DESIGN OF SURFACE FORCES APPARATUS FOR TRIBOLOGY STUDIES COMBINED WITH NONLINEAR-OPTICAL SPECTROSCOPY

We describe the design, calibration, and performance of surface forces apparatus with the capability of illumination of the contact interfac e for spectroscopic investigation using optical techniques. The appara tus can be placed in the path of a Nd-YAG laser for studies of the lin ear response or the second harmonic and sum-frequency generation from a material confined between the two surfaces. In addition to the stand ard fringes of equal chromatic order technique, which we have digitize d for accurate and fast analysis, the distance of separation can be me asured with a fiber-optic interferometer during spectroscopic measurem ents (2 Angstrom resolution and 10 ms response time). The sample appro ach is accomplished through application of a motor drive, piezoelectri c actuator, or electromagnetic lever deflection for variable degrees o f range, sensitivity, and response time. To demonstrate the operation of the instrument, the stepwise expulsion of discrete layers of octame thylcyclotetrasiloxane from the contact is shown. Lateral forces may a lso be studied by using piezoelectric bimorphs to induce and direct th e motion of one surface. (C) 1997 American Institute of Physics.