Interactive Surfaces

Molecular mechanisms in the shear of an ionic liquid in nanometer-scale confinement were investigated by atomic force microscopy with a laterally oscillating tip. On a single-crystal gold electrode, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]) exhibits molecular ordering in a simple cubic structure upon confinement by the nanometer-sized asperity. The strength of shear resistance decreases exponentially with increasing number of confined molecular layers. The dependence of lateral forces and of dissipation on the applied electrochemical potential confirms a mechanism in the electrolubrication by ionic liquids, namely, the change of the slippage plane from the interface with the electrode into the ionic liquid for increasing surface potential.

The influence of a single layer graphene on the interface between a polished steel surface and the model lubricant hexadecane is explored by high-resolution force microscopy. Nanometer-scale friction is reduced by a factor of three on graphene compared to the steel substrate, with an ordered layer of hexadecane adsorbed on the graphene. Graphene furthermore induces a molecular ordering in the confined lubricant with an average range of 4–5 layers and with a strongly increased load-bearing capacity compared to the lubricant on the bare steel substrate.

Wear mechanisms of three Ag–Cu eutectic alloy samples cooled at different rates from the melt have been investigated by friction force microscopy. The eutectic phase exhibits a lamellar structure where the interlamellar thickness decreases with increasing cooling rate. The hardness of the samples decreases with decreasing thickness of the lamellae. In the low normal force regime (Fn ≤ 1000 nN) friction is governed by shearing and the relevant contact area can be well described by the Johnson–Kendal–Roberts model. At higher normal force values, the surface is worn, and friction can be described by the ploughing friction coefficient. A ploughing friction coefficient is determined, which is positively correlated with the hardness of the Ag–Cu eutectic alloy samples cooled at different rates, while the wear volume negatively correlates with the hardness.

Frictional heating is common during the dry sliding of polymers against steel, which further makes it complex to understand the friction and wear performance of polymers at high temperature. Towards the goal of addressing the tribological response of polymers to such combined temperatures, the counter steel was heated crossing the glass transition (150 °C) of polyetheretherketone (PEEK), and tribological tests were conducted during temperature ramping or at constant counterbody temperatures. With increasing temperature, different friction responses were revealed depending on the variation manner of temperature (sliding during ramping or at controlled counterbody temperature). Even so, counterbody temperature around PEEK’s glass transition defined a transition, from which distinct friction and wear of PEEK was exhibited. Based on real-time analysis of temperature in the counterbody and PEEK near to the sliding interface, the completion between frictional and external heating is discussed. In combination with worn surface characterization, this also helped understand the mechanisms behind such kind of tribological response to temperatures.

A computationally lean model for the coarse-grained description of contact mechanics of hydrogels is proposed and characterized. It consists of a simple bead-spring model for the interaction within a chain, potentials describing the interaction between monomers and mold or confining walls, and a coarse-grained potential reflecting the solvent-mediated effective repulsion between non-bonded monomers. Moreover, crosslinking only takes place after the polymers have equilibrated in their mold. As such, the model is able to reflect the density, solvent quality, and the mold hydrophobicity that existed during the crosslinking of the polymers. Finally, such produced hydrogels are exposed to sinusoidal indenters. The simulations reveal a wavevector-dependent effective modulus E∗(q) with the following properties: (i) stiffening under mechanical pressure, and a sensitivity of E∗(q) on (ii) the degree of crosslinking at large wavelengths, (iii) the solvent quality, and (iv) the hydrophobicity of the mold in which the polymers were crosslinked. Finally, the simulations provide evidence that the elastic heterogeneity inherent to hydrogels can suffice to pin a compressed hydrogel to a microscopically frictionless wall that is undulated at a mesoscopic length scale. Although the model and simulations of this feasibility study are only two-dimensional, its generalization to three dimensions can be achieved in a straightforward fashion.

Event related potentials (ERPs) represent a noninvasive means for studying sensory and cognitive processes that occur in response to particular stimuli. Here we report on a phase measure for estimating single trial interaction of late somatosensory potentials (LSPs) following a tribological well defined mechanical stimulation of the human fingertip. Stimuli are presented via a programmable Braille-display with actively switchable pins which was slid along the apex of the passive fingertip, i.e., the fingertip rested stationarily in a finger holding system with circular opening at the bottom. The event was the raising and the lowering of either one, three or five lines of pins. Differences were identified by measures based on instantaneous phase synchronization to the stimuli across trials, in particular the wavelet phase synchronization stability (WPSS) measure for single trial sequences of LSPs. In particular, we show that the higher the friction the stronger and more localized the induced phase coherency is. We concluded that the WPSS analysis of single sequences of LSPs represents a reliable method which allows for the quantification of brain responses upon distinct tactile stimuli.

The tribological properties of poly (ether ether ketone) (PEEK) were investigated at different length scales in order to elucidate commonalities and differences in friction and wear. To achieve this goal, the PEEK/steel tribo-system was studied by block-on-ring, block-on-disc, cone-on-disc, and cylinder-on-disc tests as well as by asperity scratching. For better comparability, asperities were prepared from the counter-body steel of the macroscopic experiments. Friction and wear properties were compared on the basis of the pv level. Friction coefficients in macro sliding can be related to the interfacial shear strength in asperity scratching by material's yield pressure. The study confirms that friction and wear of PEEK at different scales can be correlated, despite differences of characteristic velocity and pressure in different experiments.

A model for the contact area of a single asperity sliding in a groove after repeated cycles is presented. Based only on the asperity geometry and on data from friction experiments, the model predicts the area of the asymmetric elliptical contact of the asperity sliding in its own groove. It thus allows to determine the shear stress of the steel–polymer couple in the relevant geometry without need for further microscopy of indenter or groove. The model was validated by experiments with an indenter manufactured from slide bearing steel and polyether-ether ketone (PEEK) as substrate. In experiments of 1000 repeated cycles, the contact area was found to vary with varying load and sliding velocity, while the shear stress was 20.5 MPa at a normal pressure of 50–70 MPa, independent of velocity when friction heating is still negligible. Model and experimental confirmation advance single-asperity friction experiments into an efficient method to extract shear stress and contact area for an understanding of sliding friction in metal-polymer contacts.

Friction forces between human fingertip and a Braille display were recorded simultaneously with electroencephalographic (EEG) signals related to the somatosensory cortex. The correlation between frictional stimuli and event-related EEG signals was analyzed. Raising and lowering the dots of the Braille display caused significant N50 and P110 waves in the event-related EEG signal, but variations in the force stimulus by a factor of two between different Braille pattern did not cause significant differences in the EEG responses related to early tactile processing. Raising and lowering the dots of the Braille display triggers a characteristic temporal development of friction due to viscoelastic skin relaxation.

Friction force microscopy was performed with oxidized or gold-coated silicon tips sliding on Au(111) or oxidized Si(100) surfaces in ultrahigh vacuum. We measured very low friction forces compared to adhesion forces and found a modulation of lateral forces reflecting the atomic structure of the surfaces. Holding the force-microscopy tip stationary for some time did not lead to an increase in static friction, i.e., no contact ageing was observed for these pairs of tip and surface. Passivating layers from tip or surface were removed in order to allow for contact ageing through the development of chemical bonds in the static contact. After removal of the passivating layers, tribochemical reactions resulted in strong friction forces and tip wear. Friction, wear, and the re-passivation by oxides are discussed based on results for the temporal development of friction forces, on images of the scanned area after friction force microscopy experiments, and on electron microscopy of the tips.