Surface forces apparatus

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. The resonance method The jump method is difficult to execute mainly due to unaccounted vibrations entering the instrument. To overcome this, researchers developed the resonance method which measured surface forces at larger distances, 10 nm to 130 nm. In this case, the bottom cylinder is oscillated at a known frequency, while the frequency of the top cylinder is measured using a piezoelectric bimorph strain gauge. To minimize the dampening due to the surrounding substance, these measurements were originally done in a vacuum. In this way, interactions in various media can be carefully probed, and the dielectric constant of the gap between the surfaces can be tuned. Moreover, use of water as a solvent enables the measurement of interactions between biological molecules (such as lipids in biological membranes or proteins) in their native environment. In a solvent environment, SFA can even measure the oscillatory solvation and structural forces arising from the packing of individual layers of solvent molecules. It can also measure the electrostatic 'double layer' forces between charged surfaces in an aqueous medium with electrolyte. Dynamic mode 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. == Theory ==

The force measurements of the SFA are based primarily on Hooke's law, F = kx where F is the restoring force of a spring, k is the spring constant and x is the displacement of the spring. Using a cantilevered spring, the lower surface is brought towards the top surface using a fine micrometer or piezotube. The force between the two surfaces is measured by \Delta F(x)=k(\Delta x_{\text{applied}}-\Delta x_{\text{measured}}) where \Delta x_{\text{applied}} is the change in displacement applied by the micrometer and \Delta x_{\text{measured}} is the change displacement measured by interferometry. The spring constants can range anywhere from 30\times10^5 \frac{N}{m} to 5\times10^5 \frac{N}{m}. When measuring higher forces, a spring with a higher spring constant would be used. ==See also==