Sticking pressure

Sometimes one estimates the forces of adhesion by reference to the sticking pressure Ps = Fad/S, where S is the true area of contact of the particle with the surface. 

The true area of contact of a particle with a surface cannot be exactly calculated or determined experimentally under real conditions. One can hardly consider it correct to determine the area of contact under the microscope by reference to the trace left after the detachment of the particles, since this trace may be established in a tacky, or at any rate inelastic, substrate, contradicting the very concept of a sticking pressure. For adhesion in an air medium, the elastic deformation of the zone of contact will be different at different points. The mean integral value of the sticking pressure may be expressed in the form... 

Even if it were possible to measure the area of contact between a particle and the surface, the mean value of the sticking pressure would not be an objective characteristic of adhesion, owing to the nonuniformity of the pressure distribution in the contact zone [see formulas (1.2)-(1.4)]. 

Fq is the force of interaction between the surfaces in contact, ju is the coefficient of friction, S is the area of true contact, and Ps is the sticking pressure acting on the area S. 

The rate of physical adsorption may be determined by the gas kinetic surface collision frequency as modified by the variation of sticking probability with surface coverage—as in the kinetic derivation of the Langmuir equation (Section XVII-3A)—and should then be very large unless the gas pressure is small. Alternatively, the rate may be governed by boundary layer diffusion, a slower process in general. Such aspects are mentioned in Ref. 146. 

The Z-trajectory ensures excellent separation of ions from neutral molecules at atmospheric pressure. In line-of-sight or conventional electrospray sources, the skimmer is soon blocked by ions and molecules sticking around the edges of the orifice. In Z-spray sources, the final skimmer, being set off to one side, is not subjected to this buildup of material. 

FIOR Process. In the FIOR process, shown in Figure 5, sized iron ore fines (0.04—12 mm) are dried in a gas-fired rotary dryer. A skip hoist dehvers the dry fines to lock hoppers for pressurizing. The fines pass through four fluidized-bed reactors in series. Reactor 1 preheats the ore to 760°C in a nonreducing atmosphere. Reactors 2, 3, and 4 reduce the ore at 690—780°C. At higher (ca 810°C) temperatures there is a tendency for the beds to defluidize as a result of sticking or hogging of the reduced material. 

CeUulosic fibers, powdered limestone, gHsonite, and asphalt are frequently added to both water and oH muds at levels of 10 to 25 kg/m (4—10 lb /bbl) when high differential pressures are encountered to control seepage losses to the formation. This treatment also is used to improve the quaHty of the mud filter cake to reduce the chance of differential pressure sticking. 

The melt flows from the extmder iato the die where it flows around the bend and around the core tube. On the far side of the core tube, it forms a weld. Melt sticks to and is pulled by the moving wire. Details of the sizes and shapes of the die parts ia contact with the melt are important ia obtaining a smooth coating at high rates. The die exit usually is the same diameter as that of the coated wire and there is Httle drawdown. Die openings are small and pressures iaside the die are high at ca 35 MPa (5000 psi). Wire takeup systems operate as high as 2000 m /min. 

For fine powders that tend to bridge or stick and are of low bulk density, some form of forced feed, such as the tapered screw feeder shown in Figure 9, must be used to deaerate, precompact, and pressurize the feed into the nip. Large machines are available with up to five screw feeders to spread the flow across the roUs, and vacuum hoppers are also used to remove air when densifying low density feeds. 

Methods for determining fiber-to-fiber friction have been developed (29—31). The friction coefficient can also be measured in terms of the force required to pull entwined fibers apart (32—34) or the force necessary to remove a single fiber from a mass of fibers under pressure (35). Another test involves an apparatus wherein one or a series of parallel fibers are mounted across a small bridge similar to a violin bridge. This is then pressed against a surface that may be another fiber or some other material, and the fibers alternately sHp and stick as they sHde across each other (36,37). 

Preflmdizing has the advantage of reducing the volume of conveying air needed consequently, less power is required. The characteristics of the rest of this system are similar to those of regular pressure-or vacuum-type conveyors. Of special concern is the tendency of material to stick to and build up on surfaces of the system compo-... 

Solvent-borne CR adhesives allow almost any two substrates to stick together. The adhesives are extensively used in bonding high-pressure plastic laminates, automotive adhesives, adhesives for construction and shoe adhesives. 

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