Soap films black spots

The physicochemical properties of foam and foam films have attracted scientific interest as far back as a hundred years ago though some investigations of soap foams were carried out in the seventeen century. Some foam forming recipes must have been known even earlier. The foundations of the research on foam films and foams have been laid by such prominent scientists as Hook, Newton, Kelvin and Gibbs. Hook s and Newton s works contain original observations on black spots in soap films. 

The black spots on soap films, which are not more than 10 to 20 molecules thick, can remain for weeks in equilibrium with the thicker, coloured parts of the film,4 and hence it is assumed that they have the same vapour pressure as the normal liquid, and that Thomson s formula can be applied for a radius of curvature of 200 x 10 cm. or less. Bakker<5 gave reasons for supposing that the surface tension is independent of the radius of curvature of the capillary layer, although he recognised that in very thin films it has abnormal values, and he calculated that the maximum ascent of a liquid occurs in a tube of 2 5 m[jL radius. Woodland and Mack found no change of surface tension in a tube of 6 7 [I radius. 

Bingham s viscosity equation, 117 black spots on soap films, 373 blending factor, 119... 

The phenomena described above have been known for a long time indeed, Newton reported on the black spots in soap films. In the past 25 years, however, these thin, liquid structures have become a subject of intensive scientific studies. One of the main reasons is that the interaction forces between colloidal particles suspended in a liquid are of the same nature as those operating in soap films. Because the film geometry is well defined (i.e., a thin, flat liquid sheet, macroscopic in lateral extension), it is an attractive experimental subject for studying these forces, in particular with optical means. 

It was found that up to 0.004% concentration of the iron soap in n-decane, which corresponds to the formation of a saturated adsorption layer at the interface, black spots are formed. However, these black spots do not produce a black film and interfacial film is broken quickly. Soap concentrations of up to 0.1% produce a film whose thinning-out stops when a thick stable grey film is created. Similar results have been observed for films stabilized by aluminum soap. During the formation of the film, monochromatic light showed alternative dark and bright bands corresponding to the interference maxima and minima. By measuring the parameters of the latter, the film thickness could be estimated. 

In our work we also compared stabilities of model emulsions with those of thin hydrocarbon films. Soap concentrations corresponding to black spot formation did not lead to stable emulsions. To obtain them, concentrations much higher than those at which stable films are formed are necessary. This is due to the fact that during emulsification a sharp increase of the interfacial adsorption surface occurs, which results in considerably lower concentration of surfactant in solution. 

AU physicists know that Newton used soap bubbles in his admirable research on the colors of thin films. The experiments that he carried out by this means, and which are described in his Optics (year 1704), are too well known for me to point them out here I will insist only on the following points Newton employed, not complete bubbles, but film caps resting on liquid he observed the black spot at the top, the small colored spots which go up and go down on the cap, as well as the little black spots which climb up to the top spot, with which they merge he noted the appearance of the blue of the 1st order only with one solution very charged with soap, and, in this case, he sometimes saw the blue in question invading aU the cap finally, one can infer from his description that the uniformity of color, and consequently the uiuformity thickness of the film, were shown sometimes also for colors other than the blue of the 1st order. 

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