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Self-pouring

Question: Recently I saw a demonstration of an amazing liquid that could pour
itself out of a jar. All you had to do was tip the jar slightly so it began to
run out. It then continued until it was all gone, even though the jar was
straightened. What was the liquid and how on Earth does it do this?

Answer: This is an example of non-Newtonian rheology called the open siphon
effect. It can be demonstrated by using a dilute solution of a suitable
long-chain polymer. A typical example is polyethylene oxide which is soluble in
water and has a molecular weight in the range of 100,000 to 5,000,000.

These highly viscoelastic systems can be likened to a large pan of cooked
spaghetti. Attempts to divide and share out the spaghetti are continually
frustrated by the entanglement and alignment of the pasta strips that seem to
give the collected material a mind of its own.

Similarly, when you start to pour a solution of long-chain polymer
molecules, the fluid is subjected to extensional stresses that cause the
molecules to align. This alignment substantially increases the extensional
viscosity of the solution compared with its shear viscosity, and when stretched
the fluid becomes relatively strong (more like a solid, in fact). However, it is
still able to flow over the lip of the container because the aligned molecules
can slip past each other. Once started, the flow continues like a normal siphon,
the stresses produced by the falling liquid pulling more fluid from the
container.

The extensional viscosity of normal Newtonian fluids, such as water, is not
increased by flow. The stream of liquid rapidly elongates and breaks into drops
before any siphon action can start. In this case, siphon action can only occur
when elongation has been prevented by the artificial support of a tube.

Photographs of this and other intriguing non-Newtonian effects, including the
related ductless or tubeless siphon effect and the Fano effect can be seen in
the book Rheological Phenomena in Focus by D. V. Boger and K. Walters
(Elsevier Science, 1993).

Martin Whittle

University of Sheffield

South Yorkshire

Answer: Many solutions of large molecules and suspensions of flexible
thread-like solids behave in ways similar to that described, though perhaps not
as spectacularly. A good everyday example is the uncooked white of an egg.

A related phenomenon, known as the Weissenberg effect, is seen when a
vertical rod is rotated in a viscoelastic fluid. The effect is similar to
turning a fork in spaghetti: the threads become entangled and pull others after
them.

This generates a local pressure that causes the fluid to climb up the rod,
rising above the free surface. Solutions of polyacrylamide or polyethylene oxide
in mixtures of glycerol and water can reproduce a wide range of viscoelastic
behaviour.

These fluids can also recover their shape after deformation. Try looking at
the way that glutinous soup unwinds when you stop stirring it. Or see how the
residual egg white retracts up into the remains of the shell when an egg is
cracked into a frying pan.

Another way to tell if a fluid is elastic is to shake it up and look at the
shape of the larger air bubbles as they rise. In elastic fluids bubbles tend to
be pear-shaped with a tail, rather than being spherical or ellipsoidal.

Such simple demonstrations do not reflect the difficulty (and associated
cost) of accurately determining, describing and controlling the basic physical
properties of viscosity and elasticity of fluids encountered in some industries.
This is a particular problem for those involved with the manufacture, supply or
use of polymers, paints, adhesives, lubricants, foodstuffs, medicines and
natural biological fluids.

John Smith

Cranleigh, Surrey

Solid state

Question: According to The Guinness Book of Records, the lowest
temperature on the Earth’s surface was -89.2 °C at Vostok Station in
Antarctica. Does carbon dioxide fall as snow at this temperature, and if not,
why not?

Answer: The temperature at which a component of a gas mixture
condenses—its dew point—depends on the concentration of that gas in
the mixture. For example, the temperature at which you get condensation of water
from the air is not 100 °C (the boiling point of water) but the temperature
at which the vapour pressure of water would be equal to its partial pressure in
the gas mixture. The dew point is lower as the concentration of the condensate
gets lower.

Hugh Wolfson

Altrincham, Cheshire

Answer: Carbon dioxide will only condense from a gas if its partial pressure
equals or exceeds the saturated vapour pressure of pure CO₂ at the
given temperature.

In the case of air at a pressure of 1 bar (10⁵ pascals) containing 0.03 per
cent CO₂, the partial pressure of the CO₂ is 30 pascals. Pure
CO₂ has to be cooled below -143 °C for its vapour pressure to fall
as low as this.

This figure would be the saturation temperature (or the dew point) of that
air with respect to the CO₂. It will not condense from the air and form
a solid if it is above this temperature.

John Smith

Cranleigh, Surrey

This week’s questions

New life: How good is the human body as a fertiliser? If I was buried under a
young apple tree would it receive sufficient nitrogen, phosphorus and potassium
to grow and mature?

What other breakdown products—from my body’s fats, proteins and other
substances—would help to sustain the soil microorganisms?

John Bond

Worcester

Dotty bottles: Near the base of every glass drinks bottle, be it used for
milk, wine or beer, there is a horizontal row of five to eight raised dots with
varying spacing. What are they for?

All of the beer bottles that I have seen, for example, have different dot
patterns, so they are no use to sight-impaired people.

Tom Newsom

Bath