Surface forces apparatus

The Surface Force Apparatus (SFA) is a

Position measurements are typically made using multiple beam interferometry (MBI). The transparent surfaces of the perpendicular cylinders, usually mica, are backed with a highly reflective material usually silver before being mounted to the glass cylinders. 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. Distance between the two surfaces can be determined by analyzing these patterns. Mica is used because it is extremely flat, easy to work with, and optically transparent. Any other material or molecule of interest can be coated or adsorbed onto the mica layer.

The jump method

In the jump method, the top cylinder is mounted to a pair of cantilever springs, while the bottom cylinder is brought up towards the top cylinder. While the bottom cylinder approaches the top, there comes a point when they will "jump" into contact with each other. The measurements, in this case, are based on the distance from which they jump and the spring constant. These measurements are usually between surfaces 1.25 nm and 20 nm apart.^[6]

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.^[6]

Solvent mode

Early experiments measured the force between

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.^[8]

Theory

The force measurements of the SFA are based primarily on Hooke's Law,

${\displaystyle F=kx}$

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

${\displaystyle \Delta F(x)=k(\Delta x_{applied}-\Delta x_{measured})}$

where ${\textstyle \Delta x_{applied}}$is the change in displacement applied by the micrometer and ${\displaystyle \Delta x_{measured}}$ is the change displacement measured by interferometry.

The spring constants can range anywhere from ${\displaystyle 30\times 10^{5}{\frac {N}{m}}}$ to ${\displaystyle 5\times 10^{5}{\frac {N}{m}}}$.^[2] When measuring higher forces, a spring with a higher spring constant would be used.

See also

• Atomic force microscope
• Colloidal probe technique

References