Aniline Films or Skins.—We have previously concluded, largely from circumstantial evidence, that a liquid drop is encased in a skin or what is equivalent to a skin, and I propose now to show by experiments with aniline how we can construct a drop, commencing with a skin of liquid. Here is some aniline in a vessel, covered by water. I lower into the aniline a circular frame of wire, which I then raise slowly into the overlying water; and you observe that a film of aniline remains stretched across the frame. By lifting the frame up and down in the water the skin is stretched, forming a drop which is constricted near the frame [pg 20] ([Fig. 9]). On lifting the wire more suddenly, the skin of aniline closes in completely at the narrow part, and a sphere of water, encased in an aniline skin, then falls through the water in the beaker, and comes to rest on the aniline below—into which, however, it soon merges. You were previously asked to regard a drop of liquid as being similar to a filled soap-bubble; and this experiment realizes the terms of the definition. And it requires only a little imagination to picture a drop surrounded by its own skin instead of that of another liquid. It is easy to make one of these enclosed water-drops by imitating the blowing of a soap-bubble—using, however, water instead of air. In order to do this I take a piece of glass tubing, open at both ends, [pg 21] and pass it down the vessel, until it reaches the aniline. Water, in the meantime, has entered the tube, to the same height as that at which it stands in the vessel. On raising the tube gently, a skin of aniline adheres to the end; and as we raise it still further, the water in the tube, sinking so as to remain at the level in the vessel, expands the skin into a sphere ([Fig. 9])—the equivalent of a filled soap-bubble. On withdrawing the tube gradually, the composite sphere is left hanging from the surface of the water.
Fig. 9.—Aniline skins enveloping water.
Surface Tension.—Before proceeding further, it will be advisable to introduce and explain the term “surface tension.” We frequently use it, without attaching to it any numerical value, to express the fact that the free surface of a liquid is subjected to stretching forces, or is in a state of tension; and thus we say that certain phenomena are “due to surface tension.” But the physicist does not content himself with merely observing occurrences; he tries also to measure, in definite units, the quantities involved in the phenomena. And hence surface tension is defined as the force tending to pull apart the two portions of the surface on either side of a line 1 centimetre in length. That is, we imagine a line 1 centimetre long on the surface of the liquid, dividing the surface into two portions on opposite sides of the line, and we call the force tending to pull these two portions away from each other the surface tension. Experiments show that this force, in the case of cold water, is equal to about 75 dynes, or nearly 8/100 of a gramme. If we choose a line 1 inch long on the surface of water, the surface tension is represented by about 3 1/6 grains. It is always [pg 22] necessary to specify the length when assigning a value to the surface tension; and unless otherwise stated a length of 1 centimetre is implied. The values for different liquids vary considerably; and it is also necessary to note that the figure for a given liquid depends upon the nature of the medium by which it is bounded—whether, for example, the surface is in contact with air or another liquid. The following table gives the values for several liquids when the surfaces are in contact with air:—
| Liquid. | Tension at 15° C. (59° F.), dynes per cm. |
|---|---|
| Water | 75 |
| Aniline | 43 |
| Olive Oil | 32 |
| Chloroform | 27 |
| Alcohol | 25 |
When one liquid is bounded by another, the interfacial tension, as it is called, is generally less than when in contact with air. Thus the value for water and olive oil is about 21 dynes per centimetre at 15° C.
We are now in a position to speak of surface tension quantitatively, and shall frequently find it necessary to do so in order to explain matters which will come under our notice later.
Figs. 10, 11 and 12.—The Diving Drop. Three stages.
The “Diving” Drop.—In order to illustrate the tension at the boundary surface of two liquids, I now show an experiment in which a drop is forcibly projected downwards by the operation of this tension. I pour some water into a narrow glass vessel, and float [pg 23] upon it a liquid called dimethyl-aniline, so as to form a layer about 1 inch in depth. A glass tube, open at both ends, is now passed down the floating liquid into the water, and then raised gradually, with the result that a skin of water adheres to the end, and is inflated by the upper liquid, forming a sphere on the end of the tube ([Fig. 10]). On withdrawing the tube from the upper surface, the sphere is detached and falls to the boundary surface, where it rests for a few seconds, and is then suddenly shot downwards into the water ([Figs. 11 and 12]). It then rises to the interface; breaks through, and mingles with the floating liquid, thereby losing its identity. Why should the drop, which is less dense than water, dive below in [pg 24] this manner? The explanation is that the drop (which consists of a skin of water filled with dimethyl-aniline), after resting for a time on the joining surface, loses the under part of its skin, which merges into the water below. The shape of the boundary of the two liquids is thereby altered, the sides now being continuous with the skin forming the upper part of the drop. This is an unstable shape; and accordingly the boundary surface flattens to its normal condition, and with such force as to cause the drop beneath it to dive into the water, although the liquid is lighter than water and tends to float. The result is the same as that which would occur if a marble were pressed on to a stretched disc of rubber, and then released, when it would be projected upwards owing to the straightening of the disc. I now repeat the experiment, using paraffin oil instead of dimethyl-aniline; but in this case the drop is only projected to a small depth, and the effect is not so marked. The experiment furnishes conclusive evidence of the existence of the interfacial tension.