Physicists shed new light on how liquids behave with other materials

ScienceDaily | 10/15/2019 | Staff
stefaniastefania (Posted by) Level 3
Their findings, published in the Proceedings of the National Academy of Sciences (PNAS), challenge the accepted wisdom on wetting and drying phase behaviour.

The authors provide a firm conceptual framework for tailoring the properties of new materials, including finding super-repellant substrates, such as expelling water from windscreens, as well as understanding hydrophobic interactions at the length scale of biomolecules.

Liquid - Water - Substrate - Drop - Contact

When a liquid such as water is repelled from a solid substrate, the drop created exhibits a large contact angle. This is known as a hydrophobic state, or superhydrophobic if the contact angle is very large, so that the drop forms a near spherical shape.

By contrast, if the substrate attracts the liquid sufficiently strongly -- in other words, a hydrophilic substrate -- this creates a small contact angle and the drop spreads over the surface.

Surface - Degree - Attraction - Substrate - Liquid

Whether a surface is hydrophobic or hydrophilic is determined by the degree of molecular attraction between the substrate and the liquid.

Controlling the attraction is key to the wettability of substrates, which determines how many physical and biological systems function. For instance, plant leaves are often hydrophobic, allowing them to remain dry during rain so that gas exchange can occur through their pores. However, liquids such as paints, inks and lubricants are required to spread out to coat or 'wet' surfaces.

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