Wetting and Spreading Phenomena

It has been shown that contact angle hysteresis might arise as a result of inhomogeneities of the surface wetted by the liquid phase or surface roughness [70]. When surface roughness plays a considerable role, the observed contact angle may depend on the exact position of the contact line with respect to the microscopic or 

While the above refers mainly to the static limit, new effects come into play when a moving contact line, i.e. spreading, is considered. It has been observed experimentally that the contact angle of a moving contact line 0, the dynamic contact angle, deviates from the corresponding static value 0. As an example, for a completely wettable surface (i.e. 6(, = 0), a relationship of the form 

Perhaps the most complete description was given by Cox [74], who derived an expression for the dynamic contact angle which is also valid in the case of large angles. The Cox prediction was tested experimentally and a good agreement between theory and experiment was found. 

Many of these results, most of them more than 10 years old, have attracted some renewed interest recently. In some pTAS and Lab-Chips small liquid volumes are transported as plugs in micro channels. Owing to the smallness of the volumes. 

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Since the atomic migration and crystallite migration mechanisms have been amply discussed in the previous proceedings on Catalyst Deactivation, the emphasis of the present paper is on the wetting and spreading phenomena, which appear to play a major role in sintering and redispersion. 

It is clear that wetting and spreading phenomena are significantly involved in all steps indicated in Fig. 3. Rather than report on the many studies on sintering and redispersion in practical supported metal catalysts, the following discussion briefly highlights some model studies which have been nicely reviewed by Rucken-stein et al. [1],... 

Surface thermodynamic considerations can be helpful in an understanding of the complex phenomena which occur in supported metal catalysts. Indeed, the physical and chemical interactions between metal, substrate and atmosphere lead to wetting and spreading phenomena (of the active catalyst over the substrate and of the substrate over the metal) which are relevant for the physical (sintering, redispersion) as well as chemical (suppression of chemisorption, modification of selectivity, enhanced activity) manifestations of supported metal catalysts. 

Despite considerable advances in the investigations carried out on wetting and spreading phenomena over several decades, a complete fundamental understanding is still elusive, particularly at the contact line where significant con-cepmal and mathematical difficulties arise. Although it has been seen above that the weak... 

Equation (17.3) may be taken as a fundamental definition of the effect of surface roughness on wetting and spreading phenomena. 

Although the mathematical relationships encountered in wetting phenomena are usually quite simple, they are found to be very useful in many practical applications. Their combinations and variations have given rise to still more relationships, which further expand their utihty without expanding the amount of information necessary for their application. Two thermodynamic relationships that can be useful in the analysis of wetting and spreading phenomena are the works of cohesion and adhesion. 

Several interfacial aspects must be considered when dealing with agrochemical formulations (i) Both equilibrium and dynamic aspects of adsorption of surfactants at the air/liquid interface. These aspects determine spray formation (spray droplet spectrum), impaction and adhesion of droplets on leaf surfaces as well as the various wetting and spreading phenomena, (ii) Adsorption of surfactants at the oil/water interface which determines emulsion formation and their stability. This subject is also important when dealing with microemulsions, (ill) Adsorption of surfactants and polymers at the solid/liquid interface. This is important when dealing with dispersion of agrochemical powders in liquids, preparation of suspension concentrates and their stabilization. 

Wetting and spreading phenomena are classical areas which have known a renewed interest over the last few years due both to their important industrial applications and to some recent theoretical and experimental developments. 

In 1974 Victor M. Starov met Prof. Nikolay V. Churaev, the beginning of a collaboration that has continued for more than 30 years and for which author Starov would like to express very special thanks. Churaev involved Starov in the investigation of wetting and spreading phenomena in the former Surface Forces Department, Moscow Institute of Physical Chemistry (MIPCh), Russian Academy of Sciences. This collaboration soon included a number of other colleagues from MIPCh appreciation is extended to these, especially professors Boris V. Deqaguin, Georgy A. Martynov, Vladimir D. Sobolev, and Zinoviy M. Zorin. 

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