The tire-road contact: A mechanical mixing seen as a shear-induced diffusive process

In this paper, the mechanical mixing of road minerals and tire tread rubber is studied experimentally and numerically. A rubber specimen on an experimental set-up is used. SEM observations show that particles on the surface are composed of an intimate mixture of rubber compounds and minerals. A longitudinal section is also observed, showing that the minerals have migrated into the surface. This phenomenon can be seen as a diffusive process driven by shear. To understand the underlying mechanisms, a numerical model is proposed using soft bodies to represent the rubber top layer. A layer of minerals is then sheared onto this surface. The minerals migrate progressively into the rubber, forming a mixed layer of rubber and minerals. The results show also that the mixing mainly depends on the shear rate in the layer containing the minerals. Due to a reinforcing effect of the minerals, velocity is increasingly accommodated in the layer without minerals. As a result, the shear rate in the mixed layer decreases, and therefore the mineral migration. This coupled mechanism explains why the mixing process stops before reaching a homogenized state in the entire domain. To support this analysis, a diffusion coefficient is derived as a function of the shear rate in the mixed layer.