Surface forces apparatus
Israelachvili had adapted the original design to operate in liquids, notably aqueous solutions, while at the Australian National University,[4] and further advanced the technique to support friction and electro-chemical surface studies[5] while at the University of California Santa Barbara.
[6] Using these sensitive elements, the device can resolve distances to within 0.1 nanometer, and forces at the 10−8 N level.
The SFA, however, is more ideally suited to measuring surface-surface interactions, can measure much longer-range forces more accurately, and is well-suited to situations where long relaxation times play a role (ordering, high-viscosity, corrosion).
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.
Position measurements are typically made using multiple beam interferometry (MBI).
When a white-light source is shined normal to the perpendicular cylinders the light will reflect back and forth until it is transmitted at where the surfaces are closest.
These rays create an interference pattern, known as fringes of equal chromatic order (FECO), which can be observed by microscope.
[6] The jump method is difficult to execute mainly due to unaccounted vibrations entering the instrument.
To minimize the dampening due to the surrounding substance, these measurements were originally done in a vacuum.
[6] Early experiments measured the force between mica surfaces in air or vacuum.
[6] The technique has been extended, however, to enable an arbitrary vapor or solvent to be introduced between the two surfaces.
[7] In this way, interactions in various media can be carefully probed, and the dielectric constant of the gap between the surfaces can be tuned.
It can also measure the electrostatic 'double layer' forces between charged surfaces in an aqueous medium with electrolyte.
The SFA has more recently been extended to perform dynamic measurements, thereby determining viscous and viscoelastic properties of fluids, frictional and tribological properties of surfaces, and the time-dependent interaction between biological structures.
