Bernd 



in practice. Moreover, foreign materials in liquid form were found not to cause 

 cavitation where a similar material in solid form did. Thus a hydrocarbon oil 

 did not limit tensile strength, whereas a hydrocarbon solid surface did. 



A few seaweed samples in sea water were tested. The weakest caused 

 cavitation at a 16.2-psi tensile stress; others were appreciably higher. The 

 cause of rupture appeared to be gas in pores on the surface of the seaweed. 

 Thus it may be necessary to prevent marine growths from accumulating in 

 order to maintain a "high- strength" surface. 



In conclusion, high purity, or removal of organic or inorganic debris, were 

 found not to be necessary for high tensile strength.* It was found impossible to 

 cavitate a sea-water sample in a glass container. The sample had previously 

 stood long enough so that gas nuclei were not present to weaken the water. 



GAS NUCLEI AND SURFACE FILMS 



Gas nuclei are generally invisible to the unaided eye. Because of this, their 

 existence is largely an abstraction. Their presence is inferred from acoustic 

 absorption measurements, or the increase in tensile strength obtained by apply- 

 ing pressure to dissolve the nuclei or by allowing water to stand so that nuclei 

 rise to the surface. Investigations associated with these approaches are Harvey 

 et al. (4), Knapp (5), Strassberg (6), and Iyengar and Richardson (7). In an in- 

 vestigation of nuclei action, one must be certain that nuclei are the sole variable 

 under observation. 



Because of their small size, and the internal pressure created as a con- 

 sequence of surface tension, gas nuclei should dissolve within a few minutes in 

 still water (8), but paradoxically they do not appear to do so. It has been sug- 

 gested that this is due to organic surface films forming a shell about the gas 

 nuclei (9), creating a barrier that prevents diffusion of the gas from the high- 

 pressure area within the bubble into the surrounding water. To determine the 

 extent to which dissolving might thus be retarded, different waters were cavi- 

 tated to create gas nuclei; and the behavior of the nuclei was followed by meas- 

 uring the tensile strength of the water. 



Tensile Strength 



The tensile strength of water containing nuclei is 



4a 



P = 



D X 69,000 



where o is the effective surface tension (dynes/cm), D is the diameter of the 

 nucleus (cm), and P is the tensile strength (psi). If no surface films are 



♦The above tests were done at room temperature. No measurements were made 

 close to O'C, where Briggs found the inherent tensile strength of water to be 

 300 psi as contrasted to a maximum of about 4000 psi at 10°C (27). Briggs' 

 measurements were made in a glass tube, presumably without nuclei present. 



88 



