Research clarifies the physics of water repelling surfaces

July 3, 2015
Research clarifies the physics of water repelling surfaces
"Water droplet at DWR-coated surface1" by Brocken Inaglory. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Researchers have gained valuable insights into the behaviour of water on strongly hydrophobic (water-repelling) surfaces. Understanding this behaviour should help scientists develop new types of surfaces with applications ranging from textiles to surgical tools.

In a GW4 collaboration, Professor Nigel Wilding and Professor Robert Evans of the University of Bristol used Bath's High Performance Computer to simulate the properties of water at at a molecular level.

The work, published today in Physical Review Letters, reveals that the physics of hydrophobic surfaces is controlled by a phenomenon known as critical drying.

When water is placed on a substance that is hydrophobic, it reduces its contact with the substance by rolling up into a drop, like rain on a freshly waxed car. The more hydrophobic a surface is, the larger the between the drop and the surface becomes, making the drop more round.

For extreme hydrophobicity the contact angle is 180°, the drop is spherical in shape, and we say that the substance is "dry".

Professor Wilding explained: "Previously the nature of the dry state has been poorly understood.

"Our simulations have established that it is an example of a surface critical phenomenon. This is because as the contact angle approaches 180°, the compressibility of water close to the surface diverges to infinity.

"At a microscopic level this means that the density of water molecules near the surface undergo huge fluctuations: some regions have a liquid-like density, while others will have a much lower vapour-like density.

"We have shown that critical drying causes these near hydrophobic surfaces, even for contact angles much less than 180°."

Explore further: Explained: Hydrophobic and hydrophilic

More information: Physical Review Letters,

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not rated yet Jul 05, 2015
When the contact angle is 180 and the surface morphology is such that heterogenous superhydrophobicity is on the surface. This means the contact of the water with the surface occur only at certain nano-sized points.

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