Believe it or not, “wettability” is actually a scientific term. It describes the ability of a liquid to adhere to or spread on a solid surface: in other words, how wet it makes whatever it lands on. Wettability is usually measured by the surface angle at which a drop of liquid sits on a surface: a high angle means the liquid has formed a spherical bead and things stay relatively dry, a low angle means it’s spread out, and the surface has become wet.
Water likes to form spherical beads because of something you’re probably familiar with: surface tension. H2O has a high surface tension because its molecules are strongly attracted to each other via a web of hydrogen bonds; because the molecules on the outer edge of the droplet don’t have any water molecules above them to cling to, they form a stronger bond with the molecules next to them – thus surface tension. That’s why water forms drops as it falls through the air. But when that drop lands on something, surface tension isn’t quite strong enough to keep the water in that spherical shape, and it spreads out, making the surface wet.
But there are ways around this! The interior of your maze is coated with a “nanoparticle superhydrophobic surface.” This surface increases the contact angle of the drop, basically turning it into a complete sphere. Compared to the flattened bubble of a low-angle drop, a sphere only touches the surface with a tiny area, so it rolls around super easily! When the drop is like this, the water is said to be in a “Cassie-Baxter State,” named after the equation that governs contact between liquids and surfaces.
What’s amazing is that the key to making that kind of superhydrophobic effect isn’t to make the surface smoother, but rougher. The secret is that the coating covers the surface in tiny bumps. And we mean really tiny: each one is around 100 nm tall; that’s about the size of a virus (a human hair is around 90,000 nanometers wide). The bumps affect the way the water rests on the surface, changing that all-important contact angle. They also leave space for some air, called a gaseous plastron, to be trapped between the water droplet and the surface, which causes that silvery sheen you see beneath the drop.
The development of these incredible coatings was inspired by two examples found in nature: the lotus leaf and the Stenocara gracilipes, also know as the Namib Sternocara Desert beetle. The self-cleaning hydrophobic qualities of the lotus leaf have been known for almost 2,000 years; as it says in the Bhagavad Gita, “just as a lotus leaf is untouched by water.” In fact, the rolling action of droplets on a superhydrophobic surface is often called “the lotus effect.”
Water on Lotus leaf
The Stenocara gracilipes beetle lives in one of most arid regions on Earth, and has evolved superhydrophobic channels on its back to channel tiny amounts of moisture that it harvests from fog into its mouth. Scientists designed superhydrophobic coatings by mimicking the pattern found in electron microscope scans of the beetle’s’ carapace.
Check out the superhydrophobic channels the Stenocara Gracilipes beetle uses to drink.
Full of paths to be discovered and dead ends avoided, rough when it looks like it should be smooth, and inspired by nature: the hydrophobic maze is a labyrinth of wonders!