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

by Charles R. Darling

Fig. 35.—Forces acting on a floating globule.

Let us study the forces at work on the floating globule a little more closely. Its upper surface is in [pg 61] contact with air, and the surface tension tends, as usual, to reduce the area to a minimum. The top of the globule is not flat, but curved ([Fig. 35]), and its surface meets that of the water at an angle; and the counter-pull exerted against the stretching-pull of the water surface is not horizontal, but inclined in the direction of the angle of contact, as shown by the line B. The under part of the globule is also curved, and meets the water surface from below at an angle; and here also is exerted a pull in opposition to that of the water surface, different in magnitude to the force at the upper surface, but also directed at the angle of contact as shown by the line C. This tension at the joining surface of two liquids is called the “interfacial” tension, to distinguish it from that of a surface in contact with air. Acting against these two tensions is that of the water, which is directed horizontally along the surface, as shown by the line A. The lines A, B, and C indicate the forces acting at a single point; but the same forces are at work at every point round the circle of contact of the globule and the surface of the water. And therefore the tendency on the part of the water [pg 62] tension is to cause the globule to spread out in all directions, whereas the other two tensions tend to prevent any enlargement of its surface. The result depends upon the magnitudes and directions of the conflicting forces. We can imagine a kind of tug-of-war taking place, in which one contestant, A, is opposed to two others, B and C, all pulling in the directions indicated in [Fig. 35]. Although A is single-handed, he has the advantage of a straight pull, whereas B and C can only exert their strength at an angle, and the larger the angle the more they are handicapped. If A be more powerful than B and C, the globule will spread; but the result of the spreading is to diminish the angles at which the pulls of B and C are inclined to the surface, and hence their effective opposition to A will be increased. Moreover, the spreading of the liquid diminishes the surface tension of the water—that is, weakens A—and hence it becomes possible for B and C to prevail and draw back the surface of the globule which A had previously stretched. If, in spite of these disabilities, A should still be the stronger, the globule will be stretched until it covers the whole surface; whereas if B and C overcome A, the globule will shrink, increasing the angles at which B and C operate, and therefore reducing their effective pulls, until their combined strength is equal to that of A, when the globule will remain at rest. Bearing these facts in mind, we can understand why a small drop of oil placed on a clean water surface spreads across; for in this case A is stronger than B and C combined. But when the surface of the water is covered with a layer of oil, A is weakened, and can no longer overcome the opposing pulls of B and C. Hence [pg 63] a further drop of oil poured on to the surface remains in the form of a globule.

Movements due to Solubility.—When small fragments of camphor are placed on the surface of water some remarkable movements are seen.[³] The bits of camphor move about with great rapidity over the surface, and generally, in addition, show a rapid rotary motion. The explanation usually given is that the camphor dissolves in the water at the points of contact forming a solution which possesses a less surface tension than pure water. This solution is in consequence stretched by the tension of the rest of the surface, and the camphor floating on its solution is therefore made to move in the direction of the line along which the stretching force happens to be the greatest. But the camphor continues to dissolve wherever it goes, and is therefore continuously pulled about as a result of this interplay of tensions. Touching the surface with a wire which has been dipped in oil immediately arrests the movements, owing to the tension of the water being diminished to such an extent by the skin of oil that it is no longer competent to stretch the part on which the camphor floats. No doubt this explanation is correct so far as it goes, but it is highly probable that when the floating substance dissolves, other forces are called into action in addition to the tensions.

Fig. 36.—Aniline globules on a water surface.

Movements of Aniline Globules on a Water Surface.—If we allow a small quantity of aniline to run on to the surface of water, it forms itself into a number of floating globules. I now project on the screen a [pg 64] water surface on which a little aniline has been poured, and we are thus enabled to watch the movements which occur. All the globules appear to be twitching or shuddering; and if you observe closely you will notice the surface of each globule stretching and recoiling alternately. The recoil is accompanied by the projection of tiny globules from the rim, which becomes scalloped when the globule is stretched. The small globules thrown off appear to be formed from the protuberances at the edge ([Fig. 36]), and after leaving the main globule they spread out over the surface, or dissolve. This process continues for a long time, gradually diminishing in vigour, until small stationary globules are left floating on the surface, which is now covered with a skin of aniline. This action is in [pg 65] striking contrast to the tranquil formation of floating globules of oil, and calls for some special comment.

Let us recall again the three forces at work at the edge of a floating globule ([Fig. 35]). The surface tension of the water, acting horizontally, tends to stretch the globule, and is successful momentarily in overcoming the opposing tensions, each of which pulls at an angle to the surface. Enlargement of the upper surface of the globule, however, reduces the angles at which the tensions B and C act, and in consequence their effective strength is increased. The spreading of the aniline over the water surface diminishes the pull A, which B and C combined now overcome, and hence the surface of the globule shrinks again. For some unexplained reason both the stretching and recoil of the globule occur suddenly, there being an interval of repose between each, and these jerky movements result in small portions of the rim being detached, each of which forms a separate small globule. The aniline which spreads over the surface of the water dissolves, and the water tension A, which had been enfeebled by the presence of the aniline skin, recovers its former strength, and again stretches the globule; and so the whole process is repeated. When the surface of the water becomes permanently covered with a skin, which occurs when the top layer is saturated with aniline, the globule remains at rest, and has such a shape that the tensions B and C act at angles which enable them just to balance the weakened pull of A. Why the edge of the globule becomes indented during the movements, and why these movements are spasmodic instead of gradual, has not been clearly made out. It [pg 66] is interesting to recall that a spheroid of liquid on a hot plate also possesses a scalloped edge, and it may be that the two phenomena have something in common.

Fig. 37.—Orthotoluidine globules on a water surface.