New technique grows tiny 'hairy' materials at the microscale

Feb 03, 2014 by Louise Lerner
Argonne materials scientists announced a new technique to grow these little forests at the microscale (the scale shows 100 micrometers, which is about the diameter of a single human hair). Credit: Alexey Snezhko and Igor Aronson.

(Phys.org) —Scientists at the U.S. Department of Energy's Argonne National Laboratory attacked a tangled problem by developing a new technique to grow tiny "hairy" materials that assemble themselves at the microscale.

The key ingredient is epoxy, which is added to a mixture of hardener and solvent inside an electric cell. Then the scientists run an alternating current through the cell and watch long, twisting fibers spring up. It looks like the way Chia pets grow in commercials.

"The process is very simple, the materials are cheap and available and they can grow on almost every surface we've tried," said Argonne physicist Igor Aronson, who co-authored the study.

By tweaking the process, the team can grow many different shapes: short forests of dense straight hairs, long branching strands or "mushrooms" with tiny pearls at the tips. Interestingly, though the structures can be permanent, the process is also instantly reversible.

"This is a completely new kind of structure," said Argonne physicist Alexey Snezhko, also a co-author. "With this method, you can support more complex structures that have unique properties."

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Scientists are very interested in materials with tiny fibers for technologies like batteries, or sensors. For one, "hairy" materials offer up a lot of surface area. Many chemical reactions depend on two surfaces making contact with one another, so a structure that exposes a lot of will speed the process along. (For example, grinding coffee beans gives the coffee more flavor than soaking whole beans in water.) Micro-size hairs can also make a surface that repels water, called superhydrophobic, or dust.

The tiny-fiber structure is so useful that it's evolved several times in nature, Aronson pointed out. For example, blood vessels are lined with a layer of similar tiny protein "hairs," thought to help reduce wear and tear by blood cells and bacterial infections, among other properties.

Currently, the primary methods of creating interesting shapes at small scales is lithography, a type of "printing" where researchers lay a pattern on the material and the rest of it is melted or etched away. But it's hard to make very with this method; it's hard to control; and the results aren't always uniform.

"These polymers assemble themselves," Snezhko explained, "which is much easier and less labor-intensive than lithography."

These tiny “mushrooms” could be useful in new energy technologies. By tweaking the process, scientists can grow a variety of shapes and sizes. The scale bar shows 20 micrometers, which is about the size of a single bacterium. Credit: Alexey Snezhko and Igor Aronson.

In one experiment the researchers used a process called that deposits a molecule-thick layer of material over the entire hairy structure, like a fresh blanket of snow, to add a layer of semiconductor material. Semiconductors are essential ingredients in many technologies, such as solar cells and electronics.

This provided proof of concept that the polymer could be incorporated into semiconductor-based renewable energy technologies. It also proved that it could survive high temperatures, up to 150°C, an essential property for many manufacturing processes.

These networks assemble themselves into fantastic shapes when an alternating electrical current is applied. The scale bar shows 50 micrometers, which is smaller than the diameter of a human hair. Credit: Alexey Snezhko and Igor Aronson.

Right now the structures are about a single micron thick—you could stack 100 of them to reach the width of a sheet of paper. Aronson and Snezhko said their next goal is to get them even smaller, to the nanoscale.

Explore further: New research could lead to less expensive solar panels

More information: "Self-assembled tunable networks of sticky colloidal particles." Arnaud Demortière, Alexey Snezhko, Maksim V. Sapozhnikov, Nicholas Becker, Thomas Proslier & Igor S. Aranson. Nature Communications 5, Article number: 3117 DOI: 10.1038/ncomms4117 . Received 28 August 2013 Accepted 16 December 2013 Published 21 January 2014

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