Local Shearing Force Measurement during Frictional Sliding Using Fluorogenic Mechanophores

When two macroscopic objects touch, the real contact typically consists of multiple surface asperities that are deformed under the pressure that holds the objects together. Application of a shear force makes the objects slide along each other, breaking the initial contacts. To investigate how the microscopic shear force at the asperity level evolves during the transition from static to dynamic friction, we apply a fluorogenic mechanophore to visualize and quantify the local interfacial shear force. When a contact is broken, the shear force is released and the molecules return to their dark state, allowing us to dynamically observe the evolution of the shear force at the sliding contacts. We find that the macroscopic coefficient of friction describes the microscopic friction well, and that slip propagates from the edge toward the center of the macroscopic contact area before sliding occurs. This allows for a local understanding of how surfaces start to slide.

Automated summary

When two macroscopic objects touch, their real contact is often made up of multiple surface irregularities, which deform under pressure. Shear force causes the objects to slide and breaks the initial contact. This study uses a fluorescent mechanophore to visualize and quantify local interfacial shear force, and investigates the evolution of microscopic shear force from static to dynamic friction. The findings show that the macroscopic coefficient of friction accurately describes microscopic friction, and that sliding begins at the edge and propagates towards the center of the contact area.