Liquid Drops and Globules, Their Formation and Movements / Three lectures delivered to popular audiences

by Charles R. Darling

Formation of Falling Drops of Liquid.—We will now direct our attention to one of the most beautiful of natural phenomena—the growth and partition of a drop of liquid. Let us observe, by the aid of the lantern, this process in the case of water, falling in drops from the end of a glass tube. The flow of water is controlled by a tap, and you observe that the drop on the end gradually grows in size, then becomes narrower near the end of the tube, and breaks across at this narrow part, the separated drop falling to the ground. Another drop then grows and breaks away; [pg 25] but the process is so rapid that the details cannot be observed. None of you saw, for example, that each large drop after severance was followed by a small droplet, formed from the narrowed portion from which the main drop parted. But the small, secondary drop is always present, and is called, in honour of its discoverer, Plateau's spherule. Nor did any of you observe that the large drop, immediately after separation, became flattened at the top, nor were you able to notice the changing shape of the narrow portion. To show all these things it will be necessary to modify the experimental conditions.

Mr. H. G. Wells, in one of his short stories, describes the wonderful effects of a dose of a peculiarly potent drug, called by him the “Accelerator.” While its influence lasted, all the perceptions were speeded up to a remarkable degree, so that occurrences which normally appeared to be rapid seemed absurdly slow. A cyclist, for example, although travelling at his best pace, scarcely appeared to be making any movement; and a falling body looked as if it were stationary. Now if we could come into possession of some of this marvellous compound, and take the prescribed quantity, we should then be able to examine all that happens when a drop forms and falls at our leisure. But it is not necessary to resort to such means as this to render the process visible to the eye. We could, for example, take a number of photographs succeeding each other by very minute intervals of time—a kind of moving picture—from which the details might be gleaned by examining the individual photographs. This procedure, however, would be troublesome; and evidently [pg 26] the simplest plan, if it could be accomplished, would be to draw out the time taken by a drop in forming and falling. And our previous experiments indicate how this may be done, as we shall see when we have considered the forces at work on the escaping liquid.

A liquid issuing from a tube is pulled downwards by the force of gravitation, and therefore is always tending to fall. At first, when the drop is small, the action of gravity is overcome by the surface tension of the liquid; but as the drop grows in size and increases in weight, a point arrives at which the surface tension is overpowered. Then commences the formation of a neck, which grows narrower under the stretching force exerted by the weight of the drop, until rupture takes place. Now if we wish to make the process more gradual, it will be necessary to reduce the effect of gravity, as we cannot increase the surface tension. We have already seen how this may be done in connexion with liquid spheres—indeed, we were able to cancel the influence of gravity entirely, by surrounding the working liquid by a second liquid of exactly equal density. We require now, however, to allow the downward pull of the drop ultimately to overcome the surface tension, and we must therefore form the drop in a less dense liquid. If this surrounding liquid be only slightly less dense, we should be able to produce a very large drop; and if we make its growth slow we may observe the whole process of formation and separation with the unaided eye.

Fig. 13.—Apparatus for forming ascending or descending drops of liquids.

Now it so happens that we have to hand two liquids which, without any preparation, fulfil our requirements. Orthotoluidine, at temperatures below 75° F. [pg 27] or 24° C., is denser than water of equal temperature. At 75° F. their densities are identical; and as the ordinary temperature of a room lies between 60° and 70° F., water, under the prevailing conditions, will be slightly the less dense of the two, and will therefore form a suitable medium in which to form a large drop of orthotoluidine. I therefore run this red-coloured liquid into water from a funnel controlled by a tap ([Fig. 13]), and in order to make a large drop the end of the stem is widened to a diameter of 1½ inches. It is best, when starting, to place the end of the stem [pg 28] in contact with the surface of the water, as the first quantity of orthotoluidine which runs down then spreads over the surface and attaches itself to the rim of the widened end of the stem. The tap is regulated so that the liquid flows out slowly, and we may now watch the formation of the drop. At first it is nearly hemispherical in shape; gradually, as you see, it becomes more elongated; now the part near the top commences to narrow, forming a neck, which, under the growing weight of the lower portion, is stretched until it breaks, setting the large drop free ([Figs. 14 to 18]). And then follows the droplet; very small by comparison with the big drop, but plainly visible ([Figs. 19 and 20]). The graceful outline of the drop at all stages of the formation must appeal to all who possess an eye for beauty in form; free-flowing curves that no artist could surpass, changing continuously until the process is complete.

Slow as was the formation of this drop, it was still too rapid to enable you to trace the origin of the droplet. It came, as it always does come, from the drawn-out neck. When the large drop is severed, the mass of liquid clinging to the delivery-tube shrinks upwards, as the downward pull upon it is now relieved. The result of this shrinkage—which, as usual, reduces the area of surface to the minimum possible—is to cut off the elongated neck, at its upper part, thus leaving free a spindle-shaped column of liquid. This column immediately contracts, owing to its surface tension, until its surface is a minimum—that is, it becomes practically a sphere; and this constitutes the droplet. In a later experiment, in which the formation is slower still, and [pg 29] the liquid more viscous, the origin of the droplet will be plainly seen, and the correctness of the description verified. The recoil due to the liberation of the stretching force after rupture of the neck was visible on the top of the large drop, and also on the bottom of the portion of liquid which remained attached to the tube, both of which were momentarily flattened ([Figs. 19 and 20]) before assuming their final rounded shape. This is exactly what we should expect to happen if a filled skin of indiarubber were stretched until it gave way at the narrowest part.

Fig. 14.

As a variation on the two liquids just used, I now take the yellow liquid nitrobenzene, and run it into nitric acid (or other suitable medium) of specific gravity 1·2, and you observe the same sequence of events as in the previous experiment, even to the details. Very rapid photography shows that the breaking away of a drop of water from the end of a tube in air is in all [pg!30] [pg 31] respects identical with what we have just seen on a large scale.