Autohesive detachment

Denudation of Erosion. In the detachment of an adherent layer of dust by an air stream, the following processes may take place removal of upper particles, i.e., overcoming the forces of autohesion detachment of the dust layer, i.e., overcoming the forces of adhesion of the layer and detachment of the individual particles remaining after removal of the layer. The removal of the upper layers is possible when Fad >Faut- In this case, the dust is raised only a relatively short distance above the original surface. The autohesion process of dust-layer removal is termed erosion [279]. 

On the basis of the properties and sizes of the particles forming these particular adherent layers of dust, we can assume that in these studies the phenomenon being investigated was autohesive detachment of particles, i.e., an erosion process, and that the values shown for the velocities were those serving to overcome the forces of autohesion since at velocities of 3-10 m/sec there is practically no removal of a monolayer of adherent particles with diameters smaller than 100 jum (see data presented on p. 322). Air-flow velocities greater than 100 m/sec are required to detach a monolayer of adherent particles. 

As can be seen from these data, with decreasing size of the adherent particles, the greatest increase in velocity should be found for the detachment of a monolayer, and the least increase in velocity for the autohesive detachment of a layer of particles (in erosion). 

The regeneration of a dust-covered surface depends on the ratio between the forces of adhesion and autohesion. As reported in [155], in the case of polyamide fibers that were covered with dust from an air stream containing quartz particles, an evaluation of the adhesive and autohesive interaction was performed on the basis of the median forces and Faut- The regeneration of the surface through autohesive detachment, i.e., when Fad >Faut, takes place when the particles are relatively small. At a flow velocity of 0.42 m/sec, this condition is valid for particles with diameters of 5.1-14.8 jum, and at a velocity of 0.28 m/sec for particles with diameters of 5.1-12.5 jum. For larger particles, adhesive detachment takes place, i.e., the condition Faut > Fad is observed. When the airflow velocity is increased to 0.84 m/sec, the finer particles undergo autohesive detachment. 

The amount of moisture in soils has a considerable effect on adhesion. With increasing moisture content, the adhesive force increases because the soil becomes more sticky (see Table XII.3). For soils of the chernozem type, when the moisture content is above 70%, the strong adhesion of soil to a metal surface brings about an autohesive type of detachment when tilling the soil, so that friction of metal on soil is replaced by friction of soil on soil. According to data from other sources [341] autohesive detachment for well-structured clay and loam soils is observed at a moisture content of 80-85%, and for light soils at... 

In the case of the adhesion of a monolayer (Fig. I.la), the detaching force acts on each particle, and if > F j (the latter being the adhesive force) the adhering particles will be detached from the surface. When whole layers adhere to the surface (Fig. I.lb), the force acts on all the particles forming the layer or layers. The strength of this layer depends not only on its adhesion to the surface, but also on the autohesion of the particles themselves. If ad det aut latter being the force of autohesion), adhesive detachment will occur if Fad > det > aut > autohesive detachment will take place. If F ad Faut, there may be mixed adhesive—autohesive detachment. ... 

By a comparison of the forces of adhesion of particles Fad as calculated from Eq. (1.42) with experimental data on the detachment of a monolayer, it is easy to establish that ad corresponds to the force of the most weakly held particles of the monolayer, i.e., the initial section of the integral curves for adhesive force (see Fig. 1.2). Consequently, in the detachment of a powder layer by tilting a dust-covered surface, we measure the average force of adhesion of the readily removable particles. As they sUde, these particles produce an avalanchelike removal of the remaining particles. If the force of adhesion of the layer to the substrate is greater than the autohesion in the layer, the detachment will take place across the weakest autohesive bonds. 

Detachment of a Layer of Particles. When a layer is removed, particles slide along the surface. The particles in the layer form a continuous mass under the influence of forces of autohesion, and this eliminates rolling of the particles upon detachment. 

When the forces of autohesion are large, greater than the forces of adhesion, the detachment will take place at the boundary between the surface and the dust layer. In this case, it is the forces of adhesion that must be overcome [280]. This process is termed denudation. In denudation, particle detachment begins at the leading edge of the dust deposit, and a cloud rapidly fills the entire passage. 

The denudation velocity is shown as a function of the parameter (FautP) in Fig. X.9. In Eq. (X.65), only the force of autohesion and the particle density are taken into account no consideration is given to the force of adhesion, even though Davies notes that dust is detached from polished brass surfaces more readily than from surfaces covered with Grade 0 emery paper [280]. 

In erosion a considerable amount of the adherent dust remains even 18 sec after the start of an air flow at a velocity of 25 m/sec. This means that erosion depends not only on the air-flow velocity, but also on the time during which the air stream is acting on the adherent dust. For this reason, the erosion process may be evaluated in terms of a certain arbitrary parameter E, which indicates the amount of dust (in g/sec) removed by an air stream with a flow velocity of 25 m/sec acting over a period of 5 sec. With this air-flow velocity, no adhesive-type detachment of the layer will take place in 4-6 sec [280], the area of the remaining layer of adherent dust being equal to the original area. The parameter E can be expressed in terms of the density of the particle material and the force of autohesion of the dust layer in the following form ... 

