Fluorescence nanoscopy: Applications in tribology

This thesis presents two new methods of measurements beyond the diffraction limit using fluorescence microscopy and introduces their application to friction and lubrication sciences. A new direct method is presented for measurements of surface 3D structure using fluorescence microscopy with molecular axial resolution based on a fluorescent liquid. This rapid method also allows measurement of gaps between the surfaces with no need of mathematical postprocessing and deep optical background. The first application of this technique to lubrication studies is presented. Using the new 3D imaging method, we test how the change in surface roughness influences the transition between different lubrication regimes. Using the same method, we verify that the main reason behind the transition from elastohydrodynamic to mixed lubrication is the strong local viscosity increase due to confinement of the lubricant. The obtained result is valid for different types of materials. Another new super-resolution method for measurements of contact area is presented. We discovered a single-molecule switching process of the contact-sensitive fluorophores previously used in our laboratory for contact imaging. The switching is based on spontaneous fluorescence recovery after photobleaching. The method allows imaging of mechanical contacts with a resolution of ca. 80 nm. Application of this super-resolution method for estimation of shear strength of polymer surfaces during friction is presented. The static friction force is shown to be larger than the dynamic friction force, not only due to contact area increase, but also due to increase in density at the frictional interface. This result is shown to be valid for different polymer surfaces on the molecular scale