Use of capacitance to measure surface forces .1. Measuring distance of separation with enhanced spatial and time resolution

We describe a simple method to measure the separation and forces between two macroscopic surfaces mounted in a crossed cylinder arrangement. It is shown that by measuring the capacitance between silver layers deposited on the backside of thin mica substrates, it is possible to achieve spatial resolution of less than 0.1 nm and time resolution of 1 ms. The growth of capacitance with decreasing distance of separation follows the predicted dependence for an ideal sphere and plate capacitor. To demonstrate the capability of obtaining force-distance profiles with this technique, octamethylcyclotetrasiloxane (OMCTS) has been confined between the mica sheets. We find that the process of collapse of the confined liquid, from four molecular layers to three, occurs more rapidly than 1 ms. This technique is also shown to give excellent results for opaque surfaces used in the surface forces apparatus (SFA), and we show that the adhesion energy between two surfaces of oxidized silicon is 3 mJ/m(2). Applications of this technique to the study of contact mechanics and dielectric properties of confined materials are discussed.