Detachment of Adhesive Particles

Figure 13-12. Surface energy effects in the detachment of wear particles. (a) Adherent particle is compressed between two surfaces. (b) Elastic compression strain has been relaxed but volume tensile strain has been locked in by adhesion. (c) Direct detachment of wear particle by influence of surface pressure on zone of potential detachment (dashed line).

Despite the criticisms above, the vOCG approach has been frequently and successfully used over recent years to interpret polymer solubility in water [14] (this is not possible using the y approach ), protein adsorption on clays [57] and conducting polymers (see Section IV.A.2 below), cell adhesion to copolymer surfaces [65], yeast-yeast and yeast-bacteria adhesion [72], fiber-matrix adhesion [69], and the hydrodynamic detachment of colloidal particles from glass plates [70]. 

The third phenomenon, that of detachment of the particles from each other by applying a pull-off force, is the test of adhesion which is most familiar to us. We increase the tension force applied to the particles until they just come apart, and define that force as the adhesion force. A large force means large adhesion. However, this is a very difficult experiment to carry out because the final detachment is an instability which is hard to reproduce exactly each time. Thus, many different values of adhesion can be found for the same samples in such tests, depending on the rate of loading, the precise moment of detachment, etc., leading to considerable unreliability in such measurements. 

The detachment of dust particles (static adhesion) depends on the magnitude and direction of the force applied to the particle. If the force is applied in a direction normal to the dusty surface (Fig. I.l.a), then for the particles to be detached we must have F et >Fad. For a tangentially directed force (Fig. I.l. c), the moment of the forces is operative, i.e., Afdet = tan (where r is the particle radius). The first stage in the detachment process in this case will be rolling or sliding of the particle i.e., friction as well as adhesion must be overcome. 

Adhesion Number for Polydisperse Particles. The adhesion number in the detachment of polydisperse particles that are subjected to a specific force can be represented as the arithmetic average of the adhesion numbers characteristic for monodisperse fractions of these particles ... 

Experimental data are listed below for the detachment of spherical particles of polyvinyl chloride from steel surfaces by a pulse method [19] (in calculating the force of adhesion, the distribution of these forces is taken into account see P. 20) ... 

When the vibration technique is used, simultaneous measurements can be made of the electric charges produced upon detachment of the particles, this information being needed to calculate the electrical component of adhesive force (see Section 15). For this purpose we can use a unit described in [82], which differs from previously used units of the electrometric type [11,14] in that the new unit uses electronic and loop oscillographs. This obviates the dependence on visual observation, giving instead a photographic record of the electrical processes taking place in the contact zone between the dust particles and the sub-... 

Determination of Adhesive Forces by Detachment of Individual Particles... 

A subsequent refinement in methods for determining adhesion by the detachment of individual particles is the use of an instrument including two electromagnets [94]. The core, which is attached to a quartz fiber, is placed between the poles of two electromagnets. The use of two electromagnets permits a more precise measurement of the force of adhesion. The single electromagnet... 

The communication of nonmetallic properties to the powder (curve 3) leads to a rise in contact potential difference and hence a rise in the adhesive forces and the charges observed upon detachment of the particles. The communication of metallic properties to the powder (curve 2) leads to the opposite result. 

Equation (IV.43) has not been confirmed by experiments on the detachment of individual particles. The validity of (IV.43) has been confirmed by experiments in which a layer of apatite was allowed to slide on an inclined surface, the apatite being shaped into pellets with a diameter of 50 mm and a height of 10 mm [141]. The shearing force on the inclined surface was found to depend upon the moisture content of the material. Hydrophobization of the powder reduced the adhesion by approximately 35% in comparison with the adhesion of untreated powder. Hydrophobization of the surface, i.e., an increase in the angle 034 in comparison with the angle 03i, reduced the adhesion to a greater degree than did hydrophobization of the powder. 

The relationship between adhesive force and sphericity factor, as determined by direct detachment of loess particles with a double-mean radius of 100-160 ixm is illustrated by the following data ... 

