1,026 research outputs found
Advanced Surfaces: Their Tailoring and Analysis
Advanced surfaces constitute an important component in numerous high technologies today, both industrial and medical. In the Laboratory for Surface Science and Technology at the ETH-Zürich, many different surface treatments and processes are being examined, and novel techniques
for the examination of such surfaces are also being developed. Among the modification methods described in this review are self-assembled monolayers, chemical vapor deposition, and surface functionalization with peptides. Novel analytical approaches include the extension of atomic force microscopy
to allow surface-chemical analysis of oxides and polymers with high spatial resolution, and a waveguide technique, adapted to enable the in situ monitoring of protein adsorption on oxides of relevance to implant applications
XPS study of the influence of temperature on ZnDTP tribofilm composition
Antiwear additives, such as zinc dialkyldithiophosphate (ZnDTP), find application in many different industrial sectors. Although it is understood that certain ZnDTP concentrations need to be used to achieve an effective antiwear performance, there has been very little work published concerning the effect of temperature on the interactions of the additive and its adsorption mechanism on steel. In this article, 100Cr6 (52100) steel ball-on-disc experiments under solutions of zinc dialkyldithiophosphate (ZnDTP) in poly-α-olefin (PAO) were performed at different temperatures, ranging from 25 to 180°C. The discs were analysed after the experiments by means of small-area, imaging and angle-resolved X-ray photoelectron spectroscopy (XPS). The composition of the reaction film was found to change as a function of the applied temperature and also to vary within the film as a function of depth: Longer polyphosphate chains were found at higher temperatures as well as towards the outer part of the reaction fil
Tuning Interparticle Hydrogen Bonding in Shear-Jamming Suspensions: Kinetic Effects and Consequences for Tribology and Rheology
The shear-jamming of dense suspensions can be strongly affected by
molecular-scale interactions between particles, e.g. by chemically controlling
their propensity for hydrogen bonding. However, hydrogen bonding not only
enhances interparticle friction, a critical parameter for shear jamming, but
also introduces (reversible) adhesion, whose interplay with friction in
shear-jamming systems has so far remained unclear. Here, we present atomic
force microscopy studies to assess interparticle adhesion, its relationship to
friction, and how these attributes are influenced by urea, a molecule that
interferes with hydrogen bonding. We characterize the kinetics of this process
with nuclear magnetic resonance, relating it to the time dependence of the
macroscopic flow behavior with rheological measurements. We find that
time-dependent urea sorption reduces friction and adhesion, causing a shift in
the shear-jamming onset. These results extend our mechanistic understanding of
chemical effects on the nature of shear jamming, promising new avenues for
fundamental studies and applications alike
Influence of Solutes on Hydration and Lubricity of Dextran Brushes
The characteristic lubricity and non-fouling behavior of polymer brushes is critically dependent on the solvation of the polymer chains, as well as the chain–chain interactions. Dextran brushes have shown promise as non-toxic aqueous lubricant films, and are similar in composition
to natural lubricating systems, while their comparative simplicity allows for controlled preparation and fine characterization. This project entails measuring the solvation and lubricity of dextran brushes in the presence of additives which modify the inter-chain hydrogen bonding. The thickness
and refractive index of the film were measured during adsorption of the brush layer onto a silica substrate and the subsequent immersion in solutions of potassium sulfate and ?, ?-trehalose. We also studied the lubricity of the system as a function of normal loading using colloidal-probe
AFM. Both solutes are shown to have a minimal effect on the hydration of the brush while significantly reducing the brush lubricity, indicating that inter-chain hydrogen bonding supports the load-bearing capacity of polysaccharide brushes
Order estimation and discrimination between stationary and time-varying (TVAR) autoregressive models
Copyright © 2007 IEEEFor a set of T independent observations of the same N-variate correlated Gaussian process, we derive a method of estimating the order of an autoregressive (AR) model of this process, regardless of its stationary or time-varying nature. We also derive a test to discriminate between stationary AR models of order m,AR(m), and time-varying autoregressive models of order m,TVAR(m). We demonstrate that within this technique the number T of independent identically distributed data samples required for order estimation and discrimination just exceeds the maximum possible order mmax, which in many cases is significantly fewer than the dimension of the problem NYuri I. Abramovich, Nicholas K. Spencer, and Michael D. E. Turle
Time-varying autoregressive (TVAR) models for multiple radar observations
©2007 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.We consider the adaptive radar problem where the properties of the (nonstationary) clutter signals can be estimated using multiple observations of radar returns from a number of sufficiently homogeneous range/azimuth resolution cells. We derive a method for approximating an arbitrary Hermitian covariance matrix by a time-varying autoregressive model of order m, TVAR(m), that is based on the Dym-Gohberg band-matrix extension technique which gives the unique TVAR(m) model for any nondegenerate covariance matrix. We demonstrate that the Dym-Gohberg transformation of the sample covariance matrix gives the maximum-likelihood (ML) estimate of the TVAR(m) covariance matrix. We introduce an example of TVAR(m) clutter modeling for high-frequency over-the-horizon radar that demonstrates its practical importanceYuri I. Abramovich, Nicholas K. Spencer, and Michael D. E. Turle
The influence of molecular architecture on the macroscopic lubrication properties of the brush-like co-polyelectrolyte poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) adsorbed on oxide surfaces
ISSN:1023-8883ISSN:1573-271
Molecular mechanisms of self-mated hydrogel friction
Self-mated hydrogel contacts show extremely small friction coefficients at
low loads but a distinct velocity dependence. Here we combine mesoscopic
simulations and experiments to test the polymer-relaxation hypothesis for this
velocity dependence, where a velocity-dependent regime emerges when the
perturbation of interfacial polymer chains occurs faster than their relaxation
at high velocity. Our simulations reproduce the experimental findings, with
speed-independent friction at low velocity, followed by a friction coefficient
that rises with velocity to some power of order unity. We show that the
velocity-dependent regime is characterized by reorientation and stretching of
polymer chains in the direction of shear, leading to an entropic stress that
can be quantitatively related to the shear response. The detailed exponent of
the power law in the velocity dependent regime depends on how chains interact:
We observe a power close to for chains that can stretch, while pure
reorientation leads to a power of unity. Our simulations quantitatively match
experiments and show that the velocity dependence of hydrogel friction at low
loads can be firmly traced back to the morphology of near-surface chains.Comment: 18 pages, 6 figures, includes supplementary materia
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