Adhesion Force imaging

The retracting portion of the force curve sometimes follows the approach curve; however, there is often hysteresis. The most common type of hysteresis is due to some sort of adhesion, which appears in the force curve as a deflection below the zero-deflection line. The source of the adhesion can vary depending on the sample. In the ideal case of a sphere interacting with a flat surface, the adhesion force can be related to the radius of the sphere and the surface energies of the two surfaces.

Under ambient conditions, the main source of adhesion is the formation of a capillary bridge between the tip and the sample. In air, most samples have several nanometers of water adsorbed to the surface; this water layer wicks up the tip and forms a ‘bridge’ between the tip and the sample. Pulling the tip out of that bridge requires a large force to overcome the surface tension. In fluid, the adhesive force depends on the interfacial energies between the tip and sample surfaces, and the solution; varying the solution can thus change the force of adhesion.

A different form of ‘adhesion’ occurs when a polymer is captured between the AFM tip and the substrate. In this case, there is a very distinctive ‘adhesive’ force as the tip is pulled away. Typically, these curves initially retrace the approach curve near the surface but, away from the surface, exhibit a smooth negative deflection as the polymer is stretched until it breaks or detaches from the tip or the substrate, and the cantilever returns to the zero-deflection line. If multiple polymer molecules attach to the tip and substrate, a saw-tooth pattern can be observed as individual polymers detach. For references concerning different kinds of adhesive forces see [1, 2].

To be useful, the force curves must be transformed into descriptions of force as a function of distance, F(D). However, current SFMs do not have an independent measure of D. Instead, the transformation to D is achieved by subtracting the cantilever deflection from the z-piezo movement.

In some cases detachment of the tip under retractive motion of scanner occurs abrupt, and suitable force (force of adhesion) can measured relatively correctly.

Corresponding adhesion maps are typically produced by taking the most negative force detected during the retraction curve as the value for adhesion and plotting that value against the x–y position of each curve. Several types of spatially resolved adhesion map can be produced, for example, the spatial distribution of adhesion in grafted-polymer systems. Using a special AFM tips, modified by antibodies or ligands, one can map the distribution of specific proteins on the surfaces of living cells etc.

References

1. TIBTECH 17, 143 (1999).
2. Israelashvili, J.N. (1992) Intermolecular and Surface Forces, Academic Press.