Thursday, January 22, 2015

A New Anti-Water Metal: What It Is and What It Is Not

Scientists at the University of Rochester have created new metal surfaces that are super water repellent, using short intense laser blasts. Because these surfaces don't accumulate water, they are self-cleaning and resistant to both corrosion and ice, making them good candidates for solar panels, aerofoils, and even toilets. The researchers published their findings yesterday in the Journal of Applied Physics.

Think you've seen this effect before? We'll explore how these new surfaces were made, what they can do, and how they are very different from two other effects you may be thinking of.

A water droplet bounces off the hydrophobic laser-etched metal. The parallel micro-grooves are just visible.
Image courtesy of J. Adam Fenster, University of Rochester

A Multipurpose Surface

Two scientists at the University of Rochester, Chunlei Guo and Anatoliy Vorobyev, were inspired by examples of water-repellent biological surfaces like the lotus leaf and the wings of the Morpho butterfly to create the metal analog. Water-repellent surfaces in nature often have a pattern of micro-and nano-scale ridges which helps make them remarkably resistant to wet.

The water-repellent properties of the lotus leaf. Credit: Adapted from Dilip Muralidaran via flickr

To replicate this fine texture onto the surface of metals, Guo and his team used femtosecond laser pulses to etch parallel micro-scale ridges into platinum, titanium, and brass. This process also created nano-sized divets and structures on top of the micro ridges, similar to the pattern observed in natural water-repellent surfaces.

The team found that once this newly-etched metal was exposed to air, carbon from CO2 accumulated onto the metal and further increased its repellent properties.

As this movie shows, even water droplets with the smallest amount of energy bounce and skid off the etched metal surface. And tilting the metal by only 4 degrees is enough to cause stationary droplets to immediately slide off, taking any dust or dirt with them. These properties classify the metals as self-cleaning superhydrophobic surfaces.

Because of the micro- and nano-scale ridges, the etched metals also absorb more than 95 percent of the visible light that falls on them, making them appear velvet black.

The self-cleaning, light-absorbing, water-repellent combo means these metals could be used to create things like a self-cleaning toilet uses far less water than ordinary toilets and solar panels that efficiently trap light and resist corrosion, dirt, and ice.

Does this water-repellent effect still look familiar? Here are two water-repellent effects that achieve hydrophobicity through very different processes.

It is not Teflon

Water droplets in a teflon pan. Credit: Thomas via flickr
Since the 1950s, a Teflon coating has been gracing the surfaces of non-stick pots and pans. Telfon, or polytetrafluoroethylene, is a chain of carbon and flourine that is extremely unreactive, low-friction, and unattractive to water.

Water molecule: one oxygen (red)
bonded to two hydrogens (white).
Credit: public domain
This is because the water molecule is polar, meaning it has a positively-charged side and a negatively-charged side. Polar molecules are attracted to other polar molecules and try to minimize their contact with neutral substances like Teflon.

Therefore distinct droplets of water form over a Teflon surface, similar to the droplets on Guo and Vorobyev's etched metals. But Teflon is much less water resistant; drops on a teflon surface will stick until the surface is tilted more than about 70 degrees compared to only 4 degrees with the new etched metals.

It is not the Leidenfrost effect

Sticking with the cooking theme, you may have noticed some strange behavior when water hits a very, very hot pan. Instead of immediately evaporating, distinct droplets form and skid across the pan, hanging around for far longer than is sensible at such high temperatures.

This is known as the Leidenfrost effect, named after Johann Gottlob Leidenfrost who described the phenomenon in 1756. More recently over the past decade, physicists have found that the Leidenfrost effect allows droplets to climb uphill. This inspired University of Bath undergraduate students in 2013 to build a Leidenfrost maze, in which droplets ricochet through a complex maze of hot, racheted inclines. They also created this cool video.

The Leidenfrost Effect, where a vapor layer
levitates and insulates a drop of liquid.
Credit: Vystrix Nexoth via Wikimedia Commons

The Leidenfrost effect occurs when a surface is heated to well above the boiling point of a liquid.

When a drop hits the hot surface, the bottom layer immediately vaporizes and levitates the drop on top. Since gas is a much better insulator than liquid, the drop is protected from the high surface temperature and remains liquid much longer before evaporating.

The Leidenfrost effect is the reason that you can quickly stick a finger in liquid nitrogen without harm; a layer of nitrogen gas between your finger and the liquid insulates you for a short time.

But yesterday's paper does not describe the Leidenfrost effect, which requires a very high temperature to work. Guo and Vorobyev's etched hydrophobic metals work at any temperature, and unlike Teflon coatings, won't peel off over time.

The team hopes their technique can find practical applications in a range of water-repellent, self-cleaning products, particularly in the developing world where clean water is scarce.

By Tamela Maciel, also known as "pendulum"


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