Natural rubber

Contact area

Engineering

Materials science

Shear (geology)

Contact mechanics

Dynamical friction

Static friction

Physics

Finite element method

Composite material

Structural engineering

Mechanics

Proceedings of the National Academy of Sciences

Significance We investigate the origin of static friction, the threshold force at which a frictional interface starts to slide. For rough contacts involving rubber or human skin, we show that the real contact area, to which static friction is proportional, significantly decreases under increasing shear, well before the onset of sliding. For those soft materials, our results will impact how we use and interpret current contact mechanics and friction models.

Evolution of real contact area under shear and the value of static friction of soft materials