We use Lateral Force Microscopy (LFM) to measure frictional forces resulting from the AFM probe scanning over a sample. When the probe crosses an area of the sample which has a higher frictional coefficient, the cantilever will display a greater amount of torque. The opposite is true for areas with lower coefficients of friction. Here 'high' and 'low' are relative terms, for instance a 'sticky' contaminant on glass may have a 'higher' coefficient of friction than the surrounding substrate. Thus, the contaminant would appear brighter in the friction image. In the image below, where the contaminant could be said to be 'slippery' (having a low frictional coefficient), the contaminant appears darker in the friction image.
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updated 04/09/2007