Ellis 



consistently for this type of bubble generation. Figure 6 also indicates that 

 shock waves from liquid- solid impact occur, since the collapse could hardly be 

 called spherically symmetric enough to cause appreciable gas compression. 



The preliminary work just described on single bubble collapse in flow was 

 done by the author and Dr. B. J. S. Barnard just before both left for a year at 

 the University of Cambridge. Subsequent work done there in collaboration with 

 Dr. T. Brooke- Benjamin was on the collapse of single bubbles in the absence of 

 flow but with a new technique for bubble generation (30). In the author's opinion 

 this new method is much superior to spark generation for most experiments. 

 The basic idea was quite simple, but much time was spent in mastering the 

 technique required. The method was to generate a single hydrogen bubble by 

 electrolysis to act as a nucleus for the desired relatively large spherical cavity. 

 The hydrogen bubble, which is usually chosen to be about 10 -^ cm in radius, at 

 1/20 atmosphere pressure, is subjected to a tension wave in the liquid which 

 causes it to grow to typically 2 cm in radius, if desired. Thus, the volume dilu- 

 tion of the gas is of the order of 1 part in 10 ^°. Since the growth occurs in a 

 few milliseconds and the water must have been deaerated previously to remove 

 extraneous air nuclei, there is little chance for additional gas to diffuse into the 

 cavity. Under these conditions the bubble should contain less gas than the usual 

 flow- generated cavitation bubble in nondeaerated water. The gas content can be 

 increased, of course, and the amount present is known from the observed size of 

 the nucleus bubble at known pressure and temperature. Figure 7 shows a typical 

 bubble of about 1.5 inches diameter. A scale in the background with large divi- 

 sions of one inch may be seen. 



With such large controlled cavities available it became possible to conduct 

 experiments that shed new light on the modes of bubble collapse. Time was not 

 available for study of any but the no-flow behavior. However, the effects of 

 pressure gradient and solid wall proximity were observed. It was found that in 

 a gravity hydrostatic pressure gradient the collapse was nearly symmetric ex- 

 cept for a flat on the bottom (high-pressure side) forming very nearly at mini- 

 mum volume. Evidently this was the beginning of a vertical jet, since pictures 

 taken after collapse and during rebound exhibited the rather surprising behavior 

 evident in Fig. 8, which also appears in Ref . 30. A very thin jet has apparently 



Fig. 7 - An example of a spher- 

 ical bubble generated by the 

 tension technique 



144 



