For more complex systems that are representative of water-surface interactions in nature, the Cassie-Baxter model is used. This model takes into consideration the fact that a water droplet may trap air between itself and the surface that it is on. The Cassie-Baxter model is described by the following equation:
 = Contact angle of water predicted by Cassie-Baxter's model
 = Liquid-air fraction, the fraction of the liquid droplet that is in contact with air