008 07:10 EDT
water-ethanol_2um_nails_hr.jpg

Droplets of two liquids with very different surface tensions, 72 mN/m (water) and 21.8 mN/m (ethanol), sit next to each other on the 2-µm-pitch nanonail substrate.

With a surface composed of tightly packed nanostructures that resemble tiny nails, thus the term “nanonails,” University of WisconsinMadison engineers and their colleagues from Bell Laboratories have created a material that can repel almost any liquid. But with a jolt of electricity, the liquid on the surface slips past the heads of the nanonails and spreads out between their shanks, wetting the surface completely.

This demonstration of dynamic tuning between superhydrophobic (liquid contact angle approaching 180°) and superhydrophilic (liquid contact angle approaching 0°) states is a first, according to Professor Tom Krupenkin, Nanophase Inorganic Materials and Devices Cluster Department of Mechanical Engineering, UW-Madison.

“The novelty of this whole thing is twofold. The first comes from the fact that we developed a system where you can repel not only high-surface-tension liquids such as water but also low-surface-tension liquids like ethanol, which obviously reduces the potential risk of [surface] contamination,” said Krupenkin. “The second thing is, we also made it electrically switchable, so if you want your surface to be wetter, you can choose that by flipping a switch.”

UW-Madison researchers Krupenkin and J. Ashley Taylor and their team etched a silicon wafer to create a “forest” of conductive silicon shanks and non-conducting silicon oxide heads. The ability of the structure’s surface to repel water, oil, and solvents rests on the nanonail geometry.

“It turns out that what’s important is not the chemistry of the surface but the topography of the surface,” Krupenkin explained, noting that the overhang of the nail head is what gives the novel surface its dual personality. “So nanonails made out of, say, polymer would work as well, or even better, than those that we make out of silicon.”

A surface of posts, he noted, creates a platform so rough at the nanoscale that “liquid only touches the surface at the extreme ends of the posts. It’s almost like sitting on a layer of air.”

The new material could find aerospace and automotive application in the manufacture of self-cleaning surfaces, and it could help extend the working life of batteries as a way to turn them off when not in use.

“One benefit is that if you have a body which is ideally fluid repellant, you don’t have to wash it as often, which is a big problem in the environment, actually,” said Krupenkin. “The other thing is that if you are in a cold climate where the winter is harsh—as I happen to be—then icing is another problem. If your surface would not allow ice to form a strong interface, you would not need all of the deicing fluids which are also toxic for the environment.”

That makes this type of surface ideal for “smaller aircraft and rotary aircraft such as helicopters, which are in very substantial danger of in-flight icing problems,” he added. “So there it could really be a life-and-death situation, while for [other vehicles] it’s more a convenience situation.”

The introduction of this technology will likely be in evolutionary steps. “An ideal self-cleaning surface which never gets dirty or icy, that certainly is much further out,” he said. Applications that deal with a controlled-type of environment are “much closer”—for example, biomedical use as lab-on-a-chip technology.

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