As described above, contact angle is used to characterize surface wettability. A droplet of solvent, typically water for hydrophobic surfaces, is placed perpendicular to the surface. The droplet is imaged and the angle between the solid/liquid and liquid/vapor interfaces is measured. Samples are considered to be superhydrophobic when the contact angle is greater than 150 degrees. Refer to section on Wenzel and Cassie-Baxter models for information on the different behaviors of droplets on topographical surfaces. For drops to roll effectively on a superhydrophobic surface, Contact angle hysteresis is an important consideration. Low levels of contact angle hysteresis will enhance the self-cleaning effect of a superhydrophobic surface.