Films surface spreading force

Fig. 14 shows the comparison of the photographs from Chandra and Avedisian (1991) with simulated images of this study for a subcooled 1.5 mm n-heptane droplet impact onto a stainless-steel surface of 200 °C. The impact velocity is 93 cm/s, which gives a Weber number of 43 and a Reynolds number of 2300. The initial temperature of the droplet is room temperature (20 °C). In Fig. 14, it can be seen that the evolution of droplet shapes are well simulated by the computation. In the first 2.5 ms of the impact (frames 1-2), the droplet spreads out right after the impact, and a disk-like shape liquid film is formed on the surface. After the droplet reaches the maximum diameter at about 2.1ms, the liquid film starts to retreat back to its center (frame 2 and 3) due to the surface-tension force induced from the periphery of the droplet. Beyond 6.0 ms, the droplet continues to recoil and forms an upward flow in the center of the... 

At room temperature the value of Em for dilute Na films varied from 5.8 to 6.9 kcal./g. atom. For K films, Em fell with increasing surface concentration from 16 kcal./g. atom at a very low coverage to 6.7 kcal./g. atom for a surface concentration of 4.8 X lO atoms/cm., and the fall in Em in this case was attributed to the existence of a spreading force due to the mutual repulsion of adions. 

A two-dimensional analog of PV work can be recognized in fluid films that exhibit surface tension (tendency of the film surface to contract against an opposing spreading force). The surface tension work wsurf (of, for example, a soap film) can be measured by a rectangular wire-frame device with moveable edge, as shown in Fig. 3.5. 

The surface pressure, or outward spreading force per centimetre, of the interfacial gaseous film at the lower interface of the duplex film would be... 

Fig. 9. The rate of attack of permanganate on the double bonds of oleic acid is less when n is high enough to force some of them off the aqueous surfaces (Hughes and Rideal, 15). The oleic acid film is spread on 0.003% permanganate in N/lOO H2SO4.

A shallow tray, the Langmuir tray, is filled to the brim with water (Fig. 18.13). The film is spread in the area between the float and the barrier by adding a drop of a dilute solution of stearic acid in benzene. The benzene evaporates leaving the stearic acid on the surface. The float is attached rigidly to a superstructure that allows any lateral force, indicated by the arrow, to be measured by means of a torsion wire. 

The study of surface films of the Langmuir type covers an extremely diverse group of phenomena. Measurements of film viscosity, diffusion on the surface, diffusion through the surface film, surface potentials, the spreading of monolayers, and chemical reactions in monolayers are just a few of the topics that have been studied. One interesting application is the use of long-chain alcohols to retard evaporation from reservoirs and thus conserve water. The phrase to pour oil on the troubled waters reflects the ability of a mono-molecular film to damp out ripples, apparently by distributing the force of the wind more evenly. There are also several different types of surface films only the simplest was discussed in this section. 

The ultimate equilibrium would be that of a drop or film of liquid that spreads until it thins to a monomolecular layer. Actually, a quasi-static equilibrium will be reached when the viscous forces exceed the gravitational and surface tension forces and the thinning of the film becomes imperceptibly slow. However, as discussed below, there are surface chemical and capillary forces acting at the three phase boundary that cause liquids to diffuse from the edge of a drop or a film. There is a school of thought and some experimental evidence that when a liquid film becomes very thin its chemical characteristics become different than that of the bulk liquid because of orientation effects induced by the solid substrate. Derjaguin strong advocate... 

Figure 4.11b. Duplex film spreading is assumed. The total spreading force at the perimeter of the oil drop is assnmed to be 2%r S It is also assumed that becomes zero at the edge of the bubble in the plane of the liqnid surface so that a linear gradient in exists with respect to radial distance from the antifoam drop. The shear stress X is therefore crndely approximated as...

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. 

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