No abstract

The interaction force gradient between a micron-size polystyrene sphere and an atomically flat highly oriented pyrolytic graphite substrate has been analyzed as a function of surface-to-surface separation distance z 0 using an oscillating cantilever technique. The interaction force gradient was found to have two contributions. For z 0 у30 nm, an electrostatic force due to charges trapped on the polystyrene sphere dominates. For z 0 р30 nm, a van der Waals interaction, characteristic of a sphere near a flat plane, is observed. Fits to the data are in good agreement with theoretical expectations and allow estimates of the surface charge density triboelectrically produced on the sphere's surface.

The isothermal elastic compliances, stiffnesses, and bulk moduli of a Lennard-Jones solid organized into an fcc crystal structure (256 atoms in 4 unit cells) have been calculated as a function of testing temperature (expressed as the mean kinetic energy per atom). Tests conducted in pure shear were used to determine S44 and C~=G&oo, where 100 refers to crystallographic directions. Tests imposing axial elongation with fixed lateral dimensions established C» and C». Axial deformation with zero lateral pressure (a tension test) was used to determine S», S», E&oo and v&oo. This provided an independent set of results for comparison with the dilatational stiffnesses C» and C». The bulk modulus K was obtained by independent triaxial tension testing. The stiffnesses, compliances, and moduli were determined by regression analysis and digital filtering applied to combinations of the stress-tensor and strain-tensor data stored at each iteration during the constant-rate deformation experiments.While the cubic fcc Lennard-Jones solid expectedly obeys the Cauchy relations for central-force potentials, it is not isotropic, allowing v to take on values other than -' as originally proposed by Poisson. The present calculations show v&OO=0. 347 for the fcc Lennard-Jones solid with a Young's modulus of E&oo =61.1c/o, an initial (as indicated by superscript 0) shear modulus of G&oo =57.2c/o. , and an initial bulk modulus of K =71.2c,/o. at zero temperature. The moduli all decreased with increasing temperature. Reuss, Voigt, and Hashin and Shtrikman [J. Mech. Phys. Solids 10, 335 (1962)] bounds on the isotropic elastic properties of polycrystalline aggregates of Lennard-Jones material were also determined. Computed values of the moduli are in reasonable agreement with experimental results for solid argon and crystalline polyethylene.

Adhesion-induced deformations of a polyurethane substrate in contact with cross-linked polystyrene spheres, having diameters ranging from less than 2 μm to approximately 12.5 μm were observed using scanning electron microscopy. The diameters of the contact areas were measured from the micrographs. It was found that the contact radius varied as the particle radius raised to the 0.75±0.05 power. Experimental results are compared to the predictions of various adhesion models. The results are also discussed in terms of the Dupré work of adhesion.

No abstract