Liquid Drops and Globules.
by Chas. R. Darling.
PREFACE
The object of the present little volume is to reproduce in connected form, an account of the many interesting phenomena a.s.sociated with liquid drops and globules. Much of the matter relates to experiments devised by the author during the past four years, descriptions of which have appeared in the _Proceedings of the Physical Society_; in the columns of _Nature_ and _Knowledge_; and elsewhere. The exhibition of these experiments at the conversazioni of the Royal Society and the Royal Inst.i.tution, and in the author"s lectures, has evoked such interest as to suggest the present publication. It may be added that all the experiments described may be repeated by any intelligent reader at a trifling cost, no special manipulative skill being required.
The context maintains the form of the lectures delivered on this subject by the author at various places, and the method of presentation is such as may be followed by those who have not received a training in this branch of science. It is hoped, in addition, that the book may prove of some service to teachers of science and others interested in the properties of liquids.
A number of the ill.u.s.trations used have appeared in the pages of _Knowledge_ in connexion with the author"s articles, and are here reproduced by courtesy of the Editor. Other drawings have been provided by Mr. W. Narbeth, to whom the author expresses his thanks.
CHAS. R. DARLING.
_City and Guilds Technical College,_ _Finsbury, 1914._
LIQUID DROPS AND GLOBULES
LECTURE I
*Introduction.*-In choosing a subject for a scientific discourse, it would be difficult to find anything more familiar than a drop of liquid.
It might even appear, at first sight, that such a subject in itself would be quite inadequate to furnish sufficient material for extended observation. We shall find, however, that the closer study of a drop of liquid brings into view many interesting phenomena, and provides problems of great profundity. A drop of liquid is one of the commonest things in nature; yet it is one of the most wonderful.
Apart from the liquids a.s.sociated with animal or vegetable life, water and petroleum are the only two which are found in abundance on the earth; and it is highly probable that petroleum has been derived from the remains of vegetable life. Many liquids are fabricated by living organisms, such as turpentine, alcohol, olive oil, castor oil, and all the numerous vegetable oils with which we are all familiar. But in addition to these, there are many liquids produced in the laboratory of the chemist, many of which are of great importance; for example, nitric acid, sulphuric acid, and aniline. The progress of chemical science has greatly enlarged the number of liquids available, and in our experiments we shall frequently utilize these products of the chemist"s skill, for they often possess properties not usually a.s.sociated with the commoner liquids.
*General Properties of Liquids.*-No scientific study can be pursued to advantage unless the underlying principles be understood; and hence it will be necessary, in the beginning, to refer to certain properties possessed by all liquids, whatever their origin. The most prominent characteristic of a liquid is _mobility_, or freedom of movement of its parts. It is owing to this property that a liquid, when placed in a vessel, flows in all directions until it reaches the sides; and it is this same freedom of movement which enables water, gathering on the hills, to flow under the pull of gravitation into the lowlands, and finally to the sea. If we drop a small quant.i.ty of a strongly-coloured fluid-such as ink-into a large volume of water, and stir the mixture for a short time, the colour is evenly distributed throughout the whole ma.s.s of water, because the freedom of movement of the particles enables the different portions to intermingle readily. This property of mobility distinguishes a liquid from a solid; for a solid maintains its own shape, and its separate parts cannot be made to mix freely. Mobility, however, is not possessed in equal degree by all liquids. Petrol, for example, flows more freely than water, which in turn is more mobile than glycerine or treacle. Sometimes a substance exhibits properties intermediate between those of a solid and a liquid, as, for instance, b.u.t.ter in hot weather. We shall not be concerned, however, with these border-line substances, but shall confine our attention to well-defined liquids.
There is another feature, however, common to all liquids, which has a most important bearing on our subject. Every liquid is capable of forming a boundary surface of its own; and this surface has the properties of a stretched, elastic membrane. Herein a liquid differs from a gas or vapour, either of which always completely fills the containing vessel. You cannot have a bottle half full of a vapour or gas only; if one-half of that already present be withdrawn, the remaining half immediately expands and distributes itself evenly throughout the bottle, which is thus always filled. But a liquid may be poured to any height in a vessel, because it forms its own boundary at the top. Let us now take a dish containing the commonest of all liquids, and in many ways the most remarkable-water-and examine some of the properties of the upper surface.
