The optical microscopic observation of frictional interface between high strength gels and a glass ball by ball-on-disk method

Abstract

The polymer gels with the properties of high strength and low friction show promise as the ideal materials for a living body-like soft robot joint. To date, the relationship between contact area and friction behavior of hydrogels has been revealed from the observation of flat frictional surface. Here, we designed the microscopic observation system for the friction surface of soft materials under deformation condition. A specially arranged ball-on-disk measuring part was combined with inverted microscope to observe the frictional interface during the friction. Both of transparent glass ball and moving glass stage were employed in this instrument and the microscope camera was thereby able to take the moving picture of frictional interface imaged by the refractive index differences between a soda-lime glass ball, transparent hydrogels and air. By using this customized measuring instrument, when water was not rich on the friction surface, the moving pictures of the frictional interface between glass ball and transparent polymer gels were observed while measuring the frictional force. The visualized frictional contact area due to the differences of refractive index and the frictional measurement data indicated that the local deformation by adhesion between gels and counter sample cause high friction resistant as a static friction coefficient.