Anomalous Friction Phenomena on Agar gel Surfaces under Sinusoidal Motion

Abstract

The friction phenomena on the gel surface under accelerated conditions are important to design biofunctional materials. However, there are few reports on friction under such nonlinear conditions. In the present study, the effects of velocity, vertical force, and gel hardness on the friction force were evaluated between two flat agar gels under sinusoidal motion. We found ultra-low friction conditions that the friction coefficient is almost zero on the gel surface. In addition, the profile of the friction force was different between the outward and homeward processes in the reciprocating sliding motion. For example, at a vertical force W = 0.98 N and angle rate ω = 2.1 rad s-1, the friction coefficient was 0.30 at friction velocity V =10 mm s-1 and then reduced to 0.02. Such characteristic friction profile is caused by long relaxation time on the gel surfaces. When the gel substrate is rubbed with shorter friction time than the relaxation time τ from static friction to kinetic friction, the morphology of the gel surface becomes unstable. Under such conditions, the formation and extinction of thick liquid film can induce super lubrication state and the asymmetric friction phenomena. These findings are useful not only for developing biofunctional materials but also for understanding non-equilibrium phenomena in soft biological systems.