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



present at the interface between gas and water, the surface tension acting is 

 that of water, 73 dynes/cm. The surface tension acts to pressurize the gas 

 within the nucleus and also to oppose the growth of the nucleus when a tensile 

 stress is applied to the water. When the tensile stress in the water is greater 

 than the pressure within the nucleus, the nucleus expands and the water is 

 ruptured to cause cavitation. The equilibrium stress for the incidence of rup- 

 ture (versus nucleus size) is calculated to be as follows: 



Table 6 



Limiting Tensile Strength of Water Due to Nuclei 



(Surface Films Absent) 



Although one cannot precisely relate tensile strength to nuclei size because the 

 surface tension is not constant when a surface film is present, the important 

 effect— the presence of tensile strength — can be measured directly.* 



Test Technique for Observing the Effect 

 of Surface Films 



The test setup for observing the effect of surface films on gas nuclei dis- 

 solving is shown schematically in Fig. 4a. The test consists of cavitating water 

 to create nuclei, then measuring the tensile strength after a given period of time 

 has elapsed. To assure that the water under observation is the weak link, glass - 

 housed acoustic transducers are used both for cavitating and subsequent stress- 

 ing of the sample to rupture. The tank containing the water is also of glass. 



*Rectified diffusion (the growth of a bubble in a sound field because a larger 

 surface area is present during the negative part of the pressure wave than dur- 

 ing the positive part (26)) was not in evidence where one could compare bubble 

 sizes determined by tensile strength measurements versus sizes obtained by 

 rise in accordance with Stokes law. Rectified diffusion should tend to decrease 

 the tensile strength of the water. The implication is that heavy surface films 

 make nuclei behave as solid spheres when rising. Also, the surface of a nucleus 

 when ruptured must have an effective surface tension equal to that of water. 

 This would apparently present a picture of a surface film that is rigid and 

 strong under compression but that can rupture readily under tension, leaving 

 "ice floes" of material held together by the surface tension of water. This is 

 not inconceivable from what is known of monomolecular surface films. 



89 



