On a regular surface (say a rose petal), water bonds to the surface and spreads out until the forces within the water holds it in check. On a very clean sheet of glass, the bonding between water and glass will create a very thin film. On other surfaces, the drop will be pulled into a rounded shape as bonding between surface and water will not be strong enough to overcome the cohesive power of hydrogen bonds in water at the edges. If you look at water on a rose petal, it will be curved on the top and sides, and flat on the bottom where it touches the petal.
New materials are being created that are ultrahydrophobic. This means that they repel water (ultra – very, hydro – water, phobic – against). They can be made by creating a surface covered with tiny “hairs” on the nano-scale. When water hits the surface, it can’t bond as it is not truly a surface. You can imagine the water balancing on a single hair – the attraction between water and the tiny surface area of the end of the hair is not enough to overcome the cohesion of the water drop. Below is a photo I took of water on an ultrahydrophobic surface at the laboratory of Dr Arthur Epstein at Ohio State University. Note that water forms an almost perfect sphere (as there is no bonding to the surface). This means that water will run straight off the material. Not only useful for weather proofing, this type of material can be used to create self-cleaning fabrics and bulidings – where dirt literally can’t stick.
Some plants use this method to prevent water sticking too long to their leaves and drowning them.