Effect of the Pores in Porous Rubber on Water Flow at the Contact Area
Porous rubber has a higher coefficient of friction on ice than nonporous rubber. Therefore, it is used as the tread rubber for studless winter tires. There are two reasons why porous rubber demonstrates a high coefficient of friction: (1) the modulus of rubber is low at cold temperatures, and (2) the pores that exist on the rubber surface increase the real contact area to absorb water. We have already reported water flow over pore with a diameter of about 1 mm. However, it is said that the smaller the pore diameter, the larger the influence of capillary force. The influence of capillary force in small pore size is investigated. Furthermore, the pores were made by drilling on the surface of rubber for the frictional experiment. This pore diameter mimicked the pores of porous rubber. Visualization of water flow at these pores and at those of the surface of porous rubber is conducted by “particle tracking velocimetry method.” The total internal reflection method is performed. The contact area is determined by using the difference of the critical angle of each substance: rubber, air, and water. As a result, it is found that the pores included air at the water lubrication contact area. The absorption of water in the pores is observed in this experiment. The influence of capillary force is also investigated.ABSTRACT
Experimental apparatus; 1, weight; 2, rubber specimen; 3, dove prism; 4, parallel leaf spring; 5, strain gauge; 6, prism holder; 7, linear guide.
— Pictures of rubber surface used in the experiment (before experiment).
Principle of optical systems used for the observation experiment: 1, rubber specimen; 2, dove prism; 3, high-speed camera; 4, light sources.
Example of the contact surface between the nonporous rubber and the mating prism under wet conditions.
Schematic illustration of particles tracking velocimetry with consideration of relative displacement between pore and particles.
Relationship between coefficient of friction and sliding speed.
Area of water absorption.
Water flow evidenced by particle movements around the pores.
Contact surface between the porous rubber and the mating prism under wet conditions in the leading area.
Contact surface between the nonporous rubber and the mating prism under wet conditions in the leading edge.
Sequential orthographic images of the contact surface between the nonporous rubber and the mating prism under wet conditions.
Schematic illustration of distribution of vertical load between tire and road.
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