Research School of Physical Sciences and Engineering
The Australian National University, Canberra, Australia
A capacitance dilatometry attachment has been constructed for the Surface Forces Apparatus. The distance between the active surfaces is obtained from the capacitance of a cylindrical parallel plate capacitor measured with a variable ratio transformer bridge, the surface separation is made to vary with a magnetic force transducer. Displacements of 0.1 nm can be detected.
The method permits measurements of forces between opaque surfaces over periods of time ranging in suitable circumstances from tens of milliseconds to days. The attachment is able to measure adhesion of single crystal mica surfaces with high accuracy and reproducibility and has been used to measure forces between mica surfaces in electrolyte using the droplet method.
Measurements have been made of the dispersion force between mica and silica surfaces in air and it is found that above 20 nm separation Casimir-Polder theory, that takes account of retardation, fits the results much better than non-retarded van der Waals theory. This is the first time that direct measurements of dispersion forces have been made in the retarded regime. The method may be applied to metallic surfaces to measure the Casimir effect. The device is described in:
Capacitance dilatometry in a surface force apparatus, A.M.Stewart. Measurement Science and Technology 11, 298 - 304 (2000).
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