ABSTRACT
Contact mechanics, spanning nanometer to tectonic scales, faces long-standing challenges arising from multiscale random roughness, which hinders experimental validation of theories. Understanding multi-asperity rough contacts is vital for addressing catastrophic consequences of these contacts failing such as earthquakes and for diverse technological applications. To visualize such contacts, we introduce a super-resolution microscopy method utilizing spontaneous millisecond ON/OFF fluorescence blinking of contact-sensitive molecular rotor molecules immobilized on a glass coverslip. This technique achieves â¼55 nm lateral imaging resolution for rough poly(methyl methacrylate) and glass spheres on glass contacts. For soft polymer spheres due to large plastic deformation, the resolution improvement does not significantly affect the area of real contact. However, for hard glass spheres, the real contact area is found to be 2.4 times smaller than that found by diffraction-limited imaging. This study highlights, through direct visualization, the impact of material stiffness on the nanoscale structure within the area of real contact.
ABSTRACT
We investigate the transition between different regimes of lubrication and directly observe the thickness of nanometric lubrication films with a sensitivity of a single molecular layer at a multi-asperity interface through fluorescence microscopy. We redefine specific film thickness as the ratio of the lubricant film thickness and the surface roughness measured only at those regions of the interface where the gap is "minimal." This novel definition of specific film thickness successfully captures the transition from full elastohydrodynamic lubrication to mixed and boundary lubrication. The transition can be triggered by increasing the surface roughness and is accurately predicted by using the new film thickness definition. We find that when the liquid carries part of the load, its apparent viscosity is greatly increased by confinement, and show how the transition between different lubrication regimes is well described by the viscosity increase and subsequent glass transition in the film.
ABSTRACT
We present a one-step method to produce air-stable, large-grain mixed cationic lead perovskite films and powders under ambient conditions. The introduction of 2.5 % of Zn(II), confirmed by X-ray diffraction (XRD), results in stable thin films which show the same absorption and crystal structure after 2 weeks of storage under ambient conditions. Next to prolonged stability, the introduction of Zn(II) affects photophysical properties, reducing the bulk defect density, enhancing the photoluminescence (PL), and extending the charge carrier lifetime. Furthermore, 3-chloropropylamine hydrochloride is applied as the film-forming agent. The presence of this amine hydrochloride additive results in highly oriented and large crystal domains showing an ulterior improvement of PL intensity and lifetime. The material can also be prepared as black precursor powder by a solid-solid reaction under ambient conditions and can be pressed into a perovskite pellet. The prolonged stability and the easy fabrication in air makes this material suitable for large-scale, low-cost processing for optoelectronic applications.
ABSTRACT
A novel method is presented for the rapid direct 3D visualization of the contact between two surfaces by means of fluorescence microscopy using a fluorescent liquid. Distances between the surfaces of up to several hundred nanometers can be determined with subnanometer accuracy in 3D and within seconds of measurement time. The method opens new possibilities for research in the areas of contact mechanics, friction, wear, and lubrication.
