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

by Charles R. Darling

Figs. 21 and 22.—Automatically formed aniline drops, showing the formation of droplets from the neck.

And now as to the explanation of this curious performance. When the aniline reaches the surface, and spreads out, it cools by contact with the air more rapidly than the water below. As it cools, its density increases, and soon becomes greater than that of the water, in which it then attempts to sink. The forces of surface tension prevent the whole of the aniline from falling—the water surface can sustain a certain weight of the liquid—but the surplus weight cannot be held, and therefore breaks away. But when the detached drop reaches the bottom of the vessel, it is warmed up again; and when its temperature rises above that of equi-density it floats up to the top. And so the cycle of operations becomes continuous, owing to cooling taking place at the top and heating at the bottom.

Perpetual motion, you might suggest. Nothing of the kind. Perpetual motion means the continuous performance of work without any supply of energy; it does not mean merely continuous movement. A steam-engine works so long as it is provided with steam, and an electric motor so long as it is fed with electricity; but both stop when the supply of energy [pg 37] is withdrawn. So with our aniline drop, which derives its energy from the heat of the water, and which comes to rest immediately the temperature falls below 147° F. or 64° C. But in order that the process of separation and reunion may continue, the cooling at the top is quite as necessary as the heating at the bottom. Our aniline drop is in essence a heat-engine—although it does no external work—and like all heat-engines possesses a source from which heat is derived, and a sink into which heat at a lower temperature is rejected. We might, with certain stipulations, work out an indicator diagram for our liquid engine, but that would be straying too far from our present subject.

Automatic Drops of other Liquids.—Liquids which possess a low equi-density temperature with water do not form automatic drops like aniline, as the rate of cooling at the surface is too slow, and hence the floating mass of liquid does not attain a density in excess of that of the water beneath. Aceto-acetic ether, however, behaves like aniline, if the temperature of the water be maintained at about 170° F. (77° C.), but as this liquid is fairly soluble in hot water further quantities must be added during the progress of the experiment. Results equal to those obtained with aniline, however, may be secured by using nitrobenzene in nitric acid of specific gravity 1·2 at 59° F. (15° C.), the acid being heated to 185° F. (85° C.); and here you see the yellow drop performing its alternate ascents and descents exactly as in the case of aniline and water. Other examples might be given; but we may take it as a general rule that when the equi-density temperature of the liquid and medium is above [pg 38] 125° F. (52° C.), the phenomenon of the automatic drop may usually be observed when the temperature is raised by 30° F. (17° C.), above this point.

Liquid Jets.—So far we have been observing the formation of single drops, growing slowly at the end of a tube, or breaking away from a large mass of the floating liquid. If, however, we accelerate the speed at which the liquid escapes, the drop has no time to form at the outlet, and a jet is then formed. We are all familiar with a jet of water escaping from a tap; it consists of an unbroken column of the liquid up to a certain distance, depending upon the pressure, but the lower part is broken up into a large number of drops, which break away from the column at a definite distance from the tap. There are many remarkable features about jets which I do not intend to discuss here, as it is only intended to consider the manner in which the drops at the end are formed. To observe this procedure, it is necessary again to resort to our method of slowing down the rate of formation, by allowing the liquid to flow into a medium only slightly inferior in density. For this purpose, orthotoluidine falling into water at the ordinary room temperature is eminently satisfactory; and we see on the screen the projection of a pipe, with its end under water, placed so that a jet of orthotoluidine may be discharged vertically downwards from a stoppered funnel. I open the tap slightly at first, and we then merely form a single drop at the end. Now it is opened more widely, and you observe that the drop breaks away some distance below the outlet, being rapidly succeeded by another and another ([Fig. 23]). On still [pg 39] further opening the tap the drops form at a still greater distance from the end of the pipe, and succeed each other more rapidly, so that quite a number appear in view at any given moment ([Figs. 24 and 25]). Notice how the drop is distorted by breaking away from the [pg 40] stream of liquid, and how it gradually recovers its spherical shape during its fall through the water. Finally, I increase the discharge to such an extent that the formation of the terminal drops is so rapid as to be no longer visible to the eye, but appears like the turmoil observed at the end of a jet of water escaping into air.

Figs. 23, 24, 25.—Jets of Orthotoluidine, discharged under water.

Fig. 26.—Water stretched between a tube and a plate.

Liquid Columns.—A simple experiment will suffice to illustrate what is meant by a liquid column. Here is a drop of water hanging from the end of a glass tube. I place it in the lantern and obtain a magnified image on the screen, and then bring up a flat plate of glass until it just touches the suspended drop. As soon as contact is established, the water spreads outwards over the plate, causing the drop to contract in diameter at or near its middle part, so that its outline resembles that of a capstan ([Fig. 26]). The end of the glass tube is now connected to the plate by a column of water of curved outline, which is quite stable if undisturbed. If, however, I gradually raise the tube, or lower the plate, the narrow part of the column becomes still narrower, and finally breaks across. In the same way we may produce columns of other [pg 41] liquids; those obtained with viscous liquids such as glycerine being capable of stretching to a greater extent than water, but showing the same general characteristics. A liquid column, then, is in reality a supported drop, and the severance effected by lowering the support is similar to that which occurs when a pendent drop breaks away owing to its weight.