Boing... bouncing drops make for smarter rainwear

[Buster's Uncle]

Boing... bouncing drops make for smarter rainwear
AFP
November 20, 2013 1:38 PM



A woman with an umbrella can be seen behind a car window covered in raindrops on September 15, 2013 in Ziegelstein near Nuremberg, southern Germany (AFP Photo/Sven Grundmann)



Paris (AFP) - Scientists on Wednesday said they had found a way to make raindrops bounce faster off surfaces, opening the way to new water-resistant materials from clothing to aircraft wings.

Reporting in the journal Nature, a team at Boston University and the Massachusetts Institute of Technology (MIT) said they had slashed the amount of time a water drop stays in contact with a surface.

High-speed photography shows that a droplet first spreads out like a tiny pancake on the surface, pulls back due to its surface tension and then bounces off.

When it comes to wetness, what counts is the duration of this contact, a calculation based on oscillations in the drop.

To minimise that time, materials scientists have been focussing their efforts on repellant, or hydrophobic, chemicals that make surfaces less adhesive for water.

But the new study takes a physical approach.

Tiny ridges added to the surface break up the chubby symmetry of a droplet as it hits.

The droplet splits apart, forming smaller, highly irregular shapes that recoil faster compared with a simple fat blob.

"The upshot is that the surface stays drier longer if this contact time is reduced, which has the potential to be useful for a variety of applications," said James Bird, an assistant professor of mechanical engineering at Boston.

The material used, in lab conditions, was a silicon wafer coated with a hydrophobic chemical called fluorosilane, etched with microscopic ridges.

Contact times with it were 40 percent shorter than for a non-treated surface, and the scientists hope to boost this to as much as 80 percent.

According to the authors, the texture could be replicated in other materials too -- a milling tool could do the same on steel or aluminium, and a drop-splitting surface could be made for fabrics.

One early area of use could be in airplane wings. Exposure to water droplets at high altitudes boosts the risk of dangerous ice buildup.

Butterfly wings and the leaves of some plants, such as nasturtiums, have minuscule veins on their surface that have the same drop-busting effect, the researchers found.

Tests showed that drops bounced off butterfly wings and nasturtiums faster than off lotus leaves, the fuzzy leaves considered the "gold standard" of non-wetting surfaces in nature.ri/gk


http://news.yahoo.com/boing-bouncing-drops-smarter-rainwear-183846052.html

[Buster's Uncle]

Textured Surface Could Create Ultra-Waterproof Materials
LiveScience.com
By Tia Ghose, Staff Writer  November 20, 2013 1:08 PM



The wings of the morpho butterfly contain hydrophobic ridges that cause water droplets to break up



A new way of texturing surfaces could make for materials that are ultra-waterproof.

The new surface takes advantage of the fact that rougher, uneven textures cause water droplets to bounce off of them more quickly. And the less time water stays in contact with a surface, the drier that material stays.

The new method could be used for many applications, including waterproof clothing and sports gear, as well as anti-icing tech for airplane wings.

Previously, researchers thought that water droplets recoil fastest when they hit a surface in a perfect, symmetrical shape. But Kripa Varanasi, a mechanical engineer at MIT, and his colleagues, decided to take a look for themselves.

They used a high-speed camera to film water droplets as they bounced off a silicone wafer sprayed with a highly water-repellent coating. They found that droplets that hit the surface, then spread out symmetrically before recoiling actually spent more time in contact with the surface than those that hit unevenly.

So, the researchers created a new textured rough surface with small ridges that broke up the drops when they hit a hydrophobic, or water-repellent, surface. Those smaller drops then took less time spreading out on the surface before bouncing off of it.

But it turns out they weren't the first to discover this waterproofing strategy; nature had beaten them to it.

"We discovered that both the wings of the Morpho butterfly (Morpho didius) and the leaves of the nasturtium plant (Tropaeolum majus L.) have multiple superhydrophobic ridges, or veins, on a similar scale to our macro-textured surfaces," the authors write in the research article describing the new technique today (Nov. 20) in the journal Nature.

The new materials could have multiple applications. In addition to waterproofing sporting gear and clothing, the new approach could be used for keeping airplane wings from becoming icy and improving the aerodynamics of minuscule robots flying in the rain.

"We expect that this approach could be extended to surfaces exposed to freezing rain to prevent icing," the researchers write.

That's because the freezing of raindrops onto a surface takes time, so reducing the contact time between the surface and the rain could reduce frost accumulation.


http://news.yahoo.com/textured-surface-could-create-ultra-waterproof-materials-180833093.html