Forces autohesive

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 literature, there are several reports that examine the role of conventional fillers like carbon black on the autohesive tack (uncured adhesion between a similar pair of elastomers) [225]. It has been shown that the incorporation of carbon black at very high concentration (>30 phr) can increase the autohesive tack of natural and butyl rubber [225]. Very recently, for the first time, Kumar et al. [164] reported the effect of NA nanoclay (at relatively very low concentration) on the autohesive tack of BIMS rubber by a 180° peel test. XRD and AFM show intercalated morphology of nanoclay in the BIMS rubber matrix. However, the autohesive tack strength dramatically increases with nanoclay concentration up to 8 phr, beyond which it apparently reaches a plateau at 16 phr of nanoclay concentration (see Fig. 36). For example, the tack strength of 16 phr of nanoclay-loaded sample is nearly 158% higher than the tack strength of neat BIMS rubber. The force versus, distance curves from the peel tests for selected samples are shown in Fig. 37. 

In Figure 9.2 the diffusion bonding process is schematically shown. The adhesive forces developing in this way are also called autohesion. 

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. 

A direct relationship was found by Bradley [25] in experiments on the adhesion of quartz spheres under vacuum. Corn [89] also found a direct variation of adhesive force with microparticle size. In essence, in the work of Bradley and Corn, interaction was determined under vacuum between the fused ends of glass fibers and a sphere (autohesion) or a plane surface (adhesion), the contiguous bodies having ideally smooth, clean surfaces. The elimination of electrical charges and capillary forces under vacuum provided grounds for the assumption that the measured values reflected only the molecular component of adhesive force in the interaction of the fused ends of the fiber with a sphere or a fiat surface. 

Qass I consists of electrically active dust for which, when precipitated, the adhesive forces are greater than the autohesive forces, so that there is no aggregation of particles. Such dusts include roasted zinc silicate and zinc oxide, finished cement, converter soot, zinc oxide, corn starch, ball clay, and diatomaceous earth after hot caustic treatment. 

The sign of the charge cannot affect the adhesion due to image forces (see Section 16). In the case of symmetrical charging, when a layer of adherent particles is formed, there may be a discharge and a decrease in the forces of particle autohesion in the layer and the force of adhesion of the layer to the surface, by an amount equivalent to the Coulomb component. 

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]. 

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. 

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. 

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. 

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 ... 

These impurities include clay materials, rock particles, low-solubility metal oxides, and suspensions of organic substances. The removal of such substances from water takes place as a result of particle adhesion on a filter material or on previously adherent particles this particle removal depends on the relationships among the forces of adhesion and autohesion, particle weight, and hydro-dynamic action of the water flow. In treating water to remove the other groups of impurities, adhesion processes are less important. 

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... 

Certain methods are known for improving the operating efficiency of electrofilters by increasing the forces of autohesion between particles. Ferromagnetic particles of iron oxide will stick together and thus become larger under the influence of an electric field, and will then adhere to the electrode surface in acicular formations. The particle enlargement tends to increase the resistivity of the layer and its adhesion. 

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. 

Loose deposits consist of an adherent layer of solid particles. Tacky deposits are caused by the presence of tacky (liquid or oily) components. Dense deposits consist of a single dense mass for which the autohesive interaction between particles is much greater than the adhesive force. Dense deposits are formed, for example, in the combustion of Estonian shales at temperatures of 500-1000°C and also in the combustion of lean coal from the Moscow area. Also, dense deposits are formed by the combustion of certain residual fuel oils. Under the operating conditions encountered in practice, all types of deposits may be present at the same time, and it is sometimes difficult to observe the distinctions between these types. 

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... 

Water and wind erosion take place when the forces of the water or air flow overcome the forces of particle autohesion. In this sense, the two processes are identical. As we now know (see Chapters IV and VI), however, adhesion in water and adhesion in air may have very different characteristics. 

Water erosion under the influence of rain becomes possible when the kinetic energy of the drops and the flow of rain are capable of overcoming the force of autohesion and the particle weight, in a manner similar to that observed when... 

The coefficient Kp takes into account the autohesive forces of the particles. If we give this coefficient a value of unity for sand, the values for other soils will be as follows ... 

The resistance of soil to wind erosion can be improved by raising the moisture content so as to increase the cohesion of the soil aggregates, i.e., increase the autohesion as a result of capillary forces. Also, the resistance to both wind and water erosion can be increased by eliminating the use of the moldboard plow, instead using implements such as disks that do not turn over the soil this retains the stubble, which helps to anchor the soil particles. 

No two surfaces are absolutely identical and there will be some contact electrification. The electrostatic theory considers the two surfaces to be bonded as the two plates of an electrostatic condenser, and is due to Deryaguin [30]. According to this theory adhesion occurs due to the electrostatic forces formed by interaction between the substrates. This theory explains the pressure dependence of tack/autohesion very well but it does not explain why raw and compounded rubbers lose most tack/autohesion as they are cured and brought into molecular contact under pressure. Further this theory is also not successful in explaining the time and temperature dependence of the tack/autohesion. By using potential contrast scanning electron microscopy the existence of an electric double layer at the polymer interface has been demonstrated [31]. 

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. ... 

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. 

A change in the properties of the suspension, in particular the turbidity of the water flow [27], and also the rheology of cohesive dispersed systems, are due to forces of autohesion. However, other still insufficiently understood factors also affect these processes. Hence, Kurgaev s attempt to associate autohesion with the rate of compaction of the residues of certain suspensions cannot be considered successful or his calculations of the forces of autohesion reliable [29]. 

Another method (theoretically better based) has been proposed for calculating the forces of autohesion from the limiting shear stress of the suspension (Psh) taking the number of coagulation bonds per unit area into consideration [30, 31], Yakhnin and Taubman [31] related Psh to the properties of the medium... 

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... 

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