Adhesion wetting theory

All the studies conducted on fracture of bulk polymers are certainly relevant to the adherence of polymers, the mechanisms of losses at a crack tip being the same viscoelastic losses due to moving stresses, work to extract chains or fibrils, and viscous drag in the presence of a liquid. It is probable that the various theories of adhesion, namely, theory of wetting, theory of the rheological factor, theory of the chemical bond, theory of the weak boundary layer, and theory of interdiffusion, are all valid, each corresponding to an emphasis on a dominant mechanism. 

The analysis makes use of relaxation and retardation spectra of elastomers. The estimation of the magnitude of friction has not yet been attempted. However, a major step towards that goal has been shown. In particular the wetting theory has been found to be useful in describing the strength of a countersurface in adhesion. 

Polymeric adhesives function by providing a bond between the two adherends. This bond is obtained by the adhesive wetting both surfaces. Wetting occurs when the adhesive has both a low-enough viscosity to flow over the surface and an attraction to the surface which causes it to be pulled into intimate contact with the surface topography. While in theory it is desirable to obtain chemical adhesion (covalent bonding), in practice this rarely occurs. Usually, mechanical bonding and attraction forces dominate between the adherend and the adhesive. 

The study of acid-base interaction is an important branch of interfacial science. These interactions are widely exploited in several practical applications such as adhesion and adsorption processes. Most of the current studies in this area are based on calorimetric studies or wetting measurements or peel test measurements. While these studies have been instrumental in the understanding of these interfacial interactions, to a certain extent the interpretation of the results of these studies has been largely empirical. The recent advances in the theory and experiments of contact mechanics could be potentially employed to better understand and measure the molecular level acid-base interactions. One of the following two experimental procedures could be utilized (1) Polymers with different levels of acidic and basic chemical constitution can be coated on to elastomeric caps, as described in Section 4.2.1, and the adhesion between these layers can be measured using the JKR technique and Eqs. 11 or 30 as appropriate. For example, poly(p-amino styrene) and poly(p-hydroxy carbonyl styrene) can be coated on to PDMS-ox, and be used as acidic and basic surfaces, respectively, to study the acid-base interactions. (2) Another approach is to graft acidic or basic macromers onto a weakly crosslinked polyisoprene or polybutadiene elastomeric networks, and use these elastomeric networks in the JKR studies as described in Section 4.2.1. 

The theory of viscoelastic braking in liquid spreading exposes the various possibilities that may exist for controlling wetting or dewetting speeds by changing solid rather than liquid properties. Applications may exist in the fields of contact lenses, printing, and vehicle tire adhesion. 

Thus, fundamentally the interest is in testing the limits and theory of polymer behavior in end-tethered systems, e.g., viscoelastic behavior, wetting and surface energies, adhesion, shear forces relevant to tribology, etc. It should be noted that relevant surfaces and interfaces can also refer to polymers adsorbed in liquid-liquid, liquid-gas, solid-gas, and solid-liquid interfaces, which makes these polymer systems also of prime importance in interfacial science and colloidal phenomena (Fig. 2). Correspondingly, a wide number of potential applications can be enumerated ranging from lubrication and microelectronics to bioimplant surfaces. 

Adhesion is created by primary and secondary forces according to the theory of adsorption interaction. This theory is applied the most widely for the description of interaction in particulate filled or reinforced polymers [30]. The approach is based on the theory of contact wetting and focuses its attention mainly on the influence of secondary forces. Accordingly, the strength of the adhesive bond is assumed to be proportional to the reversible work of adhesion (W ), which is necessary to separate two phases with the creation of two new surfaces. 

The same theories relevant to adhesion, developed to explain and predict the performance of glues, adhesives, and paints, have also been applied to bioadhesive systems [44], These include the electronic, absorption, wetting, diffusion, and fracture theories. 

Several mechanisms by which mucoadhesives adhere to biological surface have been suggested, including the electronic, adsorption, wetting, diffusion, and fracture theories. It is likely that water movement from the mucosa to the polymer and physical entanglement of the adhesive polymer in the mucus glycoprotein chains are important in obtaining adherence. 

As explained under the adsorption theory of adhesion,3 an adhesive must first wet the substrate and come into intimate contact with it. (A brief description of the adsorption theory of adhesion is presented in the section below.) The result of good wetting is simply that there is greater contact area between adherend and adhesive over which the forces of adhesion (e.g., van der Waals type of forces) may act. For good wetting, the surface free energy (surface tension yLV) of the liquid adhesive must be less than that (critical surface tension yc) of the solid adherend, or... 

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]. 

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