Equilibrium adhesion

For the system we studied the hysteresis of wetting was small and independent of the composition of the liquid phase (Figure 5). The average r gxp = (Tg + Tg)/2 has been therefore here considered to be a reasonable approximation for the equilibrium adhesion tension. 

Non-equilibrium adhesion Plasticity of surfaces Non-elastie processes... 

Malkina and Deryagin, using a value of equilibrium adhesion that they had determined by means of a crossed-fiber method [7, p. 206], obtained a value. 4 = 1.3 10 erg in an aqueous medium. This value of A, however, cannot be regarded as sufficiently accurate since the width of the gap separating the contiguous solids was not measured, but was rather assumed by Malkina and Deryagin to be 10 cm. The value of A can be determined on the basis of Eqs. (II.8) and (II.9). 

The final value of adhesive force of glass fibers in distilled water does not depend on the applied pressure (over a range of 14 to 1400 dyn), even though the time to establish the equilibrium adhesive force does depend on the load. With a load of 14 dyn, this equilibrium is reached in 24 h with a load of 230 dyn, it is reached in 5-6 h. 

The time to establish equilibrium of interaction forces between plane-parallel disks is directly proportional both to the ratio r bfMbik od to the boundary thickening factor (i.e., the viscosity affects the kinetic adhesion). The viscosity, however, has little affect on the absolute value of the equilibrium adhesive force, i.e., on static adhesion. This can be seen by analyzing Eq. (VI.2). On the one hand, in fact, with/ = const,... 

Static friction, unlike kinetic friction, can be related to the equilibrium adhesion force or energy. Generally, for two molecularly smooth surfaces, friction can be described by Eq. (2) ... 

Fig. 33. Three experimental arrangements for studying equilibrium adhesion (a) wedging (b) sphere contact (c) peeling.

From the standpoint of thermodynamics, the dissolving process is the estabHsh-ment of an equilibrium between the phase of the solute and its saturated aqueous solution. Aqueous solubility is almost exclusively dependent on the intermolecular forces that exist between the solute molecules and the water molecules. The solute-solute, solute-water, and water-water adhesive interactions determine the amount of compound dissolving in water. Additional solute-solute interactions are associated with the lattice energy in the crystalline state. 

Determination of the equilibrium spreading pressure generally requires measurement and integration of the adsorption isotherm for the adhesive vapors on the adherend from zero coverage to saturation, in accord with the Gibbs adsorption equation [20] ... 

Surface energies are assoeiated with formation of the adhesive bond beeause they determine the extent to whieh, at equilibrium, a liquid adhesive will eome into eontaet with a solid surfaee. This is refleeted in the value of the eontaet angle, 6, whieh is related to the surfaee energies (written, following common usage, as y) by Young s equation [9]... 

Eqs. 4 and 6 enable the extent of contact between a liquid adhesive and a solid substrate to be gauged. Some consequences are shown in Table 1 where the concept of the reduced spreading coefficient S/yw, employed by Padday [10], was used to clarify the situation. As is readily seen, if S is positive, the liquid at equilibrium will be spread completely over the solid, but if S/yi is less than —2, spontaneous dewetting will occur. 

If the contact angle is known, insight into the extent of wetting to be expected at equilibrium can be obtained from calculations for idealised rough surfaces. The conclusions may require modification when kinetic effects, such as setting of the adhesive, are taken into account. 

The discussion above assumes that equilibrium contact between liquid adhesive and rough substrate is achieved. However, adhesives set in what may be quite a short time, and so may never reach equilibrium contact. It is therefore relevant to consider the kinetics of penetration of the adhesive into a pore. 

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