Editing Adhesion (section)

==Other effects==
In concert with the primary surface forces described above, there are several circumstantial effects in play. While the forces themselves each contribute to the magnitude of the adhesion between the surfaces, the following play a crucial role in the overall strength and reliability of an adhesive device.

===Stringing===
[[File:Fingering.jpg|thumb|Fingering process. The hatched area is the receiving substrate, the dotted strip is the tape, and the shaded area in between is the adhesive chemical layer. The arrow indicates the direction of propagation for the fracture.]]

[[Stringing]] is perhaps the most crucial of these effects, and is often seen on adhesive tapes. Stringing occurs when a separation of two surfaces is beginning and molecules at the interface bridge out across the gap, rather than cracking like the interface itself. The most significant consequence of this effect is the restraint of the crack. By providing the otherwise brittle interfacial bonds with some flexibility, the molecules that are stringing across the gap can stop the crack from propagating.<ref name=Kendall/> Another way to understand this phenomenon is by comparing it to the [[stress concentration]] at the point of failure mentioned earlier. Since the stress is now spread out over some area, the stress at any given point has less of a chance of overwhelming the total adhesive force between the surfaces. 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.<ref name=Newby/>
Stringing can apply to both the diffusive bonding regime and the chemical bonding regime. The strings of molecules bridging across the gap would either be the molecules that had earlier diffused across the interface or the viscoelastic adhesive, provided that there was a significant volume of it at the interface.

===Microstructures===
The interplay of molecular scale mechanisms and hierarchical surface structures is known to result in high levels of static friction and bonding between pairs of surfaces.<ref>[https://www.researchgate.net/publication/283675011_Static_friction_at_fractal_interfaces Static Friction at Fractal Interfaces] Tribology International 2016, Volume 93</ref> 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 [[arthropod]]s and [[vertebrate]]s (most notably, [[gecko]]s). By intermixing periodic breaks into smooth, adhesive surfaces, the interface acquires valuable crack-arresting properties. Because crack initiation requires much greater stress than does crack propagation, surfaces like these are much harder to separate, as a new crack has to be restarted every time the next individual microstructure is reached.<ref name=Majmuder>{{cite journal|author=A. Majmuder|title=Microfluidic Adhesion Induced by Subsurface Microstructures|doi=10.1126/science.1145839|year=2007|last2=Ghatak|first2=A.|last3=Sharma|first3=A.|journal=Science|volume=318|issue=5848|pages=258–61|pmid=17932295 |bibcode=2007Sci...318..258M |s2cid=19769678 }}</ref>

===Hysteresis===
[[Hysteresis]], in this case, refers to the restructuring of the adhesive interface over some period of time, with the result being that the work needed to separate two surfaces is greater than the work that was gained by bringing them together (W > γ<sub>1</sub> + γ<sub>2</sub>). For the most part, this is a phenomenon associated with diffusive bonding. The more time is given for a pair of surfaces exhibiting diffusive bonding to restructure, the more diffusion will occur, the stronger the adhesion will become. 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 > γ<sub>1</sub> + γ<sub>2</sub> is true, one can also find evidence of it by performing "stop-start" measurements. In these experiments, two surfaces slide against one another continuously and occasionally stopped for some measured amount of time. Results from experiments on polymer-on-polymer surfaces show that if the stopping time is short enough, resumption of smooth sliding is easy. If, however, the stopping time exceeds some limit, there is an initial increase of resistance to motion, indicating that the stopping time was sufficient for the surfaces to restructure.<ref name=Maeda/>

===Wettability and absorption===
Some atmospheric effects on the functionality of adhesive devices can be characterized by following the theory of [[surface energy]] and [[interfacial tension]]. It is known that γ<sub>12</sub> = (1/2)W<sub>121</sub> = (1/2)W<sub>212</sub>. If γ<sub>12</sub> is high, then each species finds it favorable to cohere while in contact with a foreign species, rather than dissociate and mix with the other. 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. The interfacial tension of a liquid and a solid is directly related to the liquid's [[wettability]] (relative to the solid), and thus one can extrapolate that cohesion increases in non-wetting liquids and decreases in wetting liquids. One example that verifies this is [[dimethylsiloxane|polydimethyl siloxane]] rubber, which has a work of self-adhesion of 43.6 mJ/m<sup>2</sup> in air, 74 mJ/m<sup>2</sup> in water (a nonwetting liquid) and 6 mJ/m<sup>2</sup> in methanol (a wetting liquid).

This argument can be extended to the idea that when a surface is in a medium with which binding is favorable, it will be less likely to adhere to another surface, since the medium is taking up the potential sites on the surface that would otherwise be available to adhere to another surface. 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. This last point is actually fairly intuitive: Leaving an adhesive exposed to air too long gets it dirty, and its adhesive strength will decrease. This is observed in the experiment: when [[mica]] is cleaved in air, its cleavage energy, W<sub>121</sub> or W<sub>mica/air/mica</sub>, is smaller than the cleavage energy in vacuum, W<sub>mica/vac/mica</sub>, by a factor of 13.<ref name=Kendall/>

===Lateral adhesion===
Lateral adhesion is associated with sliding one object on a substrate, such as sliding a drop on a surface. When the two objects are solids, either with or without a liquid between them, the lateral adhesion is described as ''[[friction]]''. However, the behavior of lateral adhesion between a drop and a surface is tribologically very different from friction between solids, and the naturally adhesive contact between a flat surface and a [[Drop (liquid)|liquid drop]] makes the lateral adhesion in this case, an individual field. Lateral adhesion can be measured using the [[centrifugal adhesion balance]] (CAB),<ref name="Tadmor2009"/><ref name="Tadmor2017"/><ref name="de la Madrid2019"/><ref name="Vinod2022"/><ref name="Sadullah2024"/> which uses a combination of centrifugal and gravitational forces to decouple the normal and lateral forces in the problem.