Лако-красочные материалы — производство

In addition to friction, which is often confused with adhesion, there are several other phenomena which give attractions between bodies. These phenom­ena can therefore mix up the issues of molecular adhesion. As Newton wrote: “the attractions of Gravity, Magnetism and Electricity reach to very sensible distances, and so have been observed by vulgar Eyes, and there may be others which reach to so small distances as hitherto escape observation.”1

Gravity is an ever-present attractive force between bodies which must contribute to the adhesion between objects. This gravitational adhesion can be calculated from Newton’s famous law. Consider two spheres, 10 mm in diameter, mass 1 g each, in contact as shown in Fig. 2.4. The force of attraction is readily calculated from Newton’s gravitation law to be 6.7×10 11 , a very small force, so small that it can normally be neglected when compared with molecular adhesion. One chemical bond is typically 1 nN in force, a thousand times larger than the gravity force between two 10 mm diameter spheres. The conclu­sion is that we can usually neglect Newton’s gravitation in comparison to molecular forces.

Although it appears from this calculation that we can ignore the gravitational force, there is a subtle influence of gravity which has a large adhesive effect,

Fig. 2.4. Calculation of gravitational adhesion for small spheres.

which can also be confused with molecular adhesion. This effect arises from the gravity of the Earth which pulls the atmospheric gases (nitrogen, oxygen, argon) etc. down onto the planet’s surface. This creates an atmospheric pressure which can act to give suction pad adhesion.

Imagine a rubber suction pad sitting on a surface, as shown in Fig. 2.5(a). This seals around the rim so that gas cannot get in. When a force is applied to pull the suction pad off the surface, as in Fig. 2.5(b), a vacuum is created and the pressure of the Earth’s atmosphere resists this pull to give an apparent adhesion. Since the atmospheric pressure is l00 kPa, i. e. l00 kN per square meter, this is a surprisingly large adhesive effect which arises solely from gravitation. A pad only 40 mm in diameter can support a 10kg weight.

It is obvious that this suction effect is not true molecular adhesion. Suction pads work better as you dive beneath the ocean where the gravitational pressure increases. But they get worse as you climb a mountain, eventually becoming useless in space. Molecular adhesion should remain the same, at the bottom of the sea or out in space, independent of gravitational effects.