Adhesion

[3] In dispersive adhesion, also known as physisorption, two materials are held together by van der Waals forces: the attraction between two molecules, each of which has a region of slight positive and negative charge.

Generally, cases where the contact angle is low are considered of higher adhesion per unit area.

[9] Theoretically, the more exact relation between contact angle and work of adhesion is more involved and is given by the Young-Dupre equation.

Strong adhesion and weak cohesion results in a high degree of wetting, a lyophilic condition with low measured contact angles.

Conversely, weak adhesion and strong cohesion results in lyophobic conditions with high measured contact angles and poor wetting.

London dispersion forces are particularly useful for the function of adhesive devices, because they do not require either surface to have any permanent polarity.

However, experimental data showed that many of the compounds observed to experience van der Waals forces had no multipoles at all.

By solving the quantum mechanical system of two electrons as harmonic oscillators at some finite distance from one another, being displaced about their respective rest positions and interacting with each other's fields, London showed that the energy of this system is given by: While the first term is simply the zero-point energy, the negative second term describes an attractive force between neighboring oscillators.

Smooth surfaces of mica, gold, various polymers and solid gelatin solutions do not stay apart when their separating becomes small enough – on the order of 1–10 nm.

The effect is also apparent in experiments where a polydimethylsiloxane (PDMS) stamp is made with small periodic post structures.

These forces also act over very small distances – 99% of the work necessary to break van der Waals bonds is done once surfaces are pulled more than a nanometer apart.

[3] As a result of this limited motion in both the van der Waals and ionic/covalent bonding situations, practical effectiveness of adhesion due to either or both of these interactions leaves much to be desired.

For example, cross-linked polymers are less capable of diffusion and interdigitation because they are bonded together at many points of contact, and are not free to twist into the adjacent surface.

Uncrosslinked polymers (thermoplastics), on the other hand are freer to wander into the adjacent phase by extending tails and loops across the interface.

Scission is easily achieved by ultraviolet irradiation in the presence of oxygen gas, which suggests that adhesive devices employing diffusive bonding actually benefit from prolonged exposure to heat/light and air.

The outermost layer of each surface plays a crucial role in the adhesive properties of such interfaces, as even a tiny amount of interdigitation – as little as one or two tails of 1.25 angstrom length – can increase the van der Waals bonds by an order of magnitude.

By providing the otherwise brittle interfacial bonds with some flexibility, the molecules that are stringing across the gap can stop the crack from propagating.

If failure does occur at an interface containing a viscoelastic adhesive agent, and a crack does propagate, it happens by a gradual process called "fingering", rather than a rapid, brittle fracture.

[17] Technologically advanced adhesive devices sometimes make use of microstructures on surfaces, such as tightly packed periodic posts.

These are biomimetic technologies inspired by the adhesive abilities of the feet of various arthropods and vertebrates (most notably, geckos).

By intermixing periodic breaks into smooth, adhesive surfaces, the interface acquires valuable crack-arresting properties.

The aforementioned reaction of certain polymer-on-polymer surfaces to ultraviolet radiation and oxygen gas is an instance of hysteresis, but it will also happen over time without those factors.

In addition to being able to observe hysteresis by determining if W > γ1 + γ2 is true, one can also find evidence of it by performing "stop-start" measurements.

Results from experiments on polymer-on-polymer surfaces show that if the stopping time is short enough, resumption of smooth sliding is easy.

[14] Some atmospheric effects on the functionality of adhesive devices can be characterized by following the theory of surface energy and interfacial tension.

If this is true, then it follows that when the interfacial tension is high, the force of adhesion is weak, since each species does not find it favorable to bond to the other.

Naturally this applies very strongly to wetting liquids, but also to gas molecules that could adsorb onto the surface in question, thereby occupying potential adhesion sites.