A new look at friction

We learn in introductory physics classes that the friction force is the product of a friction coefficient and the force normal to the interface. That relationship, embodied in the first of Guillaume Amontons’s two laws of friction, has been superseded over the past 50 years by the recognition that the lateral friction or retention force is, in fact, proportional to the true contact area (see Physics Today, September 1998, page 22). Amontons’s law turns out to be a special but common case in which the contact area scales linearly with the normal force. In new measurements of liquid drops on surfaces, Rafael Tadmor and colleagues at Lamar University in Beaumont, Texas, observe the opposite behavior: a lowered lateral force despite a larger normal force and an increased contact area. Key to the observations was the ability to decouple the normal and lateral forces while monitoring the drop. To...