2.2.7.1 The nature of adhesion

In Introduction we discussed two cases of cantilever-sample interaction within the range of molecular forces action: the Van der Waals attraction if tip is out of contact with the sample and elastic interaction if they are touching. In the middle range where attraction forces between some of the probe-sample molecule pairs act (potential is proportional to ) and repulsive forces between some other pairs act too (potential is proportional to ), it is impossible to find the interaction force between the whole probe and the sample.

Moreover, in the transition region a qualitatively new phenomenon – adhesion arises. It originates from the short-range molecular forces. The character of adhesion affects the force curve parts "fitting". The fitting function is called the adhesion interaction.

We have to distinguish between two types of adhesion: probe-liquid film on a surface and probe-solid sample. If the first case turns into the capillary interaction considered in chapter 2.2.3.2 "Capillary forces", the origin of adhesion forces between the probe and the solid is the molecular electrostatic interaction.

Fig. 1. The work of adhesion. Dependences of forces acting on probe vs. tip-sample distance during approach-retract cycle are shown.

Adhesion is a nonconservative process. Forces acting during the cantilever-to-sample approach differ from the forces during the probe retraction (Fig. 1). Such an operation requires some work to be done which is called the work of adhesion. This work has the following components [1]:

(1)

The subscripts denote: – London dispersion interaction, – dipole-dipole (orientation) interaction, – induction interaction, – hydrogen bond, – bond, – donor-acceptor bond, – electrostatic interaction. Notice that the first three items represent the work of Van der Waals forces.

The reason of adhesion – electrostatic forces at two bodies interface arising from the formation in a contact zone of the electric double layer. Its origin is different for materials of different type. For metals it is determined by the contact potential, states of outer electrons of a surface layer atoms as well as by lattice defects; for semiconductors – by surface states and impurity atoms; for dielectrics – by dipole moment of molecules groups at the phase boundary.

Adhesion is the irreversible process. For example, if contact potential exists, electrons start to drift resulting, as is well known, in the entropy increase. That is why forces differ (see below) during the cantilever approach and retraction and the process is thereby nonconservative.

To describe the adhesion quantitatively some approximating models are used. For solids these are various corrections to the Hertz problem solution.

Summary.

Adhesion is sticking of two surfaces in contact due to electrostatic forces having different nature for different materials.
Adhesion is a nonconservative process, therefore, to separate surfaces one needs to expend an additional work. In the contact zone a "neck" arises.

References.

Zimon A.D. Fluid adhesion and wetting – Moscow. Chimiya Publ., 1974. – 416 pp.(in Russian).