Popular Science Monthly/Volume 11/September 1877/On Drops
By A. M. WORTHINGTON.
AMONG the many ways in which electricity is called in to give assistance in various physical investigations, one of the most elegant and interesting is the application of the electric spark to render momentarily visible a body that is rapidly moving or changing its form. The duration of the electric spark is so short—probably not more than 1⁄24000 of a second—that a body, such as a rotating wheel or oscillating rod, moving in a dark room with extreme rapidity, will, if illumined by an electric spark, seem stationary, since the wheel or rod has not time to change its position appreciably during the short instant for which it is visible. If the spark be bright, the impression is left on the eye long enough for the attention to be directed to it, and for a clear idea to be formed of what has been seen.
The writer of this article has recently applied this method to watching the changes of form in drops of various liquids falling vertically on a horizontal plate. As usually seen, a drop of water falling
from a height of ten or twelve inches on a smooth solid substance, such as glass or wood, seems to make an indiscriminate splash. The whole splash takes place so quickly that the eye cannot follow the changes of form; the impression made by the last part of the splash succeeding that of the first part so quickly as to confuse it.
A little careful observation, however, shows that the drop passes through very definite symmetrical forms, and that a splash is by no means an irregular, haphazard phenomenon.
Let the reader let fall a few drops of milk, about 1⁄4 inch in diameter, on a smooth dark surface of wood or paper, from a height of, say, six inches (milk is better than water, as it is easier to see, especially on a dark ground); he will observe that the liquid makes a blot with a more or less regular undulated edge, but the splash is too quick to follow with the eye.
Let him now substitute a drop of mercury for the milk. By watching the splash very intently he will be able to catch a glimpse of the mercury spread out in the symmetrical, star-like form of e, Fig. 9. After the drop has been thus spread out it recovers its globular form, since the mercury does not wet the plate. On increasing the height of fall a few inches, it will be noticed that small drops split off in a more or less complete circle, and are left lying on the plate, while the rest of the drop gathers itself together in the middle of the circle.
The chief reason why these appearances could not be seen with milk is, that the milk wets the glass or wood and sticks to it, while the mercury does not. But by smoking a slip of glass or card tolerably thickly in the flame of a candle, we get a finely-divided surface of lampblack to which the milk does not adhere any more than the mercury,
and by very careful watching we may notice that the same radial star is formed by the milk, but it is much more difficult to catch sight of than the mercury-star. But if the mark on the lamp-black be examined after the drop of milk or mercury has rolled away, it will be found to consist of delicate concentric rings with less fine radial striæ where the smoke has been swept away. These may be seen very well by holding the glass plate up to the light if it has not been too thickly smoked.
The marks thus made are very beautiful and. symmetrical, and it will be found, if the glass be uniformly smoked, that the same-sized drops of the same liquid falling from the same height will produce almost exactly similar marks: while if the height be changed the mark on the lampblack will be somewhat changed; and it is a fair inference, if each drop makes almost exactly the same complicated, symmetrical mark, that the splash of each drop takes place in almost exactly the same way.
The glimpse that may be caught of the drop in the way described is obtained when the drop is really almost stationary, having flattened itself out on the plate, and being on the point of contracting again to its original form.
That a drop if so flattened out will recover itself, is seen on pressing down a drop of mercury with the finger, or a drop of water with a piece of black-lead or other substance to which it does not adhere.
On removing the pressure the drop springs back to its old form; the force which causes this being exerted by the curved surface of the liquid at the edge of the flattened drop, on the liquid within. The flatter the drop becomes the greater is the curvature of the edge, and the greater the corresponding pressure tending to restore it to its original globular form. The extent to which a drop that has fallen on a plate will spread out depends on the velocity with which it strikes the plate, i. e., on the height of fall; so that as long as the drop returns to the globular form the whole phenomenon of the splash may be regarded as an oscillation similar to that of a pendulum; the velocity of the liquid outward being checked, overcome, and finally reversed by the ever-increasing pressure of the curved edge, just as a pendulum has its velocity checked, overcome, and finally reversed, by the action of gravity.
It is only when the height of fall is very great that the liquid flies off in all directions and the splash ceases to be an oscillation; this case corresponds to that of a simple pendulum started with a blow so violent as to break the string.
But the liquid star and the complicated pattern on the smoked glass show that the splash is not a simple spreading out of the drop equally in all directions, to return again.
In order to observe the form of the drop at any given instant during the splash, it is necessary to make use of the electric spark, and to take advantage of the fact that drops of the same size falling from the same height will all behave in the same way.
It will be necessary to let a drop, say of mercury, fall on a plate in comparative darkness, and to produce a strong spark at the instant the bottom of the drop comes in contact with the plate, and so illumine it; the observer will then see the drop in the form it has at that instant.
A second drop must be let fall in the same way, and be illumined by the spark not at the first moment of contact, but a shade later, say 100 second later, when the drop will have spread itself out slightly on the plate; and similarly we must illuminate a third drop a shade later than the second, and so on. The observer can, after a little practice, draw from memory on each occasion the drop in the form in which he has seen it. It will be seen that the process consists in isolating consecutive phases of the splash from those that precede and follow, and which take place in darkness, and so do not confuse what has been seen as they would do in continuous daylight.
The device adopted by the writer for so timing the appearance of the spark as to illumine the drop at any desired phase of the splash consisted essentially in breaking the current of an electro-magnet at the instant the drop began to fall, the magnet, thus ceasing to act, releases a spring which immediately begins to pull the terminal wire of a strong electric current out of the other terminal, which is a cup of mercury, and the strength of the spring and the depth of immersion of the wire in the mercury are so adjusted that the wire leaves the surface of the mercury, and the required spark is produced at the instant the drop reaches the plate.
For the next drop the spark is made to appear a shade later, either by slackening the spring or increasing the depth to which the terminal wire is immersed in the mercury.
The accompanying figures have been drawn in the way described, and show the behavior of a drop of mercury about 1⁄4 inch in diameter, falling from a height of about three inches on to a glass plate. Each figure represents a rather later stage of the splash than the preceding.
Fig. 9 was drawn from the final stages of a milk-drop 1⁄4 inch in diameter, falling four inches on to smoked glass; but the forms are almost identical with those of mercury. Of these d and h are vertical central sections of the middle part of the drop, while e and f are alternative forms of h and c.
From the ends of the rays of Fig. 4, usually twenty-four in number, small drops often split off. These are not shown in the figure. One of the most curious features of the phenomenon is the transition from twenty-four rays to twelve arms, shown in Fig. 5, The beauty of many of the forms, especially of the ridged, shell-like form shown in Fig. 4, when composed of shining quicksilver apparently rigidly fixed, is very striking. Very similar forms are obtained with milk, but, whether with milk or mercury, are liable to occasional variations. For a more detailed account the reader is referred to the "Proceedings of the Royal Society," Nos. 174 and 177, 1876-'77.—Nature.