Bernd 



wide variations in the amounts of trace materials carried in water, and differ- 

 ences in the characteristics of these materials. 



Yet, the conclusion that a double film structure exists in nature implies 

 that almost any surfactant material is capable of forming monomolecular films 

 that are effective in retarding dissolving, provided that a second film forms 

 about the monomolecular film. The second film should be producible from a 

 wide variety of ionic/polar structural building materials such as gelatins, 

 starches, and sugars. Hence, the ability to form effective surface films about 

 nuclei could be widespread — as also appears to be the case. By comparison, 

 the surfactant hydrocarbon materials used in this investigation for monomolec- 

 ular film formation were specially chosen to be capable of close packing — a 

 category that may not necessarily be widely found in nature. 



The testing of water from various water tunnel test facilities showed a 

 relative absence of film formation. As a consequence, when small nuclei are 

 produced by cavitation action, tensile strength probably can result in the water 

 approaching a test section. The characteristics of the body being cavitation 

 tested — as well as the attendant flow conditions — should markedly affect the 

 size of the nuclei produced. Testing for incipient cavitation should produce 

 small cavitation voids with small nuclei as the end product. Heavy cavitation 

 and long dwell times in a low pressure region should produce large nuclei. 

 Thus one is left with the conclusion that nuclei action in a tunnel must be cor- 

 related to the type of test at hand, as well as the film-forming tendencies of the 

 water. 



The preceding discussion points out the gross variability of surface film 

 effects, whereas one would like to be in a position to control or limit them. 

 Several approaches might be possible in a water tunnel, such as: (a) marking 

 off an undesirable region of operation to be avoided, (b) having uniform effects 

 that can be corrected for, (c) using film-forming additives to retard dissolving, 

 and (d) deliberately injecting suitable nuclei. Using film-forming materials 

 would require periodic replenishment of the materials. Depending upon the 

 materials chosen, one can probably expect some spoilage due to bacteria, some 

 breakdown of the molecules due to cavitation forces, and, for a protein, insolu- 

 bility causing withdrawal from action. Also, fatty acids, acting as a soap, can 

 combine with minerals in the water. 



REFERENCES 



1. Olson, "Cavitation Testing in Water Tunnels," St. Anthony Falls Hydraulic 

 Lab Project Report 42, Dec. 1954 



2. Crump, "Determination of Critical Pressures for the Inception of Cavitation 

 in Fresh and Sea Water as Influenced by the Air Content of the Water," 

 DTMB Report 575, Oct. 1949 



3. Langmuir, "Molecular Layers," Proc. Royal Soc. London A 170 (1939) 



106 



