Surface forces apparatus

Surface forces apparatus

The surface force apparatus (SFA) is a scientific instrument and technique pioneered by D. Tabor, R.H.S. Winterton, J.N. Israelachvili in the early 1970s at Cambridge University. By the mid-70's Israelachvili had adapted the original design to operate in liquids, notably aqueous solutions, while at the Australian National University.

In this instrument, two surfaces are carefully moved towards and retracted from one another, all the while measuring their interaction force. One surface is held by a cantilevered spring, and the deflection of the spring is used to calculate the force being exerted. This technique 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 10–8 N 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 and down to contact. 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. Position measurements are typically made with a white-light source and by analyzing the fringes of equal chromatic order (FECO) (although use of a laser is also possible). The substrate for the surfaces or molecules of interest is generally mica coated with a semi-reflective layer of silver. This optical setup enables determination of the distance between the two surfaces. 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.

Early experiments measured the force between mica surfaces in air or vacuum. The technique has been extended, however, to enable an arbitrary vapor or solvent to be introduced between the two surfaces. 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.

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.


Jacob Israelachvili, "Intermolecular and Surface Forces", Academic Press (1985-2004)

* [] Surface Science & Technology, Swiss Federal Institute of Technology

* [] Australian National University, Research School of Physical Sciences and Engineering

* [] "The X-ray Surface Forces Apparatus: Structure of a Thin Smectic Liquid Crystal Film Under Confinement" Science 24 June 1994: Vol. 264. no. 5167, pp. 1915 - 1918

* [] "The x-ray surface forces apparatus for simultaneous x-ray diffraction and direct normal and lateral force measurements" Review of Scientific Instruments, Volume 73, Issue 6, pp. 2486-2488 (2002).

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Self-contained breathing apparatus — SCBA redirects here. For other uses, see SCBA (disambiguation). Not to be confused with the very similar Self Contained Underwater Breathing Apparatus. Toronto firefighter wearing an SCBA A self contained breathing apparatus, or SCBA, sometimes… …   Wikipedia

  • Polar equation of a surface — Polar Po lar, a. [Cf. F. polaire. See {Pole} of the earth.] 1. Of or pertaining to one of the poles of the earth, or of a sphere; situated near, or proceeding from, one of the poles; as, polar regions; polar seas; polar winds. [1913 Webster] 2.… …   The Collaborative International Dictionary of English

  • Polar forces — Polar Po lar, a. [Cf. F. polaire. See {Pole} of the earth.] 1. Of or pertaining to one of the poles of the earth, or of a sphere; situated near, or proceeding from, one of the poles; as, polar regions; polar seas; polar winds. [1913 Webster] 2.… …   The Collaborative International Dictionary of English

  • Jacob Israelachvili — Dr. Jacob Israelachvili is a professor of chemical engineering and materials at the University of California, Santa Barbara (UCSB). Dr. Israelachvili received his Ph.D. in Physics from the University of Cambridge (in the United Kingdom) in 1972,… …   Wikipedia

  • Model lipid bilayer — A model lipid bilayer is any bilayer assembled in vitro, as opposed to the bilayer of natural cell membranes or covering various sub cellular structures like the nucleus. A model bilayer can be made with either synthetic or natural lipids. The… …   Wikipedia

  • SFA — can refer to: * Sales Force Automation * Scottish Football Association, governing body of football (soccer) in Scotland * Segmented Flow Analyzer, a type of AutoAnalyzer * Shanda Fantasy Arts, a comic book company * Single Frequency Approach,… …   Wikipedia

  • Timeline of diving technology — This is a timeline of underwater technology. The entries marked ## are about decompression tables.[1] Contents 1 Pre industrial 2 19th century 2.1 Rebreathers appear …   Wikipedia

  • analysis — /euh nal euh sis/, n., pl. analyses / seez /. 1. the separating of any material or abstract entity into its constituent elements (opposed to synthesis). 2. this process as a method of studying the nature of something or of determining its… …   Universalium

  • History of submarines — The history of submarines covers the historical chronology and facts related to submarines , manned autonomous boats that operate underwater. It includes the history of submersibles , which were designed primarily to operate on the surface, and… …   Wikipedia

  • Froth flotation — Diagram of a cylindrical froth flotation cell with camera and light used in image analysis of the froth surface. Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in several processing… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.