Observation of Water Behavior in the Contact Area between Porous Rubber and a Mating Surface during Sliding

Porous rubber is often used as the tread rubber of studless tires because of its higher coefficient of friction on icy surfaces, as the real contact area is larger because of its lower elastic modulus. It is said that the real contact area increases owing to the water absorption into the pores. The purpose of this study was to clarify the effect of pores on the surface of porous rubber during sliding under wet conditions. In this experiment, porous rubber was rubbed with a dove prism under wet conditions, so as to measure the coefficient of friction in concurrence with observing the friction surface. The total internal reflection method was adopted to distinguish the real contact area from the wet contact area. The real contact area was observed as a black area in captured image. Particle-tracking velocimetry was also conducted to visualize the water flow in the vicinity of pores during the sliding. The results of this study show that the absorption of water into the pores was not observed. The pore contained an air bubble during the sliding. The water flow detouring around the air bubble in the pore was also observed. In regard to contact, the front edge of the pore was not in contact with the mating dove prism. On the other hand, the rear edge of the pore was clearly seen as a black arc even if the pore left the dove prism. Thus, the rear edge of the pore contacting with the dove prism most likely wipes the water, so that the coefficient of friction of rubber with the pore was higher than that without the pore.