Surface Forces Apparatus
The surface forces apparatus (SFA) is an instrument used for measuring the forces between two surfaces at small separations . The SFA has angstrom precision for separation distances of microns down to contact . The challenges of such a measurement are accurately determining the distance between the surfaces and the forces applied. The SFA uses interferometry to obtain the distance between the surfaces and springs to determine forces .
One of the ingenious things about the SFA is the geometry of surfaces used. The classic SFA surface geometry is two cylinders brought together perpendicularly (forming an x-shape). The forces between two crossed cylinders are locally equivalent to the forces between two spheres . Two cylinders should be easier to align than two spheres.
A drawback to the SFA is its reliance on large ("several square centimetres" p. 158) molecularly smooth and semi-transparent materials. Many studies are performed on mica because it meets those requirements . Tonck et al. adapted the SFA for studying opaque surfaces in 1988 .
As of November 2009, many labs around the world were actively using SFA for research. Here are a few:
 Jacob Israelachvili. Intermolecular & Surface Forces (Academic Press, London, 1992).
 Yuncheng Liang, Nidal Hilal, Paul Langston, and Victor Starov, "Interaction Forces between Colloidal Particles in Liquid: Theory and Experiment" Advances in Colloid and Interface Science 134-135, 151-166 (2007).