Rain beads into rounded droplets on a freshly waxed car hood but spreads into a flat wet film on clean glass. What explains the contrast?

Core Concept

Droplet shape reflects a contest between water clinging to itself and water clinging to the surface. Clean glass carries polar silicate groups that adhere to water through hydrogen bonding, so the liquid maximizes contact and spreads into a film. Wax presents only nonpolar hydrocarbon, offering water almost no adhesion, so internal cohesion wins and pulls each drop into a bead with minimal exposed area. The same logic explains curved meniscuses in narrow tubes made of different materials.