Лако-красочные материалы — производство

Some interesting light has been thrown on the nature and roughness of surface layers in contact by experiments of Israelachvili and co-workers with the surface force apparatus [55,79,83,84]. This apparatus enables the surface energy to be evaluated both when the surfaces are advancing into closer contact, yA, and when they are receding further apart, уR. These two values would be expected to be the same, as indeed they sometimes are. In many cases, however, there is hysteresis, with yR>yA. Israelachvili and colleagues have studied this phenomenon in some detail.

In a typical experiment, Israelachvili deposited monolayers of surfactants onto cleaved mica sheets, and evaluated the surface energies. For example, mica coated with monolayers of L-a-dipalmitoylphosphatidylethanolamine (DMPE) showed no hysteresis (yA = уR = 27mJ/m2, but when coated with hexadecyltrimethylammonium bromide (CTAB) it was found that yA = 20mJ/m2 and yR = 50mJ/m2.

Israelachvilli argues that the hysteresis is a result of reorganization of the surfaces after they are brought into contact. This may occur at a macroscopic, microscopic, or molecular level. Here he argues that interdigitation or interpenetration occurs, roughening the interface at the molecular level. He has classified his surface layers as crystalline (solid­like), amorphous solid, and liquidlike (Fig. 9). The first tend not to reorganize, so hyster­esis is low. The liquidlike surfaces reorganize very quickly on both loading and unloading, so again hysteresis tends to be low. It is on the solid amorphous surfaces, where reorga­nization may take place over a significant time scale, that hysteresis is generally greatest. On a simplistic level, the analogy with viscoelastic loss is obvious, and it is not surprising to find that adhesional hysteresis is considered to have a temperature/rate dependence (Fig. 10). Under the experimental conditions employed, DMPE forms a crystalline ordered layer, but the CTAB layer is amorphous.

Thus this adhesion hysteresis is a result of a time-dependent roughening of the interface resulting from the intrinsic properties of the surface molecules. Israelachvili even interprets it in terms of a roughness factor effect (cf. Eq. (8), arguing that if уR « 2yA then the true contact area has become about twice the nominal area of contact. It would seem more realistic to argue that the energy loss associated with the hysteresis is related to the frictional forces involved in disentangling the rough, interdigitated surfaces.