Surface wettability theory

Restrained Layer Theory Coefficient of Friction Theory Surface Wettability Theory... 

Surface Energetics and Wettability Theory. The surface energetics and wettability theory of adhesion is concerned with the effect of intermolecular and interatomic forces on the surface energies of the adhesive and the adherend and the interfacial energy between the two. 

The wettability theory of adhesion is inextricably related to the study of contact angles of liquids on solid surfaces. A force balance at the point of contact between the liquid and the solid can be written (3)... 

Several adhesive mechanisms (see Theories of adhesion) have been proposed to account for the enhancement of interface-dominated properties (such as retained tensile strength after environmental conditioning) (1) the chemical bonding theory (2) the deformable layer hypothesis (3) the surface wettability hypothesis (4) the restrained layer hypothesis (5) the reversible hydrolytic bonding mechanism. 

Good, R. J. and van Oss, C. J., The modern theory of contact angles and the hydrogen bond components of surface energies, in Modern Approaches to Wettability - Theory and Applications, Schrader, M. E. and Loeb, G. I. (Eds), Plenum Press, New York, 1992, pp. 1-27. 

Sec. Ill is concerned with the description of models with directional associative forces, introduced by Wertheim. Singlet and pair theories for these models are presented. However, the main part of this section describes the density functional methodology and shows its application in the studies of adsorption of associating fluids on partially permeable walls. In addition, the application of the density functional method in investigations of wettability of associating fluids on solid surfaces and of capillary condensation in slit-like pores is presented. 

When a phase transition occurs from a pure single state and in the absence of wettable surfaces the embryogenesis of the new phase is referred to as homogeneous nucleation. What is commonly referred to as classical nucleation theory is based on the following physical picture. Density fluctuations in the pre-transitional state result in local domains with characteristics of the new phases. If these fluctuations produce an embryo which exceeds a critical size then this embryo will not be dissipated but will grow to macroscopic size in an open system. The concept is applied to very diverse phenomena ... 

In the oil flotation process emulsion drops to which ore particles adhere are used instead of gas bubbles. In the case of film flotation the crushed ore is dumped on a continuously moving water surface. The easily wetted particles sink while the poorly wettable ones remain at the surface and are then collected with a special device. The theory of particle extraction by flotation have been further developed due to the detailed study of the elementary stages of particle adhesion and attachment to bubbles, accounting for the interaction forces of wetting films on solid substrate [e.g. 15-17]. 

The last two theories described (chemical and thermodynamic) are intimately linked together because both of them induce a modification of the chemical composition at the surface. On the one hand, this modification can change the thermodynamic parameters (wettability) of the surface. On the other, changes in chemical composition influence the chemical adhesion established between the adherend and the adhesive layer. Numerous treatments are available for surface modification with coronas [8], plasmas [9, 10], lasers [11, 12], ion-assisted reactions [13], or coupling agents [14, 15]. All these treatments do not only change the chemical composition they can also affect the roughness, the orientation of macromolecular chains, and the mechanical behavior. 

The wetting behavior of polymers is reviewed beginning with the thermodynamic conditions for contact angle equilibrium. The critical surface tension of polymers is discussed followed by some of the current theories of wettability, notably the theory of fractional polarity and theories of contact angle hysteresis. The nonequilibrium spontaneous and forced spreading of polymer liquids is reviewed from two points of view, the surface chemical perspective and the hydrodynamic perspective. There is a wide di.sperity between these two viewpoints that needs to be resolved inorder to establish the predictive relations that govern spreading behavior. 

The poor wetting theory leaves several questions unanswered. Chen et al. (35) argued that above the minimum wetting rate, wettability and surface tension should only have a minor effect on packii efficiency. Also, high relative volatility and hi liquid viscosity q>pear to be detrimental to efficiency, especially in aqueous systems, and this cannot be explained by poor wettability. Finally, there is uncertain regarding the concetitration at which the transition firom an organic system HETP to an aqueous system HETP takes place. 

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