The name “adhesion film” usually refers to a soft plastic film made out of PVC or PE which has a very smooth underside that will adhere without the use of glue to smooth, non-porous surfaces like glass. In the meantime, however, there are also so-called “adhesive textiles” on the market (e.g. DIGITAL PRINT POLYESTER SELF-ADHESIVE TEXTILE) that have been outfitted with an adhesive backside coating. Unlike normal sticky films, adhesion film can be removed in a few seconds without the need for any auxiliary devices and without leaving any residue even after being in place for a very long time. For this reason, it is often used, for example, as a protective film for cell phone displays, as a medium to be painted or printed for temporary display window advertising or decoration or as a means to enliven the windows in a child’s room. The translucent version often serves as a light-transmitting and easily replaced visual shield in doctor’s offices, business offices or bathroom windows. The mostly opaque adhesive textiles are used, for example, as visual protection or for glare removal or even as a temporary projection screen on a window pane.
How is it that adhesion films stick to anything at all and why do they only work on smooth surfaces?
The main cause of adhesion (also called “force of attraction”) is the so-called Van der Waals force. This is the weak attractive force between molecules found on the surfaces of two different materials. These attractive forces are at work for all possible surfaces as, for example, when a hand is lain on the surface of a table top. The fundamental idea of the Van der Waals force is that the electrons in the molecules are in constant motion even in the case of so-called solid materials. The motion of the electrons has the effect that the charge within a molecule is for a short time unequally distributed so that one side temporarily has a positive charge and the other a negative one. If the positive side of one molecule happens to come into contact with the negative side of another molecule they will be attracted to one another. All molecules are always in the process of aligning themselves anew in accordance with their temporarily positive or, alternatively, negative partial charges.
The attraction that is thus produced between the surfaces of materials is comparatively weak and becomes less and less as the amount of air between them increases. For this reason the contact area between the molecules must be as large as possible which can only be the case with really flat, pore-free surfaces precisely like the underside of an adhesion film or like a piece of glass.
Strictly speaking, the Van der Waals forces are not electrostatic forces which naturally produce attraction. ElectroSTATIC forces are established statically, that is, on a sustained basis between the non-similar poles (anion and cation) of molecules. Electrostatic attraction is much stronger and a film that is electrostatically charged will also stick to somewhat rougher surfaces.
A concrete example of Van der Waals forces at work is the gecko, which can climb up vertical walls and can even hang upside down from the ceiling. A gecko has many millions of tiny hairs on the soles of its feet that are so dendritic that they produce a very large amount of surface area which is enough to develop the necessary attractive force between the molecules of the hairs and those of the surface it is climbing on.
The fact that the soles of the gecko’s feet can nonetheless be effortlessly pulled free of the surface as it walks is the result of the fact that as the foot is moved the angle of the hairs is changed which then releases the Van der Waals attractive force. Soft adhesion films are also able to be easily removed from a substrate based on the same principle.
And by the way: although gecko’s feet possess an enormous adhesive force you still won’t find any dirt or dust being stuck to them. Gecko feet are always clean! Why that´s true will have to be answered on the next Mr. Rodgers show!