Adhesion in Solvents. In removing adherent particles, frequent use is made not only of surfactant solutions but also of solvents. As reported in [190], the adhesion of quartz particles in various solvents was studied by the method of Buzagh, i.e., by detachment of the particles by tilting the surface (see p. 69). Alcohols and certain other organic solvents were used in this work. 

The adhesion of quartz particles in various solvents can be compared with the adhesion of these particles in water. Such a comparison has been made in [192]. The surface slope angle ol for detachment of quartz particles of various... 

On steel substrates painted with the chlorinated PVC enamel containing the optimal amount of stearic acid, experiments were performed on the adhesion of glass particles (Tables VIII.5 and VIII.6). The data in VIII.5 refer to detachment of the particles by centrifuging and those in Table VIII.6 to detachment under the influence of an air stream in a wind tunnel. 

Features of Particle Adhesion to Paint and Varnish Coatings with an Oil Layer. The presence of an oil layer changes the magnitude of adhesive interaction. The conditions for detachment of adherent particles will depend on the ratio between the adhesion of the particles to the oil layer and the cohesion of this layer. If the adhesion is greater than the cohesive interaction, detachment of particles will require that the cohesive forces of the oil layer be overcome. If the adhesion is less than the cohesion, the adherent particles will be detached as a result of overcoming the adhesive interaction. 

When the relative humidity of the air is increased, we see an increase in the force of adhesion and since the flow velocity at which dust particle detachment occurs will depend on as indicated in Eq. (X.l), we find that as a consequence the velocity det Iso increases. For the detachment of spherical particles with a diameter of 20 fjim with an air relative humidity of 40%, the required flow velocity as determined in [17] was about 10 m/sec with 80% air humidity, the required velocity was 14 m/sec. 

Since there is a distribution of adherent particles with respect to adhesive force, the detachment velocity will depend on this distribution and on the sizes of the adherent particles. It has been shown experimentally that in the detachment of identical particles, the velocity required for detachment will vary. In Fig. X.3 we show as an example the fractional distribution of particles removed, as characterized by adhesion number, in relation to detaching velocity. A probability-logarithmic scale has been used for these plots. Similar distributions have been obtained for other particle-surface systems [277]. On a probability-logarithmic scale, the distribution of particles removed as a function of detaching velocity is approximated by a straight line. This means that the distribution of detaching velocities, as the distribution of adhesive forces (see Section 3), follows a log-normal law. 

Thus we see that a value determined for the fraction of particles removed, i.e., a value determined for the adhesion number, corresponds to its own particular velocity of detachment. As in the determination of adhesive interaction (see p. 13), the removal of adherent particles by an air stream is characterized by two parameters the detaching velocity and the adhesion number. In addition, the detachment of adherent particles by an air stream can be determined quantitatively by means of a single parameter. The median or average velocity of detachment is such a parameter. 

We can see now that the detachment of adherent particles by an air stream can be characterized by the velocity of detachment. This velocity depends on the adhesive force, the particle size, and the properties of the contiguous bodies. The distribution of the detached particles with respect to adhesion number, in relation to the velocity of detachment, follows a log-normal law. If we know the parameters of this distribution, we can find the median and average velocities of detachment for the adherent particles the average velocity gives an unambiguous quantitative characterization of the effect of the air stream on the dusty surface across which it is blowing. 

Equation (X.45) enables us to determine the drag of an air stream flowing across a dust-covered steel surface with a Class 4 finish without making any assumption as to the velocity distribution in the boundary layer. Knowing the drag pressure and the distribution of adherent particles with respect to adhesive force, we can determine the probability of detachment of these particles and the removal coefficient... 

Fig. X.5. Adhesion number in detachment of loess particles with a diameter of 40-100 jum, by an air stream, from a cylindrical porcelain surface located vertically in a duct as a function of incident angle ip of the stream on the surface, with different flow velocities (m/sec) (1) 0 (2) 5 (3) 7 (4) 10 (5) 15.

