Frictional strength regulated by roughness alignment

Rough contacts are ubiquitous, yet the impacts of the roughness matching on frictional behavior remain insufficiently understood. We investigate how slow shear and stick-slip transition depend on the roughness alignment between contacting surfaces. By rotating one surface with respect to the other, we precisely control roughness matching, enabling nearly an order-of-magnitude variation in the maximum static friction coefficient. Both the variation amplitude and the angular range over which the frictional strength varies are governed by roughness alignment. Universal statistical patterns emerge in micromechanical quantities, including the microcontact area, orientation, deformation, and their evolutions under shear, following generalized extreme value distributions. To quantify asperity interactions, we introduce the contact fabric tensor, whose anisotropy provides a unified measure of roughness matching and a predictor for frictional strength and sliding onset. These findings advance our understanding of friction at rough interfaces and inform the design of mechanical systems, adhesives, and fault models.