Adhesion mechanics of the surface force apparatus

The surface force apparatus (SFA) comprises thin molecularly smooth mica sheets glued to glass cylinders, which are pressed into contact with their axes at right angles. It is frequently used, in conjunction with the Johnson-Kendall-Roberts (JKR) adhesion theory, to extract the surface energy of the contacting sheets. This procedure is open to possible error since the JKR theory is based on the contact of homogeneous, isotropic elastic cylinders. This paper extends the JKR theory to the layered structure of the SFA. Two approaches have been followed: (i) direct calculations for prescribed values of the layer thickness and elastic moduli; (ii) an experimental calibration procedure for an existing apparatus.