Why Do Some Surfaces Repel Water, While Others Attract It?
July 17, 2013

Understanding Why Water Repels On Certain Surfaces

Lee Rannals for redOrbit.com - Your Universe Online

Researchers at MIT are working on better understanding how surfaces attract or repel water.

When water strikes a surface, sometimes it spreads evenly while other times it beads into tiny droplets. Studying this behavior can lead to improvements in many applications.

If water strikes a material and maximizes its contact with it, then it is known as hydrophilic, but when water is naturally repelled on a material, it is called hydrophobic. Kripa Varanasi, an associate professor of mechanical engineering at MIT, said both these materials could have a significant impact on the performance of power plants, electronics, airplane wings and desalination planets.

Hydrophilic surfaces have a contact angle that is less than 90 degrees, while hydrophobic surfaces have contact angles more than 90 degrees. Surfaces where tight droplets form a contact angle of more than 160 degrees are considered superhydrophobic. If the droplets are spread out nearly flat, with a contact angle of less than about 20 degrees then the surface is superhydrophilic.

"In a lot of cases, it's the extreme behavior that's useful in engineering," says Evelyn Wang, an associate professor of mechanical engineering at MIT who specializes in superhydrophobic materials.

MIT researchers said it is the surface chemistry that creates this phenomenon. The shape of a surface can amplify the effects. So if a material is hydrophobic, creating nanopatterns on its surface can increase the contact area with a droplet, amplifying the effect and making the surface superhydrophobic. When a surface is moving, it gets even more complicated.

The stenocara beetle of Africa's Namib Desert is a good example of a bug species that utilizes both hydrophilic and hydrophobic properties. The insect's back and wings have hydrophilic bumps to encourage condensation from fog, while these areas are surrounded by hydrophobic troughs to help funnel water droplets towards the beetle's mouth.

Researchers are MIT are working on superhydrophobic surfaces in order to help lead to better heat transfer in power-plant condensers. Another group of researchers at MIT developed a ketchup bottle that allows the sauce to slip right out.

While advances in understanding hydrophobic and hydrophilic is leading to improved inventions, the phenomenon has been known about for at least two centuries.

"It's amazing," Varanasi says, "that some of the things we can validate now were predicted a century ago."