Surface Forces Apparatus
The surface forces apparatus (SFA) is an instrument developed by ... for measuring the forces between surfaces at small distances. The challenges of such a measurement are accurately determining the distance between the surfaces and the forces applied to them. The SFA uses interferometry to obtain the distance between the surfaces and a spring to calculate the force applied.
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. The forces between such objects are equivalent to the forces between two spheres. The cylinders are much easier to align than two spheres would be.
A drawback to the SFA is its reliance on molecularly smooth and semi-transparent materials. Many studies are performed on mica.
As of November, 2009, many labs were actively using SFA for research: