Asperité

Asperité is a term used in surface science and geology to refer to a localized roughness or a peak on a surface that comes into contact with another surface. The word derives from French asperité, from Latin asperitas roughness. In English contexts it is often used synonymously with asperity to describe a microscopic high point on a contact surface or a rough patch on a fault plane.

In tribology and contact mechanics, real contact between surfaces occurs at discrete asperities rather than over the nominal contact area. Under load, these asperities deform elastically or plastically, controlling the real contact area, local pressures, and the resulting friction and wear. The Greenwood–Williamson model and its successors describe the statistical distribution of asperity heights and their deformation. Surface roughness is quantified by parameters such as Ra (average roughness) and Rz, and is measured using profilometry, scanning electron microscopy, or atomic force microscopy. Manufacturing and finishing processes aim to manage asperities to achieve desired sealing, lubrication retention, wear resistance, or tactile properties.

In geology and seismology, asperities describe patches on fault surfaces that resist sliding and accumulate elastic strain. When these asperities fail, they can nucleate or influence the propagation of earthquakes, affecting rupture initiation, rupture speed, and the size of seismic events. The concept of fault asperities complements smoother-fault models and helps interpret tremor patterns, afterslip, and variability in fault strength.

Overall, asperité captures the idea that macroscopic behavior often depends on microscopic or localized roughness, whether in engineered contacts or natural faults.