If so, you’re not just imagining it. The friction between two surfaces in contact with each other does slowly increase over time. But why? A paper by two materials scientists at the University of Wisconsin in Madison, USA, suggests that the surfaces could actually be slowly chemically bonding together.
There are already several other explanations for this so-called “frictional ageing” effect. One is simply that two surfaces get squashed closer together. But a curious thing about friction is that the frictional force opposing sliding doesn’t depend on the area of the contacting surfaces. You’d expect the opposite to be the case: more contact should create more friction. But in fact two surfaces in apparent contact are mostly not touching at all, because little bumps and irregularities, called asperities, prop them apart. That’s true even for apparently smooth surfaces like glass, which are still rough at the microscopic scale. It’s only the contacts between these asperities that cause friction.'