Adhesion of Atmospheric Dust

The adhesion of atmospheric dust is largely determined by the conditions of contact between the dust particles and the surface and by the elastic properties of the paint coating. 

In view of this, let us consider the adhesion of dust resulting from the free settling of particles as a function of the elastic prop- 

Adhesion for Free Particle Settling. Dust particles situated in air are acted upon by the force of gravity which compels them to settle. When dust particle settle they come into contact with surfaces and are held on the latter by adhesion and friction. 

The rate of free settling for spherical particles may be calculated from the Stokes formula  

Let us suppose that the dustladen surface is placed at an angle to the horizontal plane, that the particles fall vertically, and that there is no deformation of the contact zone (Fig. V.l). 

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Two commercially available aerosol mass concentration analyzers have been described for the determination of aerosols and particulates [18], The instruments are both portable and relatively rugged and operate on ordinary commercial power. A portable system utilizing an electrostatic precipitator was described for mass concentration measurements in the range of airborne dust particles smaller than 10 /Am [19]. Chuan [20] also described a typical commercial instrument based on the piezoelectric quartz crystal microbalance for the rapid assessment of particular mass concentrations in the atmosphere. The adhesive coating used in the device is non-hygroscopic and non-reactive to the usual concentrations of pollutant gases in the atmosphere, such as CO, SO2, NOjt, and hydrocarbons. 

Surface flaws Cracks Adhesion Carbonization Process Rate of temperature rise Atmosphere of processing Dust free environment... 

The presence of sulfur dioxide in dry air had practically no effect on the adhesive force of the dust. Even at temperatures of the order of 50-80°C, about 84-96% of the particles remained undetached in an atmosphere of dry SO2, i.e., approximately the same number of particles as in air (particles were detached by a pulse technique). 

Thus it was found in [14, 90] that the adhesion of spherical glass particles to a steel surface was reduced by placing the surfaces under vacuum after dust coating in air. Patat and Schmid [28] reported that the force of adhesion (referred to 1 cm ) of an aluminum oxide powder layer on a glass surface dropped off from 60.5 dyn in a nitrogen atmosphere at a pressure of 760 mm Hg to 15.5 dyn in this same atmosphere at a pressure of 10" mm Hg. 

Influence of Thickness of Oil Layer on Adhesion. Atmospheric dust generally contains not only solid particles, but also oily contaminants, which settle on surfaces and make the surfaces oily. Also, the surfaces of some objects such as motor vehicles may become oily in the operating process. It has been found that 17% of the adherent particles on the surfaces of motor vehicles operating in Moscow do have such a layer of oil. Roughly the same results were obtained in an analysis reported in [234] on the contaminants appearing on exterior surfaces of railroad rolling stock. On a locomotive, for example, 23% of the adherent particles were in contact with oily surfaces, and on the railroad cars the figure was 19%. The presence of oily contaminants on surfaces tends to increase particle adhesion because of tackiness this is illustrated by data shown in Table VIII.8 [11]. 

Adhesion of Particles to an Oily Surface. Usually, atmospheric dust contains not only solid particles but also oil contaminations [166] which settle on surfaces, making them oily. In addition to this the surface of objects such as motor vehicles may become oily in the course of use. Thus it is found that 17% of the particles adhering to the motor vehicles of Moscow contain such oil layers. Similar results are obtained on analyzing the contaminations [226] adhering to the outer surfaces of railroad rolling stock. For example, on locomotives, 23% of the surfaces in contact are oily, and in trucks, 19%. 

Fig. I. Single-drum dryer (atmospheric). Dryers of this type may be dip or splash fed (not shown), or, as shown, equipped with applicator rolls. The latter is particularly effective for drying high-viscosity liquids or pasty materials, such as mashed potatoes, applesauce, fruit-starch mixtures, gelatin, dextrine-type adhesives, and various star dies. The applicator rolls eliminate void areas, permit drying between successive layers of fresh material and (omi me product sheet gradually. While single applications may dry to a lacy sheet or flakes, the multiple layers generally result in a product of uniform thickness and density with minimum dusting tendencies. (Bujiovak Division, BlawKnox Food Chemical Equipment. Inc)...

After holding a dust-covered surface in an atmosphere saturated with carbon tetrachloride vapor for 24 h, 95% of all particles in the 100-120-/xm size range are retained when a detaching force of 1.53 dyn is applied. The capillary forces calculated from Eqs. (IV.38) and (IV.39) when carbon tetrachloride vapor condenses in the contact zone are equal to 1.81 dyn. Apparently, thin layers of nonpolar liquids do not have any disjoining effect. Hence, we do not observe any reduction of the adhesive force due to the action of the liquid interlayer in the contact zone. This confirms the vahdity of Eq. (IV.46) for use in estimating the disjoining effect. 

We must note that the property of tackiness is used to trap dust particles in self-cleaning oil filters, in coarse dust filters for motor vehicles, in Rank filters made of metal screen, and on the precipitation plates in konimeters, impactors, and other such devices. Moreover, adhesion to a tacky base may be used in investigating dust content and dust compositions, particularly in trapping dust in the ground layer of the atmosphere. 

Adhesion is widely used in industry and agriculture. For example, this phenomenon plays a major role in filtration, separation of dry materials, cleaning of surfaces, electrophotography, treatment of plants with pesticides, and many other processes. Adhesion is a major factor in processes taking place in nature. If there were no adhesion, dust settling on the ground would be continuously returned to the atmosphere by air currents, and the dust concentration in the atmosphere would reach vast proportions. One the other hand, soil... 

Since the thin layer is exposed to the laboratory atmosphere and dust and there is a risk of touching or rubbing it during the deposition period, it should be protected with a glass cover plate (best attached at the sides with suitable adhesive tape or other means). 

Some UV adhesives (especially some of the earlier versions) are slightly anaerobic - that is they are inhibited from fully curing by the presence of oxygen in the surrounding atmosphere at the surface. This sticky or partially uncured adhesive layer is only a few microns thick but can attract dust and in some circumstances can contaminate adjacent components. 

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