*Properties of the Surface Skin of Water.*-Here is a flat piece of thin sheet silver, which, volume for volume, is 10 times as heavy as water, in which it might therefore be expected to sink if placed upon the surface. I lower it gently, by means of a piece of cotton, until it just reaches the top, and then let go the cotton. Instead of sinking, the piece of silver floats on the surface; and moreover, a certain amount of pressure may be applied to it without causing it to fall to the bottom of the water. By alternately applying and relaxing the pressure we are able, within small limits, to make the sheet of silver bob up and down as if it were a piece of cork. If we look closely, we notice that the water beneath the silver is at a lower level than the rest of the surface, the dimple thus formed being visible at the edge of the floating sheet (Fig. 1). If now I apply a greater pressure, the piece of silver breaks through the surface and sinks rapidly to the bottom of the vessel. Or, if instead I place a thick piece of silver, such as a shilling, on the surface of the water, we find that this will not float, but sinks immediately. All these results are in agreement with the supposition that the surface layer of water possesses the properties of a very thin elastic sheet. If we could obtain an extremely fine sheet of stretched rubber, which would merely form a depression under the weight of the thin piece of silver, but would break under the application of a further pressure or the weight of a heavier sheet, the condition of the water surface would then be realized. We may note in pa.s.sing that a sheet of metal resting on the surface of water is a phenomenon quite distinct from the floating of an iron ship, or hollow metal vessel, which sinks until it has displaced an amount of water equal in weight to itself.
[Ill.u.s.tration: __Fig._ 1.-Silver sheet floating on water._]
We can now understand why a water-beetle is able to run across the surface of a pond, without wetting its legs or running any risk of sinking. Each of its legs produces a dimple in the surface, but the pressure on any one leg is not sufficient to break through the skin. We can imitate this by bringing the point of a lead pencil gently to the surface of water, when a dimple is produced, but the skin is not actually penetrated. On removing the pencil, the dimple immediately disappears, just as the depression caused by pushing the finger into a stretched sheet of indiarubber becomes straight immediately the finger is removed.
*Elastic Skin of other Liquids-Minimum Thermometer.*-The possession of an elastic skin at the surface is not confined to water, but is common to all liquids. The strength of the skin varies with different liquids, most of which are inferior to water in this respect. The surface of petroleum, for example, is ruptured by a weight which a water surface can readily sustain. But wherever we have a free liquid surface, we shall always find this elastic layer at the boundary, and I will now show, by the aid of lantern projection, an example in which the presence of this layer is utilized. On the screen is shown the stem of a minimum thermometer-that is, a thermometer intended to indicate the lowest temperature reached during a given period. The liquid used in this instrument is alcohol, and you will observe that the termination of the column is curved (Fig. 2). In contact with the end of the column is a thin piece of coloured gla.s.s, with rounded ends, which fits loosely in the stem, and serves as an index. When I warm the bulb of the thermometer, you notice that the end of the column moves forward, but the index, round which the alcohol can flow freely, does not change its position. On inclining the stem, the index slides to the end of the column, but its rounded end does not penetrate the elastic skin at the surface. I now pour cold water over the bulb, which causes the alcohol to contract, and consequently the end of the column moves towards the bulb. In doing so, it encounters the opposition of the index, which endeavours to penetrate the surface; but we see that the elastic skin, although somewhat flattened, is not pierced, but is strong enough to push the index in front of it. And so the index is carried towards the bulb, and its position indicates the lowest point attained by the end of the column-that is, the minimum temperature. Obviously, a thermometer of this kind must be mounted horizontally, to prevent the index falling by its own weight.
[Ill.u.s.tration: __Fig._ 2.-Column and index of minimum thermometer._]
*Boundary Surface of two Liquids.*-So far we have been considering surfaces bounded by air, or-in the case of the alcohol thermometer-by vapour. It is possible, however, for the surface of one liquid to be bounded by a second liquid, provided the two do not mix. We may, for example, pour petroleum on to water, when the top of the water will be in contact with the floating oil. If now we lower our piece of silver foil through the petroleum, and allow it to reach the surface of the water, we find that the elastic skin is still capable of sustaining the weight; and thus we see that the elastic layer is present at the junction of the two liquids. What is true of water and oil in this respect also holds good for the boundary or interface of any two liquids which do not mix. Evidently, if the two liquids intermingled, there would be no definite boundary between them; and this would be the case with water and alcohol, for example.
*Area of Stretched Surface.*-We will not at present discuss the nature of the forces which give rise to this remarkable property of a liquid surface, but will consider one of the effects. The tendency, as in the case of all stretched membranes, will be to reduce the area of the surface to a minimum. If we take a disc of stretched indiarubber and place a weight upon it, we cause a depression which increases the area of the surface. But on removing the weight, the disc immediately flattens out, and the surface is restored to its original smallest dimension. Now, in practice, the surface of a liquid is frequently prevented from attaining the smallest possible area, owing to the contrary action of superior forces; but the tendency is always manifest, and when the opposing forces are absent or balanced the surface always possesses the minimum size. A simple experiment will serve to ill.u.s.trate this point. I dip a gla.s.s rod into treacle or "golden syrup," and withdraw it with a small quant.i.ty of the syrup adhering to the end. I then hold the rod with the smeared end downwards, and the syrup falls from it slowly in the form of a long, tapered column. When the column has become very thin, however, owing to the diminished supply of syrup from the rod, we notice that it breaks across, and the upper portion then shrinks upwards and remains attached to the rod in the form of a small drop (Fig. 3). So long as the column was thick, the tendency of the surface layer to reduce its area to the smallest dimensions was overpowered by gravity; but when the column became thin, and consequently less in weight, the elastic force of the outer surface was strong enough to overcome gravitation, and the column was therefore lifted, its area of surface growing less and less as it rose, until the smallest area possible under the conditions was attained.
