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 That relationship, embodied in the first of Guillaume Amontons’s two laws of friction, has been superseded over the past 50 years by the recognition that the lateral friction or retention force is, in fact, proportional to the true contact area (see Physics Today,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 achieve that separation, the researchers mounted the sample at an adjustable angle in a horizontal centrifuge arm, shown here, that could be rotated about the vertical axis at a variable speed. A comounted camera wirelessly transmitted video to a nearby computer. Comparing the situation in which the drop of liquid was on top of a horizontal substrate to that in which the drop was hanging below a horizontal substrate, the team found that the hanging drop had the larger lateral retention force, despite a smaller contact area and a smaller normal force. That counterintuitive result agrees with theories that incorporate the effects of surface deformation and molecular reorientation. (R. Tadmor et al. , Phys. Rev. Lett., in press.)