Equation (X.69) is valid for the removal of a layer of sand or coal particles 0.5-1 mm in thickness, with a particle size of 15-90 /xm, in ducts with a diameter of 100-400 mm. This formula can be used to determine the air velocity required to overcome the forces of autohesion in the process of erosion. For complete detachment of the adherent particles, i.e., in order to overcome the forces of dust-layer adhesion to the inside surface of the duct, the air velocity must be substantially greater than the value calculated by the use of Eq. (X.69). As the air-flow velocity is increased, it becomes possible to overcome the adhesive forces of the remaining particles and to clean the surface so that it is free of the adherent dust layer. Hence, for Fad > Faut, we must distinguish two different air-flow velocities, the first of which characterizes the conditions under which the forces of autohesion are overcome and the second the conditions under which the forces of adhesion are overcome. The first velocity is always lower than the second. 

A combined autohesion-adhesion type of detachment of the adherent layer of particles may be observed i.e., we may have a combination of the erosion and denudation processes. The fraction of particles removed under these conditions can be expressed by the formula [283]... 

Equation (X.70) is valid for v > det and has been verified experimentally in the detachment of coal particles [283]. For particles with a diameter of 10 /xm, as the flow velocity is increased from 5.5 to 13.6 m/sec, the value of ap increases from 7.0 to 10.4%, i.e., only very slightly. A greater increase in ap is found when the particle size is increased. For particles with a diameter of 88 pm, the values of OLp are 54.5-55.8%, and for particles with a diameter of 1000 pm, the value of otp increases to 96.2% this is explained by the decrease in forces of autohesion. 

As we have demonstrated, the adhesion-type detachment of a layer of adherent particles (denudation) depends on the air-flow velocity, while autohesion-type detachment (erosion) depends on the flow velocity and the time during which the air stream is acting on the dust, as expressed in terms of arbitrary quantities. In many studies, unfortunately, no distinctions have been made as to the specific features of removal of the adherent particle layer, the time of detachment has not been recorded (this is usually a long period, greater than 5 min), and the results of detachment are evaluated solely in terms of the air-flow velocity. The following data relate to the flow velocities at which removal of the layer of adherent particles has been observed ... 

If the dust-covered plates are set at an angle to the flow, the velocity for detachment of the upper layers of magnetite dust held by autohesive forces can be determined [251] from the empirical formula ... 

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. 

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 ... 

Dependence of Adhesion on Resistivity of Dust Layer. A particle after it has reached the electrode surface either may give up its charge or may acquire the charge of the electrode, and in some cases it will again be detached from the electrode. Such processes also take place in the autohesion of particles to a previously attached layer of dust these processes are determined by a supplementary electric force. This force depends on the resistivity of the dust layer and may be expressed by the equation... 

Adhesion and autohesion are critical factors in the following stages of the xerographic process (Fig. XII.5) the attachment of the toner particles 3 on the surface of a carrier particle 4 and formation of the developer complex the development, i.e., detachment of the toner particles and retention of these particles on the charged sections of the semiconductor layer 1 the detachment of particles from the semiconductor layer and transfer of the resultant image to paper 5 and the fixing of the image 7. 

Some rubbery materials adhere firmly to themselves (autohesive tack or auto-hesion) or to a different surface (adhesive tack) after brief contact imder light pressure. They have a liquid character which results in rapid bond formation, yet, without setting, they resist detachment like a solid, ie, they are sfrong and soft. (Tacky substances are stroft, like toilet paper.) Typically, adhesive tack involves bonding to a hard substrate and interdiffusion is absent or minimal. Adsorption is the principal mechanism of adhesion. On the other hand, autohesion involves both molecular contact and interdiffusion. Autohesion is important in the manufacture of articles, such as tires, which are built by laminating rubbery components. 

In the opinion of the authors of [22], F is the force corresponding to the autohesion of individual particles in the layer,i.e., the specific strength of the powder layer. However, this assertion does not entirely agree with the facts. For F d > F we in fact have F = Faut > but if Fad < aut > we have F = Fad > 3.nd then the detachment of the particle layer is of the adhesive type. 

The method of inclining a dust-laden surface may be used to measure the autohesive force of an adhering layer [24, 62] when Fad > aut The dust layer is deposited simultaneously on the movable and immovable parts of a glass platform (Fig. II.5). For a specific slope a the movable part of the platform detaches itself... 

Denudation and Erosion. In the detachment of an adhering dust layer by an air flow, the following processes may occur the removal of the top particles, i.e., the overcoming of autohesion, the detachment of a layer of dust, i.e., the overcoming of the adhesive forces in the layer, and the detachment of individual particles remaining after the removal of the layer. 

In this case, the dust is raised to a comparatively short distance above the original surface (Fig. Vl.lOa). The autohesive process of dust-layer detachment is called erosion [289],... 

Form of clay Depth of flow, m Cohesion acc.to Tsitovich (autohesive force), referred to 1 cm kg Coeff. of inhomogeneity Size of aggregates being detached, mm Scouring velocity, m/sec ... 

The Mackrle paper [25] took no account of such processes as the autohesion of contaminant particles to each other and their adhesion to a layer of earlier-adhering particles, nor the detachment of adhering particles by the flow of water. These deficiencies are to some extent eliminated in the papers of Mints [345, 346], who in calculating the efficiencies of grainy filters treated the adhesion processes with due allowance for the balance of forces associated with the adhesion or detachment of the adhering particles ... 

For a tangential blow only up to 7% of the energy is transformed into acceleration, while the rest is expended in deforming the electrode material. A normal blow communicates an acceleration 8-18 times as much as this to the adhering layer. Hence, detachment of the adhesive as well as the autohesive type takes place, as indicated by the high degree of cleaning of electric filters treated in this way Km = 10 (see 31). 

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