Interactive Surfaces
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Das Verständnis und die Kontrolle der Dynamik von Polymer-Oberflächen-Wechsel- wirkungen sind die Voraussetzung für das Design von Nanoobjekten und für das Verständnis biologischer Prozesse. Wir untersuchen dynamische Reibung und Adhäsion an einer Fest-Flüssig-Grenzfläche mit Hilfe des Rasterkraftmikroskops (AFM). Als Modellsystem wird ein einzelnes M13mp18-DNA-Molekül, mit einer Länge von 2.5 µm, mittels Biotin-Streptavidin-Wechselwirkung über einen Bead an einen Cantilever gebunden. Bei den Adhäsionsmessungen wird der Cantilever mehrfach gen Oberfläche gefahren, wobei er teilweise mehrere hundert Nanometer darüber verweilt, um eine Interaktion zwischen dem Bead und der Oberfläche zu vermeiden. Die Reibungsmessungen werden durchgeführt, indem der Cantilever seitlich parallel zur Oberfläche in einer Höhe von mehreren hundert Nanometern mit unterschiedlichen Geschwindigkeiten bewegt wird. Dies führt zur adhäsiven Wechselwirkung zwischen dem DNA-Molekül an verschiedenen Oberflächen und somit zu einer Verbiegung des Cantilevers. Die verwendeten Oberflächen sind eine mit Cellulosenitrat und Anti-Digoxigenin beschichtete Glasoberfläche, eine positiv geladene poröse Membran und eine mit Poly-L-Lysin beschichteter Glasoberfläche. Das Signal wird unter Berücksichtigung der Abrisskraft, Abrissposition und Frequenz sowie auf mögliche Hotspots bei den Reibungsmessungen analysiert, um typische Wechselwirkungen der DNA mit verschiedenen Oberflächen aufzudecken.
Friction between fingertip and surface is a key contribution to tactile perception during active exploration of materials. We explore the role of skin factors such as stratum corneum thickness and hydration, deformability, elasticity, or density of sweat glands and of Meissner corpuscles in friction and tactile perception. The skin parameters were determined non-invasively for the glabrous skin at the index finger pad of 60 participants. Sets of randomly rough plastic surfaces and of micro-structured fibrillar rubber surfaces were explored as model materials with well-defined parameterized textures. Friction varies greatly between participants, and this variation can be explained to 70% by skin factors for the randomly rough plastic surfaces. The predictability of friction by skin factors is much lower for micro-structured rubber surfaces with bendable fibrils, where 50% of variance is explained for the stiffest fibrils but only 20% for the most bendable fibrils. The participants’ age is the key predictor for their tactile sensitivity to perceive the fibrils, where age is negatively correlated to the density of Meissner corpuscles. The results suggest that stratum corneum hydration, skin deformability, and age are important factors for friction and perception in active tactile exploration of materials.
Fingertip friction is a key component of tactile perception. In active tactile exploaration, friction forces depend on the applied normal force and on the sliding speed chosen. We have investigated wheter humans perceive the speed dependence of friction for textured surface of materials, which show either increase or decrease of the friction coefficient with speed. Participants perceived the decrease or increase when the relative difference in friction coefficient between fast and slow sliding speed was more than 20%. The friction of comparison judgments which were in agreement with the measured difference in friction coefficient did not depend on variations in the applied normal force. The results indicate a perceptual constancy for fingertip friction with respect to self-generated variations of sliding speed and applied normal force.
The presence of mechanoreceptors in glabrous skin allows humans to discriminate textures by touch. Their quantification as clinical markers by biopsy is an invasive method of diagnosis. We report the localization and quantification of Meissner corpuscles using in vivo, non-invasive optical microscopy techniques. We discovered that regions containing Meissner corpuscles could be easily identified by laser scanning microscopy (LSM) with an enhanced optical reflectance above the corpuscles, caused by a protrusion of the strongly reflecting epidermis into the Stratum corneum with its weak reflectance.
The 2D materials exhibit excellent tribological properties due to their weak inter-plane interactions, such as the ultra-low friction, which can be further tuned by number of layers, application of electric bias, stacking of different materials into a van der Waals heterostructure, and change of substrate. In this work, the tribological properties of 2D materials were investigated experimentally by means of atomic force microscopy techniques in ultra-high vacuum and theoretically with atomistic simulations. Friction measurements on epitaxial graphene on SiC(0001) show that the ultra-low friction is limited by a normal load threshold, above which friction increases by one order of magnitude. Simulations suggest that, at contact pressures above 10 GPa, the high-friction regime is a result of an intermittent sp3 rehybridization of graphene and the formation of covalent bonds. Friction on the MoS2/graphene heterostructure is dominated by adhesion due to the out-of-plane deformation of the MoS2 layers. Increasing the number of MoS2 layers decreases friction as the flexural compliance decreases. Higher friction was recorded on MoSe2/hBN compared to graphene/hBN heterostructure or pristine hBN. Work on exfoliated materials was facilitated by the application of navigational microstructures.
Surface-grafted polymers can reduce friction between solids in liquids by compensating the normal load with osmotic pressure, but they can also contribute to friction when fluctuating polymers entangle with the sliding counter face. We have measured forces acting on a single fluctuating double-stranded DNA polymer, which is attached to the tip of an atomic force microscope and interacts intermittently with nanometer-scale methylated pores of a self-assembled polystyrene-block-poly(4-vinylpyridine) membrane. Rare binding of the polymer into the pores is followed by a stretching of the polymer between the laterally moving tip and the surface and by a force-induced detachment. We present results for the velocity dependence of detachment forces and of attachment frequency and discuss them in terms of rare excursions of the polymer beyond its equilibrium configuration.
