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



3. After a brief period of time, this rapid dissolving may be abruptly halted 

 (or greatly slowed down) by surface films. The size at which this change takes 

 place is highly dependent upon the initial size of nucleus. A small nucleus may 

 dissolve rapidly to obtain high tensile strength, whereas a large nucleus is 

 arrested from its rapid dissolving before high tensile strength can be developed. 

 The tensile strength obtained in the comparative tests are approximately in 

 accordance with the nuclei sizes obtained in our tests when surface films acted 

 to produce the above mentioned slow dissolving. 



4. The subsequent slow dissolving of a nucleus surrounded by a surface 

 film remains as a task requiring investigation. If the surface film retards dis- 

 solving primarily by acting as a barrier to gas diffusion, then for a given set of 

 conditions the time rate of gas transport should be a value sensibly independent 

 of water speed. If the surface film prevents dissolving primarily because it is 

 stressed in compression as the surface area is reduced, and this compression 

 opposes the contraction of the gas nucleus, then the time rate of transport should 

 be proportional to the water speed. 



Thus one expects tensile strength, as produced by the reduction in size of a 

 nucleus caused by dissolving, to be approximately independent of water speed, 

 or possibly to increase somewhat with decreasing water speed. 



TEFLON SURFACE TO CAUSE BOILING 

 OR CAVITATION 



Dr. Hummel notes that the Teflon coatings he used to promote boiling 

 worked well for an indefinite period of time when fresh but stopped after an 

 hour when old. It was our experience that certain rubberlike, castable poly- 

 mers cavitated profusely when freshly cast, but did not do so after considerable 

 drying. The presence of volatile solvents was blamed for this action. 



The rapid exhaustion of a Teflon coating in causing cavitation, as shown in 

 Fig. 2b, was obtained with an old Teflon coating (by Dr. Hummel's definition). 

 The ability to cavitate was lost after 2 hours of cavitation. I cannot comment on 

 the role of age as it may affect a Teflon surface, but I would suggest that the 

 gas content of the Teflon be measured and the surface nucleation sites be 

 counted. These had been found to be the primary factors causing cavitation 

 originating from polymers. As an example. Teflon sheet stock (virtually no 

 nucleation sites) did not cavitate, whereas a Teflon coating (many nucleation 

 sites) cavitated as mentioned above. Both Teflon samples had the same gas 

 content. 



THE MECHANISM OF A STABILIZED 

 GAS NUCLEUS 



Dr. Hummel questions the hypothesis of a stabilized, long-life gas nucleus 

 consisting of gas enclosed by a protective, relatively impervious organic 

 "shell." Reasonably he asks how a gas nucleus can be surrounded by a shell 

 and yet subsequently act to produce cavitation when a gas/water interface is 



113 



