Surface Forces Apparatus

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Definition

The surface forces apparatus (SFA) is an instrument used for measuring the forces between two surfaces at small separations [1]. The SFA has angstrom precision for separation distances of microns down to contact [1]. 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 [1].

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 [1]. 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 [2]. Tonck et al. adapted the SFA for studying opaque surfaces in 1988 [1].

Examples

As of November 2009, many labs around the world were actively using SFA for research. Here are a few:

Swiss Federal Institute of Technology

National University of Mexico

University of Illinois at Urbana-Champaign

KTH Royal Institute of Technology (Sweden)

References

[1] Jacob Israelachvili. Intermolecular & Surface Forces (Academic Press, London, 1992).

[2] 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).