Surface Repels Most Liquids

The surface of a new material repels virtually all liquids, then absorbs them when a jolt of electricity is applied. Made of tightly packed nanostructures resembling tiny nails, the material could be used in biomedical applications such as lab-on-a-chip technology and in the manufacture of self-cleaning surfaces.

The "nanonail" surface was sculpted by University of Wisconsin-Madison engineers and their colleagues at Alcatel-Lucent's Bell Labs in Murray Hill, N.J. UW-Madison mechanical engineers Tom Krupenkin and J. Ashley Taylor and their team etched a silicon wafer to create a forest of conductive silicon shanks and nonconducting silicon oxide heads.

Liquid beads on a surface composed of silicon "nanonails." Made by Tom Krupenkin and J. Ashley Taylor of University of Wisconsin-Madison's Department of Mechanical Engineering, the surface repels virtually all liquids, including water, oil, solvents and detergents. When an electrical current is applied, the liquid slips past the nail heads and between the shanks to wet the entire surface. According to Krupenkin, the nails create such a rough surface at the nanoscale that liquids only touch the surface at the extreme ends of the forest of nails, so the liquid is like sitting on a bed of air. (Photos courtesy Tom Krupenkin/University of Wisconsin-Madison)

The ability of the structure's surface to be superlyophobic -- to lack an affinity for solution -- is due to the nanonail geometry, with which it can repel liquids such as water, oil, detergents and solvents.

Krupenkin said the chemistry of the surface isn't as important as its topography, with the overhang of the nail head giving the surface its dual personality. A surface of posts 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," he said.

But the material does have trouble with some liquids. "The current samples have difficulty reliably repelling organic liquids with extremely low surface tensions," Krupenkin told Photonics.com, like those with tensions about and below approximately 16 mN/m (millinewtons per meter), such as hexane.

Silicon "nanonails" created by Krupenkin and Taylor form the basis of a novel surface that repels virtually all liquids. The surface may have applications in biomedical devices such as "labs-on-a-chip," chemical microreactors, and in extending battery life.

"In principle, at least in theory, the approach allows one to repel any liquid. We hope to achieve this with our next generation of samples," Krupenkin said.

Add a jolt of electricity (somewhere between 20-80 V), and the liquid on the surface slips past the heads of the nanonails and spreads out between their shanks, wetting the surface completely. Total energy consumption is extremely low because essentially no electrical current is flowing, Krupenkin said.

Potential applications for the material include droplet-based microfluidics, chemical microreactors, and helping extend battery life by turning them off when not in use.

Krupenkin said the material would be a good candidate for labs-on-a-chip because it would allow the devices more flexibility in the fluids it can use.

"In many labs-on-a-chip devices, the working fluid is some sort of an organic solvent. Such fluids have low surface tensions and thus cannot use benefits afforded by traditional superhydrophobic surfaces," Krupenkin said. He cited as an example a nanograss battery-on-a-chip that he and colleagues described in Scientific American last year that can only work with aqueous electrolytes. "However, the nanonails approach would allow to extend it to a wide range of organic electrolytes such as those encountered in lithium-based batteries, which have much higher energy density."

The next step for the research, he said, "is to try to fabricate the nanonails out of a wider variety of materials, including plastics, with the goal of eventually getting practically usable superlyophobic coatings."

The researchers report on the material this month in Langmuir, a journal of the American Chemical Society.

For more information, visit: www.wisc.edu