Bernd 



needed to initiate cavitation. I am sure that many, including myself, have asked 

 the same question. However, tests conducted by myself, as well as others, 

 force one to conclude that stabilized nuclei do exist. The problem is to obtain 

 a believable physical model. 



It is reasonable to expect a nucleus to acquire a monomolecular surface 

 film. The characteristics of surface films observed away from a nucleus imply 

 that the necessary properties for a stabilized nucleus capable of initiating cavi- 

 tation are obtainable by a surface film at a nucleus. For example, investigations 

 on the behavior of surface films at a gas/water interface have shown that some 

 monomolecular films can be highly compressed (in the order of 50:1) as the 

 interface is contracted, and then subsequently re- expand to their original con- 

 dition as the interface is expanded. Some surface films may however change 

 their physical structure upon compression, possibly acquire a permanent set, 

 slowly redissolve in the water, or yield slowly in a viscous fashion. Thus hys- 

 teresis may be exhibited for various reasons. In some cases, a slight expan- 

 sion may reduce the compressive stress to zero and cause rupture in tension. 

 Langmuir was the original worker in this field. 



Furthermore, because of the spherical shape of the nucleus, the surface 

 film should be stressed efficiently (good use of material) before failure in com- 

 pression can occur. Also, the spherical form is a stable shape; it prevents 

 failure due to buckling of the film. 



It is therefore reasonable to suppose that a film can be sufficiently com- 

 pressed to (a) greatly decrease the transport of gas because of the reduced 

 spacing between molecules, as well as (b) exert a compressive stress that 

 prevents the internal gas pressure necessary for dissolving from being 

 developed. 



If the stabilized nucleus enters a low-pressure area, the gas will attempt to 

 expand. The surface film may also elastically attempt to expand. The surface 

 film will then be expanded until essentially the surface tension of the water alone 

 acts to contract the nucleus. The protective "shell" no longer acts. It has 

 either been ruptured upon expansion, or it has expanded until it is ineffectual. 

 Thus the gas nucleus should be capable of expanding to form a cavitation void 

 or acquire additional gas in the form of water vapor or dissolved gas. 



114 



