Surface forces apparatus
The surface force apparatus (SFA) is a scientific instrument which measures the interaction force of two surfaces as they are brought together and retracted. One surface is held by a cantilevered spring, and the deflection of the spring is used to calculate the force being exerted. The technique was pioneered by D. Tabor, R.H.S. Winterton in the late 1960s at Cambridge University. By the mid-1970s, J.N. Israelachvili had adapted the original design to operate in liquids, notably aqueous solutions, while at the Australian National University.
A surface force apparatus uses piezoelectric positioning elements (in addition to conventional motors for coarse adjustments), and senses the distance between the surfaces using optical interferometry. Using these sensitive elements, the device can resolve distances to within 0.1 nanometer, and forces at the 10−8N level. This extremely sensitive technique can be used to measure electrostatic forces, elusive van der Waals forces, and even hydration or solvation forces. SFA is in some ways similar to using an atomic force microscope to measure interaction between a tip (or molecule adsorbed onto the tip) and a surface. The SFA, however, is more ideally suited to measuring surface-surface interactions, and can measure much longer-range forces more accurately. The SFA technique is quite demanding, however, and only a handful of labs worldwide have functional instruments.
In the SFA, method two smooth cylindrically curved surfaces whose cylindrical axes are positioned at 90° to each other are made to approach each other in a direction normal to the axes. The distance between the surfaces at the point of closest approach varies between a few micrometers to a few nanometers depending on the apparatus. When the two curved cylinders have the same radius of curvature, R, this so-called 'crossed cylinders' geometry is mathematically equivalent to the interaction between a flat surface and a sphere of radius R. Using the crossed cylinder geometry makes alignment much easier, enables testing of many different surface regions for better statistics, and also enables angle-dependent measurements to be taken. A typical setup involves R = 1 cm.
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