Adhesion tendency

Adhesion of individual particles to each other or to solid surfaces is controlled by the competition between volume and surface forces (A.5 in Table 1). In order to cause adhesion, certain criteria must be fulfilled. The most important of all is that any environmental forces (e.g. gravity, inertia, drag, etc.) must be smaller than the attraction forces between the adhering partners. According to Figure 8 and equation (1), the ratio between all binding forces, 5/(jr), and the sum of the active components of the environmental forces, Fjy x), involved is a measure for the adhesion tendency... 

Therefore, and because the surfaces of all particulate matter are rough, the adhesion tendency increases with decreasing particle size of at least one of the adhesion partners. Figure 9 explains schematically the conditions. Microscopically, all surfaces of solids, even macroscopically smooth ones, are rough. The depth of roughness depends on the particle size. Below a certain particle size the specific (in reference to particle size) roughness depth of smooth surfaces is approximately constant. 

Adhesion of larger particles is only possible if a binder is added or if, due to external forces, plastic deformations occur which increase the contact area and reduce the effective distance. Brittle breakage resulting in smaller particles will also increase the adhesion tendency (see, for example, Section 4.2.2.2 and Figure 181). 

It has been discussed (Section 5.1.1) that most of the attraction forces have only a short range their magnitude and strength decreases quickly with increasing distance. Therefore, because the surfaces of all particulate matter are, at least microscopically, rough (see Section 5.1.1, Fig. 5.11), and the mass of the particles decreases with the third power of the particle size, the adhesion tendency increases with decreasing particle dimensions. 

Fig. 6.2 Schematic representation of the adhesion tendency of a sphericai particie on a flat wail.

Other, already conventional methods to measure flowability and adhesion tendencies of particulate solids are based on the shear cell developed by Jenike and Johanson [B.ll] and adapted or modified by many other researchers. 

Ultrafme particles (UFP), defined as being in a size range from a few micrometers down to nanometers, feature natural adhesion tendencies, which strongly increase with decreasing linear dimensions or increasing specific surface area. On the one hand, this may be a disadvantage, because nanosized particles always exist as agglomerates (Fig. 11.1, Chapter 11) and, if individual nanoscale particles are required, special... 

Fig. 3.6 Diagram of the adhesion tendency of a spherical particle on a flat surface Fig. 3.7 Two-dimensional diagram of the failure lines derived from the three models describing strength of agglomerates with a matrix binder (Fig. 3.1a)...

Fig. 3.6 Diagram of the adhesion tendency of a spherical particle on a flat surface...

Naturally available adhesion tendencies can be considerably increased if moisture is added during the agglomeration process. Application of external forces can contribute to the enhancement of inherently present binding mechanisms. Depending on the... 

The wetting angle can be used to estimate the adhesive tendencies of powders [196]. For the adhesion of the particle layer, a definite correlation has been found between wettability and adhesive force. 

Lithography, originally performed with flat and smooth stone surfaces and nowadays with polymer-covered plates, is a printing technique for inks where the surface of the plate is structured into lipophilic and hydrophilic areas, from which the former ones show adhesion tendencies towards a greasy ink which is then printed onto the support. 

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