Adhesion phenomena

Many phenomena lead us to believe that molecular adhesion exists. We recognize that, just as a small solid particle sticks to a surface, so a droplet of water adheres to glass, and we realize, as Young and Laplace did in the early 1800s, that this ean be explained by attractions between the water molecules and the glass. It is evident that this is an intensely local and short-range attraction, because changing the size of the droplet to make it very small does not influence the adhesion, as shown in Fig. 2.1. 

The problem with adhesion is that we often use the same word to describe quite different situations. For example, when we split a log with an axe, as in Fig. 2.2(a), we can see the cellulose fibers separating by fracture within the wood, and we conclude that the wood is made up of cellulose fibres strongly adhering together. This is a valid description. 

However, when one describes the adhesion of a tire to a road surface or of a railway wheel to a steel rail, as in Fig. 2.2(b), this means something entirely different. In fact, there is no significant adhesion force in this case as one tries to lift the wheel from the track. Instead, the writer is referring to the force required to make the wheel skid along the surface. This phenomenon should be called friction and not adhesion. If the railway was built on the surface of the Moon, however, there could well be significant adhesion then between wheel and rail because the surfaces are much cleaner, giving improved contact. Friction would then also increase. 

In another example. Velcro fabric or a zip fastener is said to produce adhesion between two pieces of material as shown in Fig. 2.2(c). But such devices operate on a hook and eye principle, in which the hooks of one piece becomes entangled with the loops of another. The separation of such Velcro joints requires friction to slide the fibers across each other, plus deformation of the curved hooks to obtain release. No adhesion between the materials is necessarily needed. 

Similarly, a book on the Adhesion of piles in stiff clays is more about frictional sliding of the pile as it is pulled out of the clay, rather like the fiber pullout test described in Chapter 16. However, once the pile is removed, clay may then be seen adhering to the steel surface. Clearly, both friction and adhesion can be observed simultaneously in this case, as shown in Fig. 2.2(d). 

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AES has several characteristics that make it a valuable tool for research related to adhesion phenomena. The surface sensitivity of AES is similar to that of XPS. 

The surface of the substrate, the silicone/substrate interface, and the bulk properties of silicones all play significant and influential roles that affect practical adhesion and performance of the silicone. The design of silicone adhesives, sealants, coatings, encapsulants or any products where adhesion property is needed requires the development chemist to have a thorough understanding of both silicone chemistry and adhesion phenomena. 

In all the applications where silicones are used, the design of the polymer reactivity and the composition of the formulation require the development chemist to thoroughly understand both silicone chemistry and adhesion phenomena. 

Mieroscopic visualization techniques have also been used to investigate mucus-polymer interactions [36-39]. Transmission electron microscopy was used by Fiebrig et al. [36], whereas different microscopical techniques were used by Lehr et al. [37] for the visualization of mucoadhesive interfaces. Transmission electron microscopy in combination with near-fleld Fourier transform infrared microscopy (FT-IR) has been shown to be suitable for investigating the adhesion-promoting effect of polyethyleneglycol added in a hydrogel [38]. Moreover, scanning force microscopy may be a valuable approaeh to obtain information on mueoadhesion and specific adhesion phenomena [39]. 

Wet adhesion phenomena represent a potentially fruitful area of research since so little is known. Some of the important questions are (1) How does one measure quantitatively the magnitude of the adhesion when the coating is wet (2) What is the governing principle that determines whether or not water collects at an organic coating/metal interface (3) What is the thickness of the water layer at the interface and what determines the thickness A recent paper (1.) correlates the wet adhesion properties of a phosphated surface with the crystalline nature of the zinc phosphate at the metal surface. 

Yeast flocculation mechanism can be described as a phenomenon of adhesion to certain surfaces. The ability to adhere to surfaces and to form biofilm is the basis of the pathogenicity of Candida species. Pathogens adhere to mucous membranes and wounds, they stick to medical instruments and prosthesis, and thus contaminate surfaces in food processing facilities. The high mortality rate in disseminated fungal infections caused an increase in the amount of research on the molecular basis of the adhesive phenomena in Candida. This research discovered a considerable overlap in the molecular regulation of all forms of adhesive behavior. ... 

The forces acting between two surfaces in contact or near - contact determine the behavior of a wide spectrum of physical properties. These can include friction, lubrication, the flow properties of particulate dispersions, and, in particular, the adsorption and adhesion phenomena, the stability of colloidal system [1,2] and the ability to form Langmuir monolayer at the air - water interface. 

Podczeck F, Newton JM, James MB. Assessment of adhesion and autoadhesion forces between particles and surfaces. Part II. The investigation of adhesion phenomena of salmeterol xinafoate and lactose monohydrate particles in particle-on particle and particle-on-surface contact. J Adhesion Sci Technol 1995a 9 475-486. 

Colloidal dispersions and other related systems are present in many applications, e.g., in paints and coatings and detergents. Here, phase equilibrium and surface phenomena are equally important. A unified representation of such phenomena, e.g., of adhesion phenomena and liquid-liquid equilibria with the same model/concepts is of interest. Thermodynamic models can be used to calculate certain surface properties such as surface tension. hi addition, properties such as the solubility parameters can be equally well employed for bulk and surface thermodynamic properties. ... 

The surface tension, interfacial tension and adhesion phenomena will be discussed, and a new correlation for the molar parachor will be presented, in Chapter 7. The calculation of the interfacial tension from the surface tensions of the components will also be discussed, and shown to be in need for significant improvements. In this context, an introduction will also be provided to advanced numerical simulation methods that are becoming increasingly useful in modeling the interfacial phenomena and phasic behavior of polymer-containing systems. 

Replacement of A-acyl and 0-acyl by A-thioacyl groups in sialo-sugar chains of cell surface glycoproteins can influence certain biological functions mediated by sialic acids, such as cellular adhesion phenomena or virus specificity for host cells. Interestingly, sialyltransferases specific for different... 

Macro scale (CCM level) adhesion phenomena, polymer segregation in catalyst layer, mechanical properties of electrode and membrane... 

This paper wiU build on previous reviews which have sought to explore the marmer in which surface analysis methods can be purposefully employed to understand adhesion phenomena [4—6], with an emphasis on the elucidation of interphase chemistry. The rationale behind such an approach is that it is this critical region of a polymer/metal or polymer/polymer couple that will influence the performance of the overall system, be it the durability of an adhesive joint or the corrosion protection performance of an organic coating. 

Local and Global Aspects of Adhesion Phenomena in Soft Polymers... 

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