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  • 1
    Electronic Resource
    Electronic Resource
    Shear-induced dilation of confined liquid films (2000)
    Springer
    Tribology letters 9 (2000), S. 55-62 
    Details
    ISSN: 1573-2711
    Keywords: shear-induced dilation ; confined liquid films ; surface forces apparatus ; stick–slip
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We demonstrate that sheared molecularly-thin fluid films dilate at the point of stick-to-slip (which is the transition from static to kinetic friction), indicating that density decreases when sliding occurs. This contrasts with incompressibility characteristic of bulk fluids when they are deformed. The magnitude of dilation was less than the size of the molecule and was larger in a polymer system (large molecules) than for small-molecule fluids. The experiments employed a surface forces apparatus modified to measure, using piezoelectric methods, sub-angstrom variations of film thickness during dynamic shear excitations that were performed at rates too rapid to allow fluid to enter and exit the zone of shear contact during the period of shear excitation. To demonstrate generality of the dilation effect, the specific systems studied included nonpolar fluids whose complexity was varied (a globular-shaped molecule, OMCTS; a branched alkane, squalane; a tethered diblock copolymer, polyvinylpyridine–polybutadiene) and also an aqueous electrolyte, MgCl2 dissolved in water. Extensive analysis is also presented of the piezoelectric methods that were employed to detect volume changes too small to observe by the methods of multiple beam interferometry that are traditional for thickness measurement in a surface forces apparatus.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1018852326571
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  • 2
    Electronic Resource
    Electronic Resource
    Exploring the “friction modifier” phenomenon: nanorheology of n‐alkane chains with polar terminus dissolved in n‐alkane solvent (1999)
    Springer
    Tribology letters 6 (1999), S. 207-214 
    Details
    ISSN: 1573-2711
    Keywords: friction modifiers ; thin film lubrication ; nanorheology ; molecular tribology ; surface forces apparatus
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Dilute solutions of two polar end‐functionalized linear alkanes (1‐hexadecylamine and palmitic acid), each dissolved in tetradecane, were confined between two mica surfaces and investigated using a surface forces apparatus modified to study shear nanorheology. These two solutions showed similar nanorheological properties that differed from those observed for pure n‐alkanes. In static measurements, a “hard wall”, rather than an oscillatory force, was observed as a function of film thickness. The polar alkane component formed a weakly adsorbed single layer at each mica surface, disrupting the layered structures found in neat n‐tetradecane. In dynamic experiments at low shear amplitude, the storage modulus G' exceeded the loss modulus G" at low frequencies; above some characteristic frequencies G' increased such that g' ≈ G", indicating significantly more energy loss through viscous modes at higher frequency. When the amplitude was varied at fixed frequency, no stick–slip was observed and the limiting value of the shear stress at high effective shear rate was an order of magnitude less than for unfunctionalized n‐alkanes at similar loads. Together, these results show that the addition of a small amount of polar alkane component, by disrupting the layered structures that would have been formed in the neat n‐alkane, is effective in suppressing static friction and reducing kinetic friction in the boundary lubrication regime.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019136429128
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  • 3
    Electronic Resource
    Electronic Resource
    Apparent hydrodynamic thickness of densely grafted polymer layers in a theta solvent (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 35 (1997), S. 2961-2968 
    Details
    ISSN: 0887-6266
    Keywords: anchored coils ; hydrodynamic thickness ; surface forces apparatus ; interfacial rheology ; interface ; viscosity ; theta solvent ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: We study the drainage of a near-theta solvent through densely grafted polymer layers and compare to recent notions that these layers display little permeability to solvent flow at surface separations less than a “hydrodynamic thickness.” The solvent is trans-decalin (a near-theta solvent at the experimental temperature of 24°C). The polymer is polystyrene (PS) end-attached to two opposed mica surfaces via the selective adsorption of the polyvinylpyridine (PVP) block of a PS-PVP diblock copolymer. The experimental probe was a surface forces apparatus modified to apply small-amplitude oscillatory displacements in the normal direction. Out-of-phase responses reflected viscous flow of solvent alone - the PS chains did not appear to contribute to dissipation over the oscillation frequencies studied. The value of the hydrodynamic thickness (RH) was less than the coil thickness (Lo) measured independently from the onset of surface-surface interactions in the force-distance profile, implying significant penetration of the velocity field into the polymer layer. As the surface-surface separation was reduced from 3Lo to 0.3Lo, the apparent hydrodynamic thickness (RH*) decreased monotonically to values RH* ≪ RH. Physically, this indicates that the “slip plane” moved progressively closer to the solid surfaces with decreasing surface-surface separation. This was accompanied by augmentation of the effective viscosity by a factor of up to approximately 5, indicating somewhat diminished permeability of solvent through the overlapping polymer layers. Similar results hold for the flow through surface-anchored polymers in a good solvent. It is interesting to note the strong stretching of densely end-grafted polymers in a theta solvent. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2961-2968, 1997
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
  • 4
    Electronic Resource
    Electronic Resource
    PMMA adsorption over previously adsorbed PS studied by polarized FTIR-ATR (1995)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 33 (1995), S. 2429-2437 
    Details
    ISSN: 0887-6266
    Keywords: surface ; adsorption ; exchange ; FTIR-ATR ; histogram ; hydrogen-bond ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Polarized infrared spectroscopy in attenuated total reflection was used to investigate the adsorption of PMMA (polymethylmethacrylate) onto PS (polystyrene) that was previously adsorbed onto oxidized silicon from dilute solution in carbon tetrachloride at 30°C. The carbonyl group of PMMA forms hydrogen bonds with surface silanol groups, giving segment-surface interaction energy of 4 kT as against 1 kT for PS (k is the Boltzmann constant, T the absolute temperature). The formation of hydrogen-bonding by PMMA was unaffected, in rate or amount, by preadsorbed PS, but a lesser total amount of PMMA adsorbed onto PS, resulting in a higher average bound fraction. The histogram of the mass adsorbed, attributable to subpopulations of chains with different bound fractions, was inferred by subtracting infrared spectra acquired at successive times. Whereas this histogram was broad and bimodal for adsorption onto bare surface, it was narrower and unimodal for adsorption onto preadsorbed PS. © 1995 John Wiley & Sons, Inc.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/polb.1995.090331714
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    Paper (German National Licenses)
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