A NOVEL METHOD FOR MEASURING THE FORCE BETWEEN 2 SURFACES IN A SURFACE FORCE APPARATUS

The well-known surface force apparatus of Israelachvili, or modifications thereof, has been limited in application because of the small number of surfaces available for study. Only mica and a few other materials provide suitable substrates. A new technique is reported which eliminates this restriction and opens up a range of possibilities. A piezoelectric bimorph has been used as a single cantilever spring to measure the force between two surfaces. The bimorph produces a charge proportional to the applied force. A technique is described for measuring the charge while accurately accounting for leakage. The device can then be used as a static force sensor. Static and dynamic measurements of attractive and repulsive surface forces are described and found to be in excellent agreement with data obtained by traditional techniques. Surface separation is usually measured using interferometry, and this requires that the surfaces studied be transparent. Since the deflection of the bimorph can be used to measure both separation and force, the forces between any type of surface can now be studied with the new technique. To illustrate, measurements of the van der Waals attraction between two opaque silver surfaces in air are presented.