25 No. 6
The Use of AFM in Direct Surface Force Measurements
The atomic force microscope (AFM), more correctly described as a scanning probe microscope, can trace its origins to the scanning tunneling miscroscope originally invented by Binning and Roehrer. The AFM is aimed at providing high-resolution topographical analysis for both conducting and non-conducting surfaces. The basic imaging principle is rather simple. In a common configuration, a sample attached to a piezoelectric positioner is rastered beneath a sharp tip attached to a sensitive cantilever spring. The alternative arrangement of a moving tip and a fixed sample is also possible. Undulations in the surface of the sample lead to a deflection of the spring, which is monitored optically. The images obtained are critically dependent on the pressure, or force acting or applied, between the tip and the sample. Furthermore, optimal imaging conditions are inevitably sample dependent.
|AFM—how to best obtain reliable images?
Whilst the AFM is important for imaging investigations, quantifying the tip-sample interaction has become increasingly important in order to improve the quality and reliability of images obtained. However, it was realized in 1991 that the AFM could be used to measure interaction forces directly between a small sphere, attached to the cantilever, and a flat substrate, resting on the piezoelectric positioner, forming an interesting and highly flexible adjunct to the conventional surface forces apparatus (SFA). Force-distance profiles can now be readily obtained for a variety of soft and hard interfaces and the “colloid probe” technique, as it has become widely known, has also been extended to the measurement of friction forces.
The technique is widely used in a large number of research laboratories, in industry, universities, and research institutions. The purpose of this IUPAC project is to produce a timely technical report that recommends experimental procedures for performing colloid probe surface force measurements, in normal and lateral (frictional) modes. This project commenced in 2001 and is now in its final stages, with a second draft report prepared. The project team consists of John Ralston (University of South Australia, Adelaide, Australia), Ian Larson (Victoria College, Australia), Mark Rutland and Adam Feiler (Institute for Surface Chemistry, Stockholm, Sweden), Per Claesson (Royal Institute of Technology, Stockholm, Sweden), and Mieke Kleijn (Wageningen, Netherlands).
The second draft report, now in circulation, deals, with the following:
• AFM development
• calibration of cantilever spring constant for normal (z) deflection
• calibration of piezoscanner
• colloid probe attachment
• force measurements
• construction of force versus distance curves
• different materials
• latest developments
A section on frictional forces is presently being expanded.
The penultimate report will be completed by late December of this year at which time a project meeting will be held in Adelaide to agree on the final version.
For more information, contact the Task Group Chairman John Ralston
last modified 30 October 2003.
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