Deflation policy
Question: Why do helium balloons deflate so quickly? When my children bring
balloons home from parties, the ones that are filled with helium are often small
and wizened by the following morning. I realise that some of the size reduction
is caused by deflation but something else must be at work because standard
air-filled balloons stay inflated for much longer.
Answer: Helium gas is not only very light, it is monatomic鈥攊ts
particles are all made of a single atom. As a result, helium is made up of the
smallest gaseous particles possible. The atoms are only 0.1 nanometre in
diameter, and are quite capable of diffusing through metal films. Because it so
readily diffuses through small pores, helium is used to help test for leaks in
industrial and laboratory vacuum systems. Nitrogen and oxygen molecules have a
much larger diameter than helium atoms which means that they are much less
capable of diffusing through the balloon walls. It鈥檚 like the difference between
trying to get sand and small pebbles to pass through a sieve鈥攖he sand goes
through much more easily because it鈥檚 made from smaller particles.
The second factor which helps to increase losses by diffusion is that
balloons are made from viscoelastic materials whose structure is a tangled mass
of polymer strands鈥攁 bit like a plate of spaghetti. The polymer strands
cannot pack closely together, and have channels through which the helium can
diffuse, so even at low pressure the helium will diffuse out through the walls.
When the balloon is inflated, the polymer stretches, so the balloon walls become
thinner (the helium has a shorter distance to diffuse out), the molecular
structure becomes slightly more open (making diffusion much
easier), and the increased pressure provides a driving force for the
diffusion. These are the reasons why deflation is very rapid to begin with, but
then gradually slows down as the balloon gets smaller.
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Commercial helium balloons are made from nonporous inelastic materials and
are coated to reduce the losses even further, although even they still lose a
significant percentage of helium per day, certainly enough to disappoint
children (and grown-ups) the morning after buying a balloon.
Gavin Whittaker
Heriot, Borders
Answer: The helium atom is very small and very light. It is able to diffuse
through the thin, stretched rubber of the balloon quite easily, finding its way
through atomic-size pores. Air molecules (oxygen and nitrogen mainly) are larger
and heavier and diffuse through much more slowly. In addition to the increased
pressure inside the balloon which pushes helium out through the sides, there is
another factor that increases helium flow outwards.
Because there is almost no helium in the air, far more helium atoms are
hitting the inside of the balloon than the outside, and there is a net flow
outwards. However, you will notice that the balloon does not completely deflate.
This is because some air moves in as, conversely, more air molecules hit the
outside than the inside.
This leads to a truly bizarre effect if the balloon is filled with the gas
sulphur hexafluoride, which has large, very heavy molecules which hardly diffuse
through the rubber at all, and so cannot get out. But once again, as in the
helium example, there are more air molecules outside than in, so air diffuses
inwards and the balloon slowly increases in size.
Harvey Rutt
University of Southampton
Clotty spice
Question: What is the substance in turmeric that causes blood to clot and how
does it work? Apparently chefs who cut themselves use it all the time.
Answer: Standard turmeric is made from the dried and powdered plant
Curcuma longa. Coagulation is not mentioned among turmeric鈥檚 traditional
applications. However, blood absorbed into the microscopically rough powder is
immobilised and loses water. Possibly aided by proteins and astringent
components in the turmeric, this causes rapid clotting.
Many powders do this. A handful of calcium sulphate (plaster of Paris), or a
piece of plaster bandage slapped onto serious bleeding and held in place, stops
blood pretty smartly. I don鈥檛 recommend using turmeric in lieu of more
conventional materials: the crust is brittle and on ragged wounds it might
complicate matters for a surgeon cleaning the wound afterwards. Still, it can
save lives if it is the best you have.
Any powder of doubtful provenance on an open wound invites infections,
poisoning and allergies, but the range of substances that people make use of for
that purpose is amazing: raw earth, dung, snuff, cobwebs, plaster of Paris (as
suggested above) and, of course, turmeric. That people survive folk remedies
attests to the toughness of the human body.
Jon Richfield
Dennesig, South Africa
This week鈥檚 questions
Aim and pour: When I open a carton of milk I have to pour the liquid quickly
from the opening so that it goes into my glass.
If I tip the carton too slowly, the milk runs out of the opening and then
runs down the underside of the carton and pours from the bottom of the container
onto my foot or the floor. Orange juice and other liquids do the same. Why does
it stick to the carton when poured slowly?
Tom Khan
Bradford, West Yorkshire
Pluck for luck: I live on land recently reclaimed from dry bushland. I seeded
the garden with grass and clover seed, and in the past three years I have found
hundreds of four-leaved clovers and numerous versions with five, six, seven,
eight and nine leaves.
Sometimes there can be as many as 12 leaves all wound tightly round the stem
so there is room for each leaf to grow. Is my garden exceptional?
What conditions affect the number of leaves grown, bearing in mind that
anything more than a three-leafed clover is considered out of the ordinary? And
do I live on the luckiest patch of land in Australia?
Alison Venugoban
Ngunnawal, ACT
Tone deaf: If I yawn while listening to music on my headphones, the perceived
pitch falls by about half a semitone.
The pitch can鈥檛 really be going down because it鈥檚 determined by the CD and
player to which the headphones are attached. So does time slow as I yawn or is
there another explanation?
Richard Gledhill
London