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 (on the order of centimeters) 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).