A hydrophobic material is one that repels water and a hydrophilic material is one that likes water. Pretty obvious. It's not so obvious how to go about testing where a material falls on the hydrophobicity/hydrophilicity scale. This is where something called the contact angle comes into play. It's the angle made at the interface of a water droplet with air at the surface of a solid material. Here's a picture that will clarify.
This test can be done with any two fluids (air and water are the most common) at a solid surface to compare surface tensions of different materials. When the contact angle is less than 90 degrees the surface is considered hydrophilic and if the contact angle is more than 90 degrees it is considered hydrophobic. If the contact angle is greater than 150 degrees then the surface is considered super-hydrophobic. There's not too much work on superhydrophilicity but a contact angle of nearly 0 degrees is the general consensus.
So why are super-hydrophobic surfaces worth writing a blog post about? Well, they can be used to reduce drag anywhere that a surface is pulled against water (hulls of ships, inside of pipes, etc). They can also be used as a self cleaning surface coating, so anytime it rains, dirt will be easily swept away. Imagine cleaning your car (well) by leaving it out in the rain.