In order to bring about identical conditions for the detachment of adherent particles, it is necessary that the force of stream action on each adherent particle be the same. The detachment of particles under the influence of an air stream will take place if the flow is able to overcome the adhesion and weight of the particles, i.e., if conditions (X.l) and (X.2) are fulfilled. The force with which the stream acts on the particle will depend on the density p and viscosity 17 of the medium, the particle diameter d, the flow velocity u, and the conditions of flow around the adherent particles, as expressed by the coefficient y... 

From Eq. (X.51) it follows that the particle detachment depends on the properties of the medium (p, t ), the force of adhesion the particle size d, and the flow velocity v. Particle detachment, under otherwise equal conditions (P, ad> and d = const) will be determined by the flow velocity, which in turn depends on the conditions of flow around the objects or plates of different sizes. Hence, in order to create identical conditions of particle detachment from the model (small plate) and the object in nature (larger plate), the process of flow around the object must be modeled. Such modeling should bring about the establishemnt of an identical drag for the detachment of adherent particles under the model conditions and natural conditions in the case of laminar and turbulent boundary layers. 

The detachment of adherent particles by an air stream is accompanied by removal of these particles from the dust-covered surface, which eliminates the possibility of secondary adhesion. In the general case, the process of freeing a surface from dust is determined by the probability of particle detachment and the probability of particle removal from the surface Pj- (see p. 18). 

The probability of detachment of adherent particles by an air stream drops off sharply as the adhesive force increases. This increase is particularly great in the case of oily surfaces (see Section 38). With an air stream giving a detaching force of 2 g-units on the adherent particles, the probability of detachment for particles of different sizes will vary as shown below (standard deviation 0.35) ... 

The probability of removal of adherent particles by an air stream (after detachment) is near unity. The fact is that the velocity required to detach adherent particles is generally greater than the velocity at which adhesion will take place. Hence, particles that have been torn from the surface are unable to reattach themselves to this same surface. If the probability of particle removal is equal to unity, i.e., if = 1, then according to Eq. (1.33), the coefficient will be given by Kj = 1/(1 - P )- For the conditions under which the detachment of adherent particles were determined (detaching force of 2 g-units), the values of the removal coefficient will be as follows ... 

Detachment of Adherent Particles of Different Sizes. The velocity required for detachment of adherent particles will vary with particle size in a complex manner. The type of variation is governed by the ratio between the weight of the adherent particles and the force of adhesion. If the force of adhesion is greater... 

The removal of a layer of particles will depend on the relationship between the forces of adhesion and autohesion. Adhesion-type detachment of an adherent layer (denudation) is determined by the air-flow velocity and the adhesive force. Autohesion-type detachment (erosion) depends not only on the force of autohesion and the air velocity, but also on the time during which the air stream is acting on the surface. Consequently, the detachment of either a monolayer or layer of adherent particles, under otherwise equal conditions, is determined by the air-flow velocity. In turn, the air-flow velocity required for detachment of adherent particles will also be determined by the size of these particles. 

The surface material also affects particle detachment. Data are listed below for the detachment of glass particles by a water stream (variation of adhesion number) in relation to the type of surface material ... 

In the work of the Mackrles [69], no account was taken of such processes as autohesion of contaminant particles to each other, adhesion of particles to the layer adhering previously, or detachment of adherent particles by the water flow. These deficiencies were eliminated to some extent in the work of Mints [302], who based his calculations of efficiency of granular filters on an analysis of the adhesion processes with due regard for the balance of forces responsible for adhesion or detachment of the adherent particles ... 

The quantities c, v, and can be measured. The first term on the right-hand side of the equation accounts for the adhesion of th suspended particles (b is the adhesion parameter), and the second term accounts for detachment of these particles under the influence of hydrodynamic forces. If bc>ap x, the adhesion processes predominate over the hydrodynamic processes, i.e., filtration takes place. If up x/v > be, previously adhering particles are removed. 

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