Local Shearing Force Measurement during Frictional Sliding Using Fluorogenic Mechanophores

Abstract

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.

Molecular Probing of the Stress Activation Volume in Vapor Phase Lubricated Friction

“…By applying the Otsu thresholding method, we defined the contact area at the lubricated surface, as shown in Figure 2. 30,[36][37][38]41,42 Comparing those results to the contact area of a dry contact as simulated using the open-source boundary element method (TriboSolver) 43 based on the elastic-fully plastic half-space approximation (Figure 2), 44 the contact areas show only minor differences (<5%) between lubricated and dry conditions. Furthermore, we evaluated the topography of the polymer bead before and after the frictional experiment and found no severe wear or deformation (Figure S2).…”

“…This concept is widely used to explain i.e. tribofilm formation and atomic scale wear. − On the other hand, in boundary lubrication, the stress activation volume is interpreted as the volume of boundary layer molecules that are moved at the interface during the thermally activated shear process. ,,− In the remainder of the paper, the stress activation volume is always referred to in the latter meaning. According to eq , the effective energy barrier for discrete processes underlying relative motion of the boundary layer decreases with the product of the stress activation volume and the applied shear stress. ,, For single-asperity contacts, the dependence of the shear stress on sliding velocity can be accurately captured by the Eyring model. ,,,, However, for larger multi-contact interfaces, it remains challenging to connect the theoretical description of boundary lubrication to experimental observations due to limitations in our ability to measure the area of real contact and, thus, the shear stress. , Moreover, there is no direct method available to locally probe the mobility of boundary layers and connect the activation volume to the free volume of the lubricant.…”

“…Although diffraction limited fluorescence microscopy has a finite resolution of ∼250 nm, previous studies have demonstrated that for plastic-on-glass interfaces as those studied here, most of the contact structure can be resolved. , As the DCDHF molecules at the non-contact area can freely rotate around specific bonds in the molecule, the fluorescence intensity outside the area of real contact is much lower than within the contact. By applying the Otsu thresholding method, we defined the contact area at the lubricated surface, as shown in Figure . ,− ,, Comparing those results to the contact area of a dry contact as simulated using the open-source boundary element method (TriboSolver) based on the elastic-fully plastic half-space approximation (Figure ), the contact areas show only minor differences (<5%) between lubricated and dry conditions. Furthermore, we evaluated the topography of the polymer bead before and after the frictional experiment and found no severe wear or deformation (Figure S2).…”

Why Teflon is so slippery while other polymers are not

“…The contact is immersed in a small quantity of refractive-index-matched liquid [dimethyl sulfoxide (DMSO) or formamide] to suppress optical reflections at the interface. Contact area measurements with a "dry" contact visualization method for similar polymer contact have yielded very comparable results [20], precluding an influence of the liquid on contact area. Fluorescence images of the polymer-substrate interface are point-scanned using 488-nm excitation light, and a suitable emission bandpass filter is used to prevent reflected light from reaching the detector.…”

A force controlled tribometer for pre-sliding measurements at the nanometer scale

“…Boundary Element Method calculations have been developed to enable calculation of the pre-slip that results from an externally applied force under the assumption that the local friction coefficient equals the global friction coefficient (Mishra et al, 2019). This assumption was recently verified experimentally through the use of shear stress sensitive fluorescent molecular probes (Hsu et al, 2022).…”