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Shifts in molecular vibrational frequencies are used to measure intermolecular forces in liquids and solids as a function of external pressure. The force along a particular bond within a molecule is derived from its measured vibrational frequency shift using an expression for the perturbation of a quantum anharmonic oscillator in a classical bath. New pressure induced frequency shift and force measurements are performed on the CC bond in 1-octene, trans-2-octene and trans-4-octene (in both pure liquids and methanol solutions), and on the Si−O bond in three methylsiloxanes. Comparison of these and previous gas-to-liquid (or solid) and high-pressure vibrational frequency shift results reveal a large variation in the force on different types of bonds, while families of similar bonds experience a similar force at a given external pressure, with only a weak dependence on the location of the bond within the solute or the molecular structure of the surrounding solvent. Physical interpretations of the results using both continuum and perturbed hard sphere fluid models are suggested.

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Spatial pressure variations in synthetic lubricants contained in a static high-pressure diamond anvil cell (DAC), as well as in a loaded model bearing contact device, have been measured using the frequency shift of the lubricant’s Raman vibrational modes. Long-lived pressure fluctuations of ±0.5 GPa, with a relaxation time of several days, are observed m the static high pressure systems at an average pressure of 2.5 GPa. Evidence for rapidly varying pressure fluctuations in a concentrated contact is inferred from the increase in lubricant Raman linewidths. These results raise questions about key assumptions made in modeling EHD contacts. It is suggested that the present results are closely linked to recent observations of shear localization made by Winer and Bair.

The Raman scattering intensity from a polyphenylether fluid film entrapped between a diamond window and a one-inch-diameter steel ball is found to correlate linearly with fluid film thickness over a 0.1 to 10 μm thickness range. The calibration and precision of the technique, as well as comparisons with other methods, are discussed.