THE SPLASH OF A DROP--LOW FALL
We will now turn to the photographic record itself. The first series
shows the splash of a drop of water weighing .2 of a gram, and therefore
7.36 millimetres (or rather less than one-third of an inch) in diameter,
falling 40 cm. (about 16 inches) into milk mixed with water. The object
of adding milk to the water was to make it more visible. The addition of
milk makes, as we shall see, a little but not much difference in the
general character of the splash. The scale of the figures is
three-quarters of the actual size.
The number written against any figure gives, on the assumption that no
unobserved error has crept in, the time in decimal parts of a second
that has elapsed since the stage marked "_T = 0_," which is nearest to
the first instant of contact. The reader will understand from what has
been said that the error in any of these times may be as much as
two-thousandths of a second, but is not likely to be more than that,
when all precautions were taken.
It will be observed that as the drop descends into the liquid the upper
portion is at first not appreciably distorted, but that a little cup or
crater of liquid is thrown up round it. As the drop descends further,
this crater grows wider and higher and thicker in the wall, and jets are
shot out from its edge or rim. These jets are visible even in the second
figure. The black marks on the inside wall of the crater are due to the
lamp-black carried down with the drop from the smoked surface of the
supporting cup: though in one sense a disfigurement, they serve to show
by their presence that the interior of the crater _is lined by the
original liquid which formed the drop_, and thus afford useful
information as to the nature of the flow.
The crater rises with great rapidity up to Fig. 4. In Fig. 5 the walls
are beginning to grow thicker, while the next three figures show the
crater subsiding and widening, till in Fig. 9 it lies as a mere ring of
lobes on the surface, surrounding a central hollow.
[Illustration: SERIES I
Milk into water (40 cm.). Scale 3/4.
1
2 T = 0
3 0.002 sec.
4 0.007 sec.
5 0.018 sec.
6 0.031 sec.
7 0.040 sec.
8 0.050 sec.
9 0.056 sec.]
Fig. 10 shows the beginning of the rebound, in the rising of a central
column. It will be seen that the lamp-black is now all swept to the
middle, indicating that _the liquid of the original drop emerges at the
head_ of the central column. Full confirmation of this is obtained from
Fig. 12, which represents the emergent column obtained when the
circumstances are all the same, except that we have a drop of milk
falling into water instead of water falling into milk. It will be
observed that the upper part only of the column is visible, precisely
because it contains nearly all the milk of the drop, while the lower
part, consisting chiefly of transparent water, remains invisible.
[Illustration: SERIES I--(_continued_)
10 0.064 sec.
11 0.073 sec.
12
13 0.093 sec.]
No. 15 shows the column at its greatest height, and it should be noticed
that Figs. 16 and 17 show a tendency on the part of the head of this
column to split off as a separate drop.
[Illustration: SERIES I--(_continued_)
14 0.103 sec.
15 0.116 sec.
16 0.129 sec.]
The column in subsiding forms a "cake" of liquid round the base. The
edge of this circular cake (see Figs. 17, 18, and 19) is the first
well-marked ripple spreading outwards in an ever-widening circle.
[Illustration: SERIES I--(_continued_)
17 0.153 sec.
18 0.197 sec.
19 0.217 sec.]
If Fig. 19 is reached without the top of the column having separated,
then the splash follows the course shown in Figs. 20-a to 23-a, in which
it will be observed that the disappearance of the first column is very
quickly followed by the rise of a secondary column very different in
shape, which itself subsides again, but has not yet (in 23-a) formed, as
it ultimately will, a second "cake" on the top of the first. Thus the
second ripple follows late after the first.
[Illustration: SERIES I--(_continued_)
Alternative (_a_).
20-a 0.240 sec.
21-a 0.242 sec.
22-a 0.248 sec.
23-a 0.253 sec.]
If, however, the summit of the primary column succeeds in breaking off
(as in Fig. 18-b), or even in very nearly breaking off, then the impact
of this newly-formed drop forms a second slight crater on the top of the
first cake, and we have the series (18-b to 24-b), in which it will be
observed that the rim of the secondary crater spreads rapidly outwards,
so that a second well-marked circular ripple in this case _quickly_
follows the first. The secondary column that is thrown up in Fig. 23-b
is very like that which emerged at a much earlier stage in the (_a_)
series.
The photographs of this (_b_) series show very beautifully the manner in
which the advancing edge of the ripple degenerates into smaller ripples
travelling with greater speed.
[Illustration: SERIES I--(_continued_)
Alternative (_b_).
18-b 0.214 sec.
19-b 0.237 sec.
20-b 0.242 sec.
21-b 0.244 sec.]
[Illustration: SERIES I--(_continued_)
Alternative (_b_).
22-b 0.261 sec.
23-b 0.257 sec.
24-b 0.311 sec.]
It will be readily understood that if the splitting off of the head of
the primary column happens to take place a little earlier, or on the
other hand is nearly, but not quite, complete when it descends below the
surface, then subsequent configurations will differ somewhat from either
of the sub-series here shown.
Since any figure photographed might belong to either sequence, the
disentanglement of the two series required careful consideration and
long experimenting.
The reappearance of the original drop at the head of the rebounding
column, of which the explanation has been given in this chapter, is
easily verified by naked-eye observation.
Let the reader when he next receives a cup of tea or coffee to which no
milk has yet been added, make the simple experiment of dropping into it
from a spoon, at the height of fifteen or sixteen inches above the
surface, a single drop of milk. He will have no difficulty in
recognizing that the column which emerges carries the white milk-drop at
the top only slightly stained by the liquid into which it has fallen.
In the same way naked-eye observation reveals the crater thrown up by
the entry of a big rain-drop into a pool of water. In either case what
we are able to glimpse is a "stationary" stage. The rebounding column
reaches a maximum height, remains poised for an instant, and then
descends. The same is true of the crater. It is the relatively long
duration of the moment of poise that produces on the eye a clear
impression where all else is blurred by rapid change.
But there is frequently a curious illusion. We often seem to see the
crater with the column standing erect in the middle of it. We know now
that in reality the crater has vanished before the column appears. But
the image of the crater has not time to fade before that of the column
is superposed on it.
Those who are accustomed "to believe nothing that they hear and only
half of what they see" may be glad to find at least the latter part of
their maxim so completely justified.