Understanding Adhesion with Atomic Force Microscopy (AFM)

By Thomas BarthelayMar 7 2019

Image Credits: noprati somchit/shutterstock.com

Atomic Force Microscopy (AFM) is an advanced material imaging technique, which is able to provide accurate topographic images of a surface. It was created by Gerd Binning and Heinrich Rohrer in 1986 and is an incredibly sensitive method, with a resolution that can capture less than a nanometer. AFM is a versatile technology that can carry out many different functions; including friction, capacitance, and magnetic measurements. AFM can also be used to measure the adhesion forces of a material, by examining the attraction between the atoms of a surface and the AFM probe.¹

The crucial components of the AFM apparatus are a needle or probe attached to a cantilever (typically made of silicon or silicon nitride), a laser and a position sensitive device (PSD). As the probe is brought into the proximity of a sample surface, forces between the tip and the sample cause deflections in the cantilever, which are then tracked by the movement of the laser (reflected from the cantilever) on the PSD. Crucially, the deflection of the cantilever, as measured by the PSD, is fed back into the system and the probe support is adjusted accordingly to maintain a constant, user-defined deflection as the probe scans the whole sample. This ensures that the laser point remains directed at the PSD throughout the scan, and the feedback output equals the tracked topography of the surface to within a small error.

There are three primary modes that AFM can adopt: dynamic (tapping), non-contact and contact. The dynamic model oscillates the cantilever, causing the needle to tap against the surface. When in non-contact mode, the cantilever never comes into contact with the surface; it oscillates just above the surface, measuring the interacting forces between the probe and the surface. Contact mode is the most destructive mode as it is in constant contact with the surface, but it can obtain a lot of information that other modes cannot. It is in this mode that the adhesive and frictional forces can be studied.

The adhesion force can be defined as the point where the needle and the surface are the closest. However, this force is not a finite point. As the two surfaces approach each other, the attractive force becomes greater as they are gradually moved together. This leads to a potential well, which is the distance between the two materials that have the maximum attraction. After this maximum adhesion point, when the surfaces are pushed closer together, repulsive forces rapidly take over and push the needle away, causing the cantilever to bend further away from the surface. These forces arise from an amalgamation of electrostatic, magnetic and London dispersion interactions. Adhesion on a nanoscale is measured by using the pull-off force when the tip of the needle is moved closer to the surface of the material and then suddenly repelled.¹ The adhesion force is then characterized through the bending of the cantilever by using Hooke’s law:

F = kx

F is force, k is the spring constant of the cantilever and x is the cantilever deflection.

There is, therefore, a range of values for which a material will have attractive forces, and these will vary for different types of material. The increased roughness of a surface will decrease the range of the adhesion between the materials, which occurs due to the reduction of actual contact between the two surfaces.² This can be a problem when using AFM to measure adhesion as different surfaces will vary in roughness, making it difficult to test materials fairly. Physical and chemical etching can be used to purposely create roughness so that materials can be judged by the same standard.³

AFM may also be used to conduct force spectroscopy to produce a force-distance curve for samples. In this application, the force experienced by the probe as it approaches the surface of the sample is recorded, and the resulting cantilever deflection plotted as a function of distance from the surface. This method is commonly used to study interactions such as atomic bonding and van der Waals forces in materials.

References

1. Jiang, Y. and Turner, K. (2016). Measurement of the strength and range of adhesion using atomic force microscopy. Extreme Mechanics Letters, 9, pp.119-126.
2. Ramakrishna, S., Clasohm, L., Rao, A. and Spencer, N. (2011). Controlling Adhesion Force by Means of Nanoscale Surface Roughness. Langmuir, 27(16), pp.9972-9978.
3. Sedin, D. and Rowlen, K. (2000). Adhesion Forces Measured by Atomic Force Microscopy in Humid Air. Analytical Chemistry, 72(10), pp.2183-2189.

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