The key is that impurities like dust, dirt, and stray molecules, coat nearly every surface and prevent smooth motion. The laws of static friction were first laid down by Guillaume Amontons, a 17th century French researcher who believed that surfaces have regular jagged edges like the teeth in a bicycle wheel gear. When two surfaces meet, his theory said, the teeth of the upper surface settle into the grooves of the lower surface. Accordingly, Amontons' laws say that static friction is proportional to the weight of the upper plate and independent of the plate's surface area. The gears in a car transmission will grind and lock unless they line up precisely, but a mismatch at the molecular level has the opposite effect.
Theoretical models predict zero static friction for mismatched surfaces.
To slide the upper surface, one expends energy to lift up the ping pong balls that are lying in the spaces, but recovers energy from the balls that drop from their perches. These molecules are like marbles that roll to the open niches between the strands of ping pong and tennis balls, locking the two surfaces together. The team found that the marbles always find a local energy minimum, and so it always takes energy to start sliding. Both a simple analytic expression and detailed computer simulations of sliding surfaces show that this model explains Amontons' laws. Salmeron's group is one of many that are trying to carry out the challenging experimental verification of the theory.