The strange behaviour of some liquids

D. Leela and classmates

We have a strange result to report. Strange, because we just found out
about it, and we found it fascinating. My mother tells us that this
has been known for a long time. But then she loves to spoil sport.

It happened accidentally. We were cleaning our cycles and left a mug of
water with a cloth half stuffed into it and went off to play. The next
day, when we went to collect our things, we found that the cloth was
soaking wet and there was a mess of water on the floor next to the
mug. Yet, the cloth was not left in the water; only a portion of it was
in the water.

We came back home and decided to do it again. This time we tore off a
strip of old cloth. We immersed one end in water and put the other end
in an empty glass. This way, we thought we could actually measure how
much water was flowing out through the cloth. That was mainly because of
Nithya. She did not believe that water could rise all by itself and
flow over. She told us that this was against the law of gravity where
everything falls down, not up!

The next morning, sure enough, the cloth was wet and there was a small
amount of water in the empty glass. Manisha threatened to talk to the
physics teacher about us.

We wondered whether the water would empty completely. After a day
or so, it became clear that no more water would flow: the part of the
cloth not dipping into the water was drying off in the heat of the day.
We poked and prodded the cloth and quietly wetted it again, each one
pretending that they had not seen the act! 

Next day we got another surprise! While wetting the cloth, Nivedita had
accidentally pulled it so far over that it was touching the water in
both glasses. Now we saw that the water levels were the same in both
glasses! The cloth acted as an equaliser, distributing the water equally
in both glasses. Now even Nithya was excited (and convinced).

We repeated the experiment with oil. We thought that oil is
sticky so it shouldn't work as well. However, the next day, we found the
oil distributed neatly between the two glasses as well. There were so
many friends who came in to see this amazing display that Mother got
suspicious. "What are you all doing in there?", she asked. We showed her
the experiment with everyone talking at once, in our excitement. Mother
cut us off. "Oh, capillary action, yes. It can be quite fascinating."

We gaped at her. "Aunty, did you know about this already?" asked Navya.

Mother laughed. "How do you think Diwali "diyas" or "deepams" work? Don't
you put a small cotton wick into a diya of oil? Doesn't the wick get
all oily?"

"Yes", said Nithya, "and if the wick is not properly soaked, it just
burns up instantly. But it it is well soaked, then it burns as long as
the oil lasts."

"That's another story", said Mother, "but you are right. You have to
soak it well because it takes too long otherwise: you want the lamps lit
that night, not the next day! But the oil continues to rise because of
capillary action."

"And what about with water. Do you know whether it works with water as
well?" I challenged. 

Mother shrugged. "For centuries gardeners who are going away on vacation
leave water cans with thick string or pieces of cloth hanging over a
plant pot. The plant is continuously watered by the water dripping from
the string and survives until you get back from vacation and start
watering it again."

We thought for a while. Then we decided, so what if everyone knows about
it. We had found it out for ourselves. Maya came wandering in with a
bottle of honey. We grabbed it from her. Does it work with honey as
well, we wondered. Well, do it and see.

BOX

Capillary action

This happens when the attraction between water molecules and the cloth
molecules is more than between the water molecules themselves. As a
result, the water gets attracted to the cloth and slowly gets drawn in
to it. A common example is newspaper which gets very wet when water is
poured on it. Paper napkins are made of a material which absorbs water
easily.

Xylem cells use capillary action to send water to the tops of trees,
large and small.

So when you dip cloth in a glass of water, the water molecules "pull"
themselves up the cloth through capillary action. But the weight of the
water keeps pushing it down (due to gravity). At some point, when
gravity can no longer be overcome, the water will stop climbing up
further.

Do you know: Albert Einstein's first scientific publication in the
journal Annals of Physics in 1901 was on capillarity.