75Fundamentals of Friction
usually results in a decrease of the real area of contact and a decrease of the
friction force.
Various dynamic models have been suggested, based on various physi-
cal effects such as, for instance, time-dependent creep-like relaxation and
viscosity (Persson 2000). Increasing velocity typically results in a decrease
of friction (the so-called “negative viscosity,” sometimes also referred to
as the Stribeck effect), although for some material combinations and fric-
tion regimes the opposite trend is observed. The decrease of friction with
increasing velocity may lead to a dynamic instability, since decreased fric-
tional resistance will lead to acceleration, a further increase of velocity, and
a decrease in friction.