Cavitation, Tensile Strength, and the Surface Films of Gas Nuclei 



specifies a minimum molecular weight. Here again, there should also be a limit 

 to the maximum molecular weight suitable. The ability of a molecule to diffuse 

 and be transported to the surface decreases as the molecule weight increases. 

 In addition, the molecule may become too cumbersome to align readily at the 

 surface. Thus, for a given type of molecule, there should be a maximum molec- 

 ular weight beyond which film formation cannot take place readily. 



3. Mechanical Properties. It would seem desirable that a surface film be 

 strong and highly elastic, so that the film in its early stages can follow the mo- 

 tions of a cavitation void or bubble surface without being destroyed. Also, 

 strength would be desirable to prevent the crushing postulated by Fox and 

 Herzfeld as the end of the life of a nucleus. Thus, different mechanical prop- 

 erties should affect nuclei performance. One would expect a thicker hydrocar- 

 bon film (higher molecular weight) to be stronger in compression and more 

 rigid. 



4. Barrier to Gas Diffusion. A thicker film offers a greater resistance to 

 gas diffusion (16). Thus, for a given type of a hydrocarbon film, increasing the 

 molecular weight decreases the permeability of the film. 



In a hydrocarbon, most of the above properties are related to molecular 

 weight. Thus finding an effective film should simplify to determining the effects 

 of molecular weight on nuclei dissolving. 



Hydrocarbons — Hydrocarbon films, if they could be formed about nuclei, 

 appear suitable in many ways. That is, certain types of hydrocarbon materials 

 are used to form monomolecular films on the surface of water which act as bar- 

 riers to the diffusion of water vapor. They also reduce tension and produce 

 solidlike films upon compression. These hydrocarbons are long chain linear 

 alcohols, fatty acids, and amines (14,15). These materials are readily obtain- 

 able. Hence they would be of interest if found to be usable in a cavitation test. 

 They would also provide a standard of known molecular composition to compare 

 against a protein. 



However, hydrocarbons are generally insoluble. As a consequence, hydro- 

 carbon films are not spread from a water solution; other techniques are used. 

 Nevertheless, nuclei can undergo a large surface contraction when dissolving. 

 This should markedly increase the concentration of hydrocarbons at the surface 

 even if ordinarily only present in nominal quantities. 



Proteins — The available evidence does point to proteins as being impor- 

 tant in the creation of surface films. That is, protein films meet the require- 

 ments of reducing surface tension and forming a solidlike film upon compres- 

 sion (12,13), Although the majority of proteins are insoluble in water, some 

 proteins (egg albumin for instance (18,19)) are slightly soluble and do form 

 monomolecular surface films by coming out of solution. These films form 

 fairly rapidly, and are strong, elastic, and durable to a greater degree than a 

 hydrocarbon film. Film possessing characteristics typical of proteins (as well 

 as hydrocarbons) have been noted in streams. That is, films are formed that, 

 upon compression, whiten, fold, and become insoluble (20). And, as previously 

 mentioned, water capable of producing surface films ceased to do so after 



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