In this study, polyetheretherketone composites were compounded using a two-screw extruder followed by injection moulding. The effects of multi-fillers on the mechanical properties and crystallization performances were investigated. Differential scanning calorimetry results indicate that the addition of fillers slightly increases the crystallization temperature and crystallinity. Compared to neat polyetheretherketone, the incorporation of inorganic filler leads to a significant improvement in matrix hardness, matrix stiffness and a slight increase in tensile strength. However, the material ductility, the impact strength and the fracture toughness of polyetheretherketone composites decrease. Fractography analyses show that the addition of fillers restraints the ductile deformation of polymers, which is responsible for the reduction of material ductility, impact strength as well as fracture toughness of polyetheretherketone composites.
Ionic liquids (ILs) represent an important class of liquids considered for a broad range of applications such as lubrication, catalysis, or as electrolytes in batteries. It is well-known that in the case of charged surfaces, ILs form a pronounced layer structure that can be easily triggered by an externally applied electrode potential. Information about the time required to form a stable interface under varying electrode potentials is of utmost importance in many applications. For the first time, probing of relaxation times of ILs by friction force microscopy is demonstrated. The friction force is extremely sensitive to even subtle changes in the interfacial configuration of ILs. Various relaxation processes with different time scales are observed. A significant difference dependent on the direction of switching the applied potential, i.e., from a more cation-rich to a more anion-rich interface or vice versa, is found. Furthermore, variations in height immediately after the potential step and the presence of trace amounts of water are discussed as well.
Plasticity experiments have been conducted with single dislocation resolution in both indentation and wear studies using atomic force microscopy (AFM). The high force sensitivity and the small tip radii in AFM permit the measurement of the nucleation of single dislocations in plastically deformed nanoscale volumes. Nanoscale mechanical testing in an ultra-high vacuum (UHV) environment allows for the preparation of oxide-free surfaces, required for direct comparison between atomistic simulation and experiment. Moreover, nanoscale mechanical testing often shows increased strength compared to what is observed in macroscale testing, motivating the use of atomistic simulation to gain insight into new deformation mechanisms. Indentation experiments show that it is possible not only to observe single dislocation events but also determine the glide vector of the dislocation in three dimensions on KBr(001). Discontinuous displacements of the tip during indentation in both normal and lateral directions are indicative of yielding events, referred to as pop-ins. The measured displacement of the tip into the material during these events is on the order of one Ångström or less when blunt diamond coated tips are used as indenters. Larger pop-in displacements are measured with sharper probes, resulting from the localization of stress near the surface. Only with the use of AFM can such small, Ångström-sized pop-in displacements be observed. Indentation creep studies indicate that creep in nanoscale volumes is accommodated only through dislocation nucleation and glide. A comparison between creep measured with AFM-based indentation and instrumented nanoindentation highlights the importance of dislocation nucleation and glide at this length-scale. High resolution imaging of the indented structure on KBr(001) allows for the identification of dislocations and charges associated with them. Wear experiments have demonstrated the contribution of dislocations to wear on the atomic scale. The role of dislocations in wear experiments has been observed through the similar dislocation structures typically surrounding scratches and indents, as well as in pop-ins observed while scratching. The measured friction coefficient in nanoscale wear experiments is closer to those typically reported in macroscopic experiments. This finding suggests that while single-asperity experiments at low loads on flat surfaces may produce no or little wear, friction of real rough surfaces always involves some wear and plastic deformation of microscopic contacts between the two surfaces.
Ein neuartiges Messgerät wurde durch die Kombination von einem AFM und zwei Tribometern entwickelt. Dieses Gerät ist in der Lage eine Probe mechanisch und tribologisch auf verschiedenen Längenskalen zu charakterisieren. Die Leistungsfähigkeit wird anhand von drei sehr unterschiedlichen Proben und Messaufgaben demonstriert, die im Folgenden beschrieben werden. Als Erstes wird das Reibungs- und Verschleißverhalten von Graphenlagen auf SiC und Cu untersucht. Die makroskopische Reibung auf SiC ist nach dem Abscheren des Graphens durch die Grenzflächenschicht bestimmt, auf Cu dominiert die Deformation der Oberfläche. Diese Arbeit zeigt außerdem, dass Wasser eine wichtige Rolle in der Reibung von Graphen auf der mikroskopischen Skala spielt. In der zweiten Untersuchung wird ein transparenter, viskoelastischer Kunststoff deformiert und die anschließende Formrelaxation profilometrisch vermessen. Zusätzlich wird mittels Polarisationsmikroskopie die Spannungsrelaxation bestimmt. Aus beidem zusammen werden die Kriechfolgefunktion und das Relaxationsspektrum berechnet. Zuletzt wird das Reibungs- und Verschleißverhalten des Periostracums der Miesmuschel im nassen und trockenen Zustand untersucht. Diese Arbeit zeigt, dass es im nass sehr abriebsresistent ist, während es im trockenen Fall schnell verschleißt. Zusätzlich wurden E-Modul und Härte mittels Nanoindentation bestimmt. Alle drei Projekte belegen die Wichtigkeit des skalenübergreifenden Ansatzes in mechanischen Untersuchungen.