[Ill.u.s.tration: __Fig._ 3.-Thread of golden syrup rising and forming a drop._]
*Shape of Detached Ma.s.ses of Liquid.*-Let us now pay a little attention to the small drop of syrup which remains hanging from the rod. It is in contact with the gla.s.s at the top part only, and the lower portion is only prevented from falling by the elastic skin around it, which sustains the weight. We may compare it to a bladder full of liquid, in which case also the weight is borne by the containing skin. Now suppose we could separate the drop of syrup entirely from the rod; what shape would it take? We know that its surface, if not prevented by outside forces from doing so, would become of minimum area. a.s.suming such extraneous forces to be absent or counterbalanced, what would then be the shape of the drop? It would be an exact sphere. For a sphere has a less surface-area in proportion to its volume than any other shape; and hence a free drop of liquid, if its outline were determined solely by its elastic skin, would be spherical. A numerical example will serve to ill.u.s.trate this property of a sphere. Supposing we construct three closed vessels, each to contain 1 cubic foot, the first being a cube, the second a cylinder of length equal to its diameter, and the third a sphere. The areas of the surfaces would then be:-
Cube . . . . 6 square feet.
Cylinder . . . . 586 ,, ,,
Sphere . . . . 49 ,, ,, ------------------------------------------------------
And whatever shape we make the vessel, it will always be found that the spherical form possesses the least surface.
[Ill.u.s.tration: __Fig._ 4.-Drops of different sizes resting on flat plate._]
Now let us examine some of the shapes which drops actually a.s.sume. I take a gla.s.s plate covered with a thin layer of grease, which prevents adhesion of water to the gla.s.s, and form upon it drops of water of various sizes by the aid of a pipette. You see them projected on the screen (Fig. 4). The larger drops are flattened above and below, but possess rounded sides and resemble a teacake in shape. Those of intermediate size are more globular, but still show signs of flattening; whilst the very small ones, so far as the eye can judge, are spherical.
Evidently, the shape depends upon the size; and this calls for some explanation. If we take a balloon of indiarubber filled with water, and rest it on a table, the weight of the enclosed water will naturally tend to stretch the balloon sideways, and so to flatten it. A smaller balloon, made of rubber of the same strength, will not be stretched so much, as the weight of the enclosed water would be less; and if the balloon were very small, but still had walls of the same strength, the weight of the enclosed water would be incompetent to produce any visible distortion. It is evident, however, that so long as it is under the influence of gravitation, even the smallest drop cannot be truly spherical, but will be slightly flattened. The tendency of drops to become spherical, however, is always present.
[Ill.u.s.tration: __Fig._ 5.-Formation of a sphere of orthotoluidine._]
*Production of True Spheres of Liquids.*-Now it is quite possible to produce true spheres of liquid, even of large size, if we cancel the effect of gravity; and we may obtain a hint as to how this may be accomplished by considering the case of a soap-bubble, which, when floating in air, is spherical in shape. Such a bubble is merely a skin of liquid enclosing air; but being surrounded by air of the same density, there is no tendency for the bubble to distort, nor would it fall to the ground were it not for the weight of the extremely thin skin. The downward pull of gravity on the air inside the bubble is balanced by the buoyancy of the outside air; and hence the skin, unhampered by any extraneous force, a.s.sumes and retains the spherical form. And similarly, if we can arrange to surround a drop of liquid by a medium of the same density, it will in turn become a sphere. Evidently the medium used must not mix with the liquid composing the drop, as it would then be impossible to establish a boundary surface between the two. Plateau, many years ago, produced liquid spheres in this manner. He prepared a mixture of alcohol and water exactly equal in density to olive oil, and discharged the oil into the mixture, the buoyancy of which exactly counteracted the effect of gravity on the oil, and hence spheres were formed. The preparation of an alcohol-water mixture of exactly correct density is a tedious process, and we are now able to dispense with it and form true spheres in a more convenient way. There is a liquid known as _orthotoluidine_, which possesses a beautiful red colour, does not mix with water, and which has exactly the same density as water when the temperature of both is 75 F. or 24 C. At this temperature, therefore, if orthotoluidine be run into water, spheres should be formed; and there is no reason why we should not be able to make one as large as a cricket-ball, or even larger. I take a flat-sided vessel for this experiment, in order that the appearance of the drop will not be distorted as it would be in a beaker, and pour into it water at 75 F. until it is about two-thirds full. I now take a pipette containing a 3 per cent. solution of common salt, and discharge it at the bottom of the water. Being heavier, the salt solution will remain below the water, and will serve as a resting-place for the drop. The orthotoluidine is contained in a vessel provided with a tap and wide stem, which is now inserted in the water so that the end of the stem is about 1 inch above the top of the salty layer. I now open the tap so as to allow the orthotoluidine to flow out gradually; and we then see the ball of liquid growing at the end of the stem (Fig. 5). By using a graduated vessel, we can read off the quant.i.ty of orthotoluidine which runs out, and thus measure the volume of the sphere formed. When the lower part reaches the layer of salt solution, we raise the delivery tube gently, and repeat this as needed during the growth of the sphere.
