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



The ability of a material to adhere to water should be determined by (a) the 

 strength of its molecular forces, (b) the fraction of these forces available at a 

 surface, and (c) the fraction of the surface forces that are actually able to pro- 

 vide a bond with the water. One might liken the situation to a riveted joint be- 

 tween metals. The strength of the joint is proportional to the basic strength of 

 the metal times a joint efficiency expressing the fraction of material that is 

 used effectively. 



The relative strength of molecular forces can be obtained in various ways, 

 for instance by comparing boiling points, melting points, heat of evaporation, 

 modulus of elasticity, etc. A readily available measure of the forces available 

 at a surface is surface tension or surface energy.* In similar types of mate- 

 rials, one would expect an increasing adhesion to water as the numerical values 

 of these properties increase. 



One can roughly categorize materials by their type of atomic bond: co- 

 valent or ionic, Covalent materials do not bond as well as ionic materials. 

 Langmuir showed this by a comparison of the heat of evaporation of alcohols 

 possessing different ratios of covalent (hydrocarbon) groups to ionic (OH) 

 groups in the alcohol molecule (3). Many materials are not strictly covalent or 

 ionic but a simultaneous combination of the two. Water is highly ionic (polar), 

 and bonds to adjacent molecules via hydrogen bonds. Thus there should be an 

 increasing adhesion to water as a material contains more ionic groups, or if the 

 groups themselves are more highly ionic. 



One would therefore expect the poorest adhesion to water to result with low- 

 surface -energy, covalent compounds. If the tensile strength of such a poor bond 

 were appreciable in magnitude, then tensile strength with water as a general 

 case should be readily obtainable, since all other materials would be likely to 

 exhibit higher tensile strengths. 



Teflon is the lowest-surface-energy material known (6 ergs/cm-^). Com- 

 mon with otherlow-surf ace -energy materials, its surface is slippery and diffi- 

 cult to cement or bond to. A common group of low-surface-energy, covalent 

 materials are waxes, paraffins, and similar hydrocarbons. Table 3 shows that 

 the lowest tensile strength obtained with covalent, low-surface-energy materials 

 is about 40 psi. This is therefore the minimum tensile strength to he expected 

 irrespective of the materials used. 



Tests on quite a few materials substantiated this. To give some examples. 

 Table 4 lists the tensile strengths obtained with metallic oxides. Higher tensile 

 strengths were generally obtained with increasing molecular forces. Table 5 

 lists tensile strengths obtained with various polymers. Increasing tensile 

 strength resulted as covalent compounds contained polar groups. 



♦ Surface energy (by definition) is the energy expended to create ideally a surface 

 where none existed before. For liquids, a related property is surface tension. 

 Water has a surface tension of 73 dynes /cm. This is numerically equal to the 

 surface energy of water. That is, 73 ergs would be expended to create a square 

 centimeter of water surface in contact with air, using water originating from 

 the interior of the liquid where no surface exists. 



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