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



conclude that surface films must be acting for all slower dissolving waters. 

 Concentrated nitric acid was therefore tested to provide a standard for ideal 

 dissolving, since it is inconceivable that the conditions required for surface 

 film formation can exist. Nitric acid is highly oxidizing and should break down 

 organic materials to simple molecules and ions, whereas relatively long, high- 

 molecular -weight surfactant molecules are necessary for film formation. Cor- 

 rections were made for differences in diffusion coefficients, viscosity, and sur- 

 face tension where necessary. 



Some results are given (Fig. 5) to show the range of nuclei dissolving en- 

 countered. Three general categories of water were identified, covering 

 approximately a 100:1 variation in the time required to reach appreciable 

 tensile strength. They were: 



Rapid tensile strength gain, typified by the artificial case of no surface 

 films obtained with nitric acid (5); approximating ideal dissolving by diffusion 

 through the water surrounding a nucleus, as solved by Epstein and Plesset (8).* 

 Distilled water dissolved almost as fast when care was taken not to contaminate 

 it. Waters of this type reached high tensile strengths (> 50 psi) in 6 to 7 

 minutes. 



A slower strength gain for natural, "prototype" waters, typified by pure 

 lake water (2). Distilled water contaminated by moderate exposure to air fell 

 on this curve (la). The sea water sample tested was somewhat similar. These 

 waters continued to dissolve for 5 to 6 hours after cavitation, slowing down 



TIME ( HOURS) 



Fig. 5 - Summary of the trends of 

 tensile strength after cavitation 



*However, at high tensile strengths close to 100 psi, dissolving did slow down 

 somewhat. The cause of this was not investigated, since it occurred at high 

 strengths beyond the range of major interest. 



91 



