Particle adhesion, to surface

Understanding particle adhesion to a surface has applications in tissue engineering and particle processing. Experimental techniques for charactering particle adhesion to surfaces include laser trapping, AFM and microscopy with force measurement. 

Vatistas, N., Particle Adhesion to surface under tuibulent flow conditions, in Particles on Surfaces, Vol. 3, (K. L. Mittal, ed.). Plenum Press, New York (1988)... 

Electrostatic Induction Potentials. Since many of our materials are semiconductors (Fc203, Mn02, CuO, silicon metal), and aluminum metal is a conductor (42-46), the possibility that potentials arising from electrostatic induced-image forces could contribute significantly to adsorption free energies should be considered. These forces have been demonstrated to be important in Xerox toner particle adhesion to photoreceptor surfaces (47). 

For a more thorough evaluation of particle adhesion to modified surfaces on the basis of the parameters o and F, which characterize the distribution of particles with respect to adhesive force, let us calculate in accordance with Eq. (1.29) (see p. 15) the average forces of adhesion (see Table II.6). 

In contrast to the capillary forces that are operative in the adhesion of spherical particles (see Section 17), we find here a number of different features in the adhesive interaction of cylindrical particles. In the first place, there is a difference in the forces acting in the case of particle adhesion to vertical and horizontal surfaces. In the second place, there is a certain distance between a cylindrical particle and a flat surface at which capillary penetration of liquid takes place, and this must be taken into account in evaluating the adhesive force. In the third place, gravitational force may change the quantity of capillary-held liquid, particularly when the contact surfaces are vertical. 

The relationships found for the adhesion of particles to surfaces in air and in hquid media (see Chapters IV-Vl) are valid in the case of particle adhesion to painted surfaces. The distribution of particles of different shapes adhering to paint and varnish coatings likewise follows a log-normal law [194]. 

We have listed below the values obtained for the median and average forces of adhesion in relation to the diameter of spherical glass particles adhering to surfaces painted with a chlorinated PVC enamel ... 

Also, the force of particle adhesion to a solid substrate coated with a sticky layer will depend on the kinetic energy of the dust particles at the moment they come into contact with the substrate. For example, in tests on an oily surface with 5 mg/cm of oil, higher velocities of particle precipitation on this surface were found to given correspondingly higher values for the centrifuge speed required to detach the adherent particles, as illustrated by the following data ... 

Thus we see that for regularly shaped particles (spherical, cylindrical, and rectangular, prismoidal), the forces of particle adhesion to oily surfaces are much greater than the forces of adhesion to painted surfaces that do not have any oil layer. 

The decrease in the force of particle adhesion to the oily surface with increasing temperature is explained by the decrease in oil viscosity and the consequent decrease in force of adhesion between the oil film and the particle. 

The increase in particle adhesion to an oily surface may be affected by impurities that are present in the liquid medium. After evaporation of the liquid, these impurities tend to increase the adhesive interaction between particles and painted surfaces. In an evaluation of such increases in particle adhesion, particles were deposited on chlorinated PVC enameled surfaces, together with a drop of liquid. The particles in the liquid drop remained on the surface after evaporation of the liquids the particle adhesion, however, was found to be different from the adhesion of the same dust in air. 

The particle adhesion can be related not only to the angle but also to the incident angle p of the particles on the surface. The relationship between the angles (p and jS has been determined [259] in the case of particle adhesion to a spherical surface (a similar relationship exists in the case of particle adhesion to a cylindrical surface). 

Certain Features of Particle Adhesion to Plates. In examining the adhesion of a particle layer to plates, two important circumstances have been neglected [259]. First, the presence of particles previously adhering is often ignored, i.e., it is considered that each contact of the particle is with the original surface. Second, the dust deposition takes place not only on the facing (upper) side of the plate, but also on the lower (rear) side. Let us now examine the influence of these factors on the adhesion of particles from an air flow. 

Thus, for small particles, it is sufficient to determine the force of particle adhesion to a surface in order to calculate the flow velocity at which detachment and removal of all particles will take place. 

The support clearly affects the rate of some Au-catalyzed reactions. The support can play various roles. First, the support can change the nature of the metal particle adhesion to the surface, and thus change the metal particle size that forms, as was discussed above. Second, the support can act to strain the metal-metal bonds, which would significantly change the electronic properties of the metal atoms near the interface and thus their catalytic properties. Third, there can be electron transfer between the metal and the support, which would change the electronic properties of the metal. Neutral and positively and negatively charged Au clusters have all been proposed to be catalytically active for specific reactions in the literature. Lastly, the interface between the metal and the support can act to create unique bifunctional sites which demonstrate enhanced reactivity. We discuss the last three effects below. The effect of particle size on the catalytic performance was discussed in detail in the previous section. 

Use of mesoporous silicon as an effective toothpaste abrasive exploits its highly tunable hardness and relatively high strength compared to silica. Remaining challenges are its cost of manufacture and color manipulation. Controlled delivery opportunities exist in oral hygiene, but particle adhesion to teeth or other oral surfaces needs to be quantified and optimized. 

Figure 31 shows the schematic of a particle of diameter d attached to a flat surface. Here, P is the external force exerted on the particle, a is the contact radius, and Fad is the adhesion force. The classical Hertz contact theory provides for the elastic deformation of bodies in contact, but neglects the adhesion force. Several models for particle adhesion to flat surfaces were developed in the past that improves the Hertz model by including the effect of adhesion (van der Waals) force. 

The effect of bubbles passages on the fouling formation was studied with a DO setup [66], The system did not capture the images of bubble passages onto the membrane surface due to camera speed limitation, but the consequences of the bubbles on the deposited cake were recorded. As expected, the introduction of bubbles into the system led to a limitation of the cake layer formation by enhancing particle back-transport and reducing the particle adhesion to the membrane surface. 

Schwarz et al. studied the effect of adhesion and pressure on the tribological properties of fullerenes [94,99]. Particles adhere to surfaces by van der Waals forces. These forces are proportional to the radius of the particles and independent of the number of layers. Deformations due to adhesion of the particles on surfaces are very weak. Adhesion supports the exfoliation of the particles, but does not trigger it. The exfoliation of fullerenes is due to the pressure exerted on the particles, which destabilizes the structure, a critical pressure being necessary. Exfoliation occurs only under adhesion conditions and concerns only the first layers (1 or 2 first layers). Layers then adhere on surfaces, which minimizes the edge effects and leads to the formation of films on surfaces [99],... 

One valid alternative to the electrolytic deposition of metal clusters is the direct electroreduction of metal ions in the presence of an appropriate stabilizer capable of preventing particle adhesion to the electrode surface. Such stabilizers adsorb onto the growing particles, thus preventing their deposition and producing structures that usually are stable within the reaction environment. These molecules can be chosen from different types of surfactant a good stabilizing agent should not interfere with the electroreduction of the metal ions, and neither should it passivate the electrode active surface. 

We will first summarize some of the approaches employed in studying particle adhesion to materials that exhibit elastic properties. Quesnel et al. (84) have discussed the similarity between the tendency of a fluid drop to wet a surface of a solid and the tendency to wet when solids are brought together (e.g., a glass particle and a plastic substrate). 

Although the principles of surface thermodynamics, the interfacial Iree energies, and the work of adhesion play an important role in models of particles adhesion to polymeric substrates the thermodynamic quantities associated with dividing lines between differing phases—the edge energy or line tension—have not been considered in these adhesion models. 

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