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Scientific Unit
Low Friction and High Solid-Solid Contact Ratio—A Contradiction for Laser-Patterned Surfaces?
(2017)
Recording of Stribeck-like curves is a common way to study the effect of laser-patterned surfaces on the frictional efficiency. However, solely relying on the coefficient of friction when identifying the lubrication regime and the underlying working principles can be misleading. Consequently, a ball-on-disc tribometer was combined with an electrical resistivity circuit to simultaneously measure Stribeck-like curves and solid-solid contact ratios for polished and laser-patterned samples. Line-like surface patterns with different periodicities were produced by direct laser interference patterning on steel substrates (AISI304). The reference shows a Stribeck-like behavior well correlating with the contact ratios. The behavior deviates for high sliding velocities (high contact ratios) due to a loss of lubricant induced by centrifugal forces pulling the lubricant out of the contact zone. In contrast, the solid–solid contact ratio of the laser-patterned samples is around 80% for all sliding velocities. Those values can be explained by higher contact pressures and the structural depth induced by the surface topography which make a full separation of the surfaces unlikely. Despite those high values for the contact ratio, laser-patterning significantly reduces friction, which can be traced back to a reduced real contact area and the ability to store oil in the contact zone.
The adhesion of fibrillar dry adhesives, mimicking nature's principles of contact splitting, is commonly characterized by using axisymmetric probes having either a flat punch or spherical geometry. When using spherical probes, the adhesive pull-off force measured depends strongly on the compressive preload applied when making contact and on the geometry of the probe. Together, these effects complicate comparisons of the adhesive performance of micropatterned surfaces measured in different experiments. In this work we explore these issues, extending previous theoretical treatments of this problem by considering a fully compliant backing layer with an array of discrete elastic fibrils on its surface. We compare the results of the semi-analytical model presented to existing continuum theories, particularly with respect to determining a measurement system- and procedure-independent metric for the local adhesive strength of the fibrils from the global pull-off force. It is found that the discrete nature of the interface plays a dominant role across a broad range of relevant system parameters. Accordingly, a convenient tool for simulation of a discrete array is provided. An experimental procedure is recommended for use in conjunction with this tool in order to extract a value for the local adhesive strength of the fibrils, which is independent of the other system properties (probe radius, backing layer thickness, and preload) and thus is suitable for comparison across experimental studies.
Ultrathin nanowires are promising nanoscale materials. They can reach length-to-diameter aspect ratios exceeding 1000, making them suitable building blocks for optoelectronic devices such as transparent conducting films. An organic ligand shell surrounds their inorganic core, provides colloidal stability, and guides their one-dimensional growth. Two unresolved issues limit their application. Nanowires can agglomerate into elongated bundles, but efficient use of this superstructure is difficult since we do not yet understand the bundling mechanisms. Furthermore, nanowires are prone to the Plateau-Rayleigh instability: thin wires tend to fragment into discrete spheroidal particles to reduce their surface energy, limiting their lifetime and reliability. This thesis investigates superstructure formation and nanowire stability and the link between both topics. Bundles are shown to emerge in non-polar solvents for entropic reasons. Solvent or unbound ligand molecules align in proximity to the ligand shell, thus losing entropy. Bundling decreases this loss in entropy by reducing contact with the bulk solvent. The structural stability of nanowires is enhanced or degraded by the ligand shell, depending on the relationship between free energy and local surface curvature. Kinetic barriers in ad- and desorbing ligands and rearrangement of surface atoms slow down the break-up. Bundling further stabilizes the wires by confining the space available to them.
The relative tendency of freely dispersed and bundled gold nanowires to break up along their length by the Rayleigh–Plateau instability is investigated both experimentally and theoretically. Small angle X-ray scattering, in combination with transmission electron microscopy, reveal that the bundling of nanowires can enhance their stability. The experimental observation is rationalized by a linear perturbation analysis of a representative unit cell of bundled wires. A stability map is constructed for a bundle of nanowires to display the sensitivity of the Rayleigh–Plateau instability to the number and size of contacts with nearest neighbors per nanowire, and to the ratio of interfacial energy to surface energy. Stabilisation is enhanced by allowing the bundle of wires to sinter freely: a criterion for this kinetically-based stabilisation is given in terms of the ratio of pinch-off time for the instability to the sintering time to form the necks between nanowires.
The break-up of a nanowire with an organic ligand shell into discrete droplets is analysed in terms of the Rayleigh-Plateau instability. Explicit account is taken of the effect of the organic ligand shell upon the energetics and kinetics of surface diffusion in the wire. Both an initial perturbation analysis and a full numerical analysis of the evolution in wire morphology are conducted, and the governing non-dimensional groups are identified. The perturbation analysis is remarkably accurate in obtaining the main features of the instability, including the pinch-off time and the resulting diameter of the droplets. It is conjectured that the surface energy of the wire and surrounding organic shell depends upon both the mean and deviatoric invariants of the curvature tensor. Such a behaviour allows for the possibility of a stable nanowire such that the Rayleigh-Plateau instability is not energetically favourable. A stability map illustrates this. Maps are also constructed for the final droplet size and pinch-off time as a function of two non-dimensional groups that characterise the energetics and kinetics of diffusion in the presence of the organic shell. These maps can guide future experimental activity on the stabilisation of nanowires by organic ligand shells.
