Spreading forced

In printing, a film of ink is formed by wetting the surface with the compression force of the rollers. This force spreads the ink over the surface and into any capillaries that may be present. Spreading and penetration are controlled thermodynamically and kinetically. Measurement of the contact angle can be used to determine the thermodynamics of wetting. This angle can be used also to determine the contribution that polarity and dispersive forces of the liquid make to the wetting of the surface. 

Feed material enters the inlet [2] and flows onto a distributor plate [5], which is part of the rotor assembly. The initiation of feed to the WFE occurs at the same time that the drive and motor [1] are started. As the rotor unit turns, centrifugal force spreads the feed from the distributor plate onto the heated wall of the WFE. Volatile components, such as glycerine, are rapidly evaporated. Slotted wiper blades [8] connected to the rotor evenly distribute the feed material into a uniform, agitated, thin film and continuously move material down the heated wall, including highly viscous materials. 

The wetting behavior of polymers is reviewed beginning with the thermodynamic conditions for contact angle equilibrium. The critical surface tension of polymers is discussed followed by some of the current theories of wettability, notably the theory of fractional polarity and theories of contact angle hysteresis. The nonequilibrium spontaneous and forced spreading of polymer liquids is reviewed from two points of view, the surface chemical perspective and the hydrodynamic perspective. There is a wide di.sperity between these two viewpoints that needs to be resolved inorder to establish the predictive relations that govern spreading behavior. 

This review will present the basic surface chemical and rheological principles involved in the spreading of polymers. First, the case of static drops or films of liquids in equilibrium on solid surfaces is discussed. Secondly, we discuss systems where the solid/liquid/vapor interfacial forces are not in equilibrium and the liquid is free to spread spontaneously. Finally, we deal as best we can with the much more difficult problem of a liquid being spread by an external force, i.e., forced spreading. 

Before starting, it is important to set some definitions. We assume that in a gravitational field when a liquid contacts a solid, some interfacial area is formed i.e., the liquid wets the solid. In the next section, we present a definition of wettability — the contact angle. In contrast, the term spreading refers to the motion of a liquid film over a solid either by spontaneous spreading or by forced spreading. 

A spigot deposits a precise amount of photoresist on the wafer surface. The wafer is then spun so that centrifugal force spreads the liquid over the surface at an even thickness. This operation takes place on every layer that is modified by a photolithographic procedure called masking. 

The thickness of the wetting liquid film (which has sometimes been called a pancake) results from a competition between the capillary forces (spreading... 

A thin film of hydrocarbon spread on a horizontal surface of quartz will experience a negative dispersion interaction. Treating these as 1 = quartz, 2 = n-decane, 3 = vacuum, determine the Hamaker constant A123 for the interaction. Balance the negative dispersion force (nonretarded) against the gravitational force to find the equilibrium film thickness. 

Apart from the techniques described in this chapter other methods of organic film fonnation are vacuum deposition or film fonnation by allowing a melt or a solution of the material to spread on the substrate and subsequently to solidify. Vacuum deposition is limited to molecules with a sufficiently high vapour pressure while a prerequisite for the latter is an even spreading of the solution or melt over the substrate, which depends on the nature of the intennolecular forces. This subject is of general relevance to the fonnation of organic films. 

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. 

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

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