Alcohol 25
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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
deg. 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.
[Illustration: __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 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 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.
*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; 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 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.
[Illustration: __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 deg. F. or 24 deg. C., is denser than water of equal
temperature. At 75 deg. F. their densities are identical; and as the
ordinary temperature of a room lies between 60 deg. and 70 deg. 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 1 1/2
inches. It is best, when starting, to place the end of the stem 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 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.
[Illustration: __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 respects identical with what we have just
seen on a large scale.
[Illustration: __Figs._ 14 to 20.--Formation of a drop of
orthotoluidine, showing the droplet. Seven stages._]
*Ascending or Inverted Drops.*--If we discharge orthotoluidine into
water when both are hotter than 75 deg. F., the former liquid will rise,
as its density is now less than that of water. If, therefore, I take a
funnel with the stem bent into a parallel branch, so as to discharge
upwards (A, Fig. 13) and raise the temperature of both liquids above 75
deg. F., we see that the drop gradually grows towards the top of the
water, finally breaking away and giving rise to the droplet. Everything,
in fact, was the same as in the case of a falling drop, except that the
direction was reversed. A slight rise in temperature has thus turned the
whole process topsy-turvy, but the action is really the same in both
cases. When, on heating, the water acquired the greater density, its
buoyancy overcame the pull of gravitation on the orthotoluidine, and
accordingly the drop was pushed upwards, the result being the same as
when it was pulled downwards. An inverted drop may always be obtained by
discharging a light liquid into a heavier one, e.g. olive oil into
water, or water into any of the liquids mentioned on p. 19, below the
equi-density temperature.