We have now run out 100 cubic centimetres, or about one-sixth of a pint, and our sphere consequently has a diameter of 5 centimetres, or 2 inches. To set it free in the water we lift the delivery tube rapidly-and there is the sphere floating in the water (Fig. 6). We could have made it as much larger as we pleased, but the present sphere will serve all our requirements.
[Ill.u.s.tration: __Fig._ 6.-The detached sphere floating under water._]
[Ill.u.s.tration: __Fig._ 7.-The Centrifugoscope._]
*The Centrifugoscope.*-I have here a toy, which we may suitably call the centrifugoscope, which shows in a simple way the formation of spheres of liquid in a medium of practically equal density. It consists of a large gla.s.s bulb attached to a stem, about three-quarters full of water, the remaining quarter being occupied by orthotoluidine. This liquid, being slightly denser than water at the temperature of the room, rests on the bottom of the bulb. When I hold the stem horizontally, and rotate it-suddenly at first, and steadily afterwards-a number of fragments are detached from the orthotoluidine, which immediately become spherical, and rotate near the outer side of the bulb. The main ma.s.s of the red liquid rises to the centre of the bulb, and rotates on its axis (Fig.
7), and we thus get an imitation of the solar system, with the planets of various sizes revolving round the central ma.s.s; and even the asteroids are represented by the numerous tiny spheres which are always torn off from the main body of liquid along with the larger ones. When the rotation ceases, the detached spheres sink, and after a short time join the parent ma.s.s of orthotoluidine. We can therefore take this simple apparatus at any time, and use it to show that a ma.s.s of liquid, possessing a free surface all round, and unaffected by gravity, automatically becomes a sphere. After all, this is only what we should expect of an elastic skin filled with a free-flowing medium.
*Effect of Temperature on Sphere of Orthotoluidine.*-I will now return to the large sphere formed under water in the flat-sided vessel, and direct your attention to an experiment which teaches an important lesson. By placing a little ice on the top of the water, we are enabled to cool the contents of the vessel, and we soon notice that the red-coloured sphere becomes flattened on the top and below, and sinks a short distance into the saline layer. Evidently the cooling action, which has affected both liquids, has caused the orthotoluidine to become denser than water. I now surround the vessel with warm water, and allow the contents gradually to attain a temperature higher than 75 F. You observe that the flattened drop changes in shape until it is again spherical; and as the heating is continued elongates in a vertical direction, and then rises to the surface, being now less dense than water. So sensitive are these temperature effects that a difference of 1 degree on either side of 75 F. causes a perceptible departure from the spherical shape in the case of a large drop. It therefore follows that orthotoluidine may be either heavier or lighter than water, according to temperature, and this fact admits of a simple explanation.
Orthotoluidine expands more than water on heating, and contracts more on cooling. The effect of expansion is to decrease the density, and of contraction to increase it; hence the reason why warm air rises through cold air, and vice versa. Now if orthotoluidine and water, which are equal in density at 75 F., expanded or contracted equally on heating above or cooling below this temperature, their densities would always be identical. But inasmuch as orthotoluidine increases in volume to a greater extent than water on heating, and shrinks more on cooling, it becomes lighter than water when both are hotter than 75 F., and heavier when both are colder. We call the temperature when both are equal in density the _equi-density temperature_. Here are some figures which show how the densities of these two liquids diverge from a common value on heating or cooling, and which establish the conclusions we have drawn:-
------------------------------------------------------------------- Temperature. Density.
Deg. F. Deg. C. Water. Orthotoluidine.
------------------------------------------------------------------- 50 10 09997 1009
59 15 09991 1005
68 20 09982 1001
Equal: 75 24 09973 0997
86 30 09957 0992
95 35 09940 0988