Abstract
EVERY student of physics has observed the motion of bubbles in tubes. Which of them has not used a big bubble to show the little ones their duty in clearing out the air when filling a barometer tube? Who has not spent his time and patience in whisking a spirit thermometer to drive a bubble out of the column? Mr. Trouton has recently communicated to the Royal Society the result of some researches on this subject. He has studied the behaviour of big bubbles and of little ones, of bubbles in large and small tubes, of bubbles of air in a liquid, and of one liquid in another, of bubbles in heavy land in light liquids, of bubbles in liquids of various degrees of viscosity and with various degrees of surface tension at their surfaces. From this enumeration it is evident that the number of different magnitudes involved is very great, and at the start it seemed almost hopeless to disentangle the effects due to each. The first matter to observe was that, as in other cases of fluid motion, two cases must be distinguished. These are the cases of slow motion and of quick motion. When the motion is slow the viscosity of the liquid causes the flow to be very simple. It entirely stops all whirling and swirling, such as is seen in the water behind a boat. When the motion is quick, on the other hand, the flow is very complicated. Whirls and swirls are set up, and the resistance is increased, owing to the increased energy that has to be communicated to the whirling and swirling liquid for each centimetre that the bubble moves. The slow kind may be described as viscous flow, and the quick as turbulent flow. The most interesting point observed in connection with the turbulent flow was that it was sometimes possible to increase the rate of flow by increasing the viscosity. Increasing the viscosity of a liquid generally makes it flow more slowly, but in some critical cases the increase of viscosity may produce more effect in decreasing the turbulence than in increasing the viscous resistance, and the result is to, on the whole, reduce the combined resistance so that the bubble moves more rapidly in the liquid of greater viscosity. Another matter that was of interest was the question of the size of the bubbles, and how it affected their rate of motion. Very long bubbles moved all at nearly the same rate, but short bubbles had a great variety of rates. Very small bubbles ran along ever so fast, while ones only a little larger went very much more slowly. These latter blocked up the tube much in the same way that a crowd blocks its own egress through a doorway. However, bubbles a little larger seemed to have more sense, for they shape themselves into a sharpish head, with the result that they can make their way along the tube more rapidly than smaller ones. Those a little larger again take up a dumb-bell sort of shape, and block the tube, and go more slowly again, though not so slowly as the smaller blocking bubbles. A little larger go somewhat more rapidly again, but as the bubbles are made longer the differences between the rates of the quick and slow sizes become rapidly less and less until pretty soon all go at the same rate, no matter how long they are. This alternation of speeds is evidently connected with the ripples that are formed at the head of the bubble as it passes through the liquid, much as a stick moving through water makes a series of ripples upon the surface. If the ripples are so long that the bubble has a pointed head it goes fast, if it has a blunt head it goes slowly. These ripples are in some cases very marked. Mr. Trouton found that when a bubble of oil was allowed to rise through water with which one fifty-thousandth part of caustic soda was mixed the ripples became quite a feature of the figure of the bubble. They at first extended as a series of rings round it, which, however, soon coalesced into a spiral wave, when the bubble rose rapidly through the liquid with a sort of corkscrew motion. If the tube were inclined the ripples were only formed on the lower surface of the bubble, the top surface floating up against the containing tube, and the ripples then looked like the feet of a caterpillar walking up the tube. This is not the only case in which surface tension motions simulate muscular actions, and it is an important question whether some of these actions are similarities or simularities.
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On the Motion of Bubbles in Tubes. Nature 49, 351–352 (1894). https://doi.org/10.1038/049351a0
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DOI: https://doi.org/10.1038/049351a0