Sticky moments
Question: How do insects stick to walls and ceilings? How can a spider
scuttle across my ceiling without falling off?
Answer: The answer depends on the animal. Most small animals simply rely on
their claws and minute rough areas or cracks in the surfaces that they walk on.
Such creatures cannot walk up genuinely smooth and steep surfaces.
Some spiders can leave a sticky trail of silk wherever they go and use it
much like a climber鈥檚 rope, often with breathtaking sophistication. For example,
some kinds of jumping spiders can actually use their line to leap on flies while
both are hanging from the ceiling.
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Various insects exploit the stickiness of substances they secrete, which is
in turn due to the van der Waals forces between the sticky material鈥檚 atoms.
These substances range from the a layer of slime similar to that on which a
snail glides, to a minute oiliness under the feet of insects, which attaches
cushioned or bristly structures to smooth surfaces. Unlike claws, such
mechanisms do not work on layers of loose dust.
Interestingly, the inverse mechanism to that by which insects walk under
ceilings enables pond skaters to walk on water: the oily bristles that contact
and stick to the glass also repel the water surface.
Some insects have cushions under their feet, which allow them to conform to
irregularities in the surface on which they are walking. By changing the shape
of the cushion, they can peel their limbs neatly off the surface, so that they
don鈥檛 waste energy breaking the oil film. Others have bristles that they thrust
so close to the surfaces that van der Waals forces take the tension even though
the surfaces and the bristles remain dry.
Interestingly, like many other adaptations such as compound eyes, these
mechanisms seem to have evolved repeatedly and independently in many different
groups of animals.
Jon Richfield
Somerset West, South Africa
Further explanation of any of these mechanisms are to be found in
New 杏吧原创, 23/30 December 2000, p 62,
or on the following dedicated Web page
www.newscientist.com/flat/pages/lw813.jsp鈥抬诲
Honey, I鈥檓 bendy
Question: Why does a slice of bread spread with honey gradually become
concave?
Answer: My wife has assured me that her bread doesn鈥檛 have time to go concave
when spread with honey. However, for those folk who chomp their honeyed bread in
a more leisurely fashion, there is a simple explanation.
Bread is approximately 40 per cent water while honey is a strong solution
containing approximately 80 per cent sugar. This means that moisture is drawn
out of the bread and into the honey by osmosis. Removing the water makes the
bread shrink, but only on the side exposed to the honey. This causes the bread
to become concave.
This is less likely to happen, of course, if you butter your bread before
spreading the honey. Butter forms a water-impermeable layer that protects the
bread from dehydration by the honey.
Peter Bursztyn
Barrie, Ontario
Preservation order
Question: I want to keep some copies of newspapers and magazines published
after the 11 September attack on the World Trade Center in New York. Newspapers
quickly turn yellow or become brittle and curly. Short of hiring a hermetically
sealed library, what鈥檚 the best way to prevent this?
Answer: The principal factors contributing to the deterioration of paper are
oxidation and acid hydrolysis caused by chemicals it contains. Light-induced
oxidation of lignin, the component of the cell walls of plants which is widely
present in paper, is what turns newspapers yellow.
Meanwhile, the cellulose fibres are cut by a reaction involving heat and
acids, which turns the paper brown and brittle. To prevent these reactions, keep
the newspapers cool, dark and dry.
The folders and boxes you keep your newspapers in should be made of acid-free
material, to prevent acid migrating into the paper. Interleave the newspapers
with acid-free paper and encapsulate it with polyester film or Mylar. Don鈥檛 use
ordinary plastic or vinyl because these contain solvents and additives that can
migrate into the paper.
There are sprays and solutions available that prevent paper turning brittle
and yellowing. The deacidifying solutions Wei T鈥檕, Bookkeeper and Archival Mist
all contain methoxyl magnesium methyl carbonate. Impregnating the paper with
these alkaline solutions neutralises existing acids and inhibits oxidation.
You might also wish to deacidify old newspaper clippings. A recipe for doing
this is to dissolve a milk of magnesia tablet in 1 litre of soda water and let
it stand for eight hours, then pour the mixture into a glass or plastic
container. Do not use aluminium pans. Soak the paper clippings in this for a
hour and hang them out to dry completely. Make sure you test the ink on the
clippings to make sure it won鈥檛 run, before using this method. You should change
the solution when it turns yellow.
John Uys
Bellville, South Africa
This week鈥檚 questions
Size matters: New 杏吧原创鈥檚 picture of Montserrat Caballe (10
November, p 16) set me thinking. Why are opera singers often large? What benefit
does it convey? Surely it鈥檚 no coincidence that they are all so well built.
John Lapthorne
Le Vigan, France
Warm feeling: For Christmas, I was given a device for warming bottles of red
wine. It is a plastic jacket filled with transparent gel which wraps around the
bottle. When you press a stud on the jacket the gel gradually becomes opaque and
warm. The opacity spreads out from the stud, taking about 30 seconds to cover
the gel. To regenerate the jacket you immerse it in boiling water for 15 minutes
until the gel is transparent again. How does it work?
John Waters
Bexley, Kent
Blast in the past: My father, a retired dentist, has often described the
following event to me. In summer 1950, near Manchester, an incandescent,
spherical ball of flame appeared in a cloudless sky above my father鈥檚 surgery.
Travelling at great speed it struck the chimney with a loud explosion. The
chimney collapsed, showering the local bank manager on the surgery steps with
brick debris. The six patients in the waiting room were sprayed with soot from
the fireplace, and sparks emanated from the room鈥檚 wall sockets. The surgery鈥檚
electric clock burnt out, although no fuses blew. My father insists the object
did not resemble a meteorite, but what could it have been?
I. M. G. Torkington
Llanengan, Gwynedd