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Expedition
Six
Space Chronicles #15
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By:
ISS Science Officer Don Pettit
The Invisible
Spoon of Marangoni
As in politics,
natural phenomena are often governed by the balance of force. Take
for example, the free surface seen in a glass of water. What you
observe depends on a subtle balance between surface tension force
and gravitational force. Surface tension force wants to make the
water curl up into a spherical ball; gravitationalforce wants to
flatten the water out into a pancake. If gravitational force wins
this game of tug-of-war, then you see a flat surface with surface
tension relegated to a consolation prize of exhibiting only a little
curvature next to the edges where the water contacts the glass.
If you reduce the effect of gravity, surface tension wins and you
see a highly curved, near-hemispherical surface.
There are several
ways to rig this contest. Gravitational force works best over large
dimensions so by making the free surface large, you ensure that
gravity dominates and you have a flat surface. Surface tension works
best over small dimensions so by shrinking the glass to smaller
diameters, you will see the surface change from flat to highly curved.
To design a good surface tension experiment, you often have to work
at small length scales, perhaps only a few millimeters in extent.
By working at such small scales, other phenomena of interest may
be suppressed and go unnoticed. Such is the case with Marangoni
convection.
Marangoni convection
is an obscure phenomenon where the liquid of interest is put in
motion as if stirred by an invisible spoon. However, there is no
spoon stirring the liquid, only surface tension. To make surface
tension forces cause convection, you need a variation in the magnitude
of the surface tension along the free surface. Surface tension is
a function of temperature, so a temperature gradient superimposed
on a free liquid surface gives an imbalance in surface tension forces
that results in a game of tug-of-war with itself.
To see Marangoni
convection requires a free surface scale of large dimension, but
to have surface tension forces dominate the system over gravity
requires a scale of small dimension. Because of these conflicting
requirements, Marangoni convection on Earth remains rather obscure.
Only a handful of examples come to mind. One such example is observed
in the molten steel left in the wake of an arc welder's torch. Here
a puddle of steel undergoes immense temperature gradients as it
cools and the observant welder will notice that this molten puddle
is stirred by the invisible spoon of Marangoni.
A way to enable
the detailed study of this phenomenon is to arrange matters so the
effects of gravity are reduced. By creating something like a fluid
mechanical equivalent to a magnifying glass, reduced gravity allows
for enlarging the length scale of the experiment so detailed observations
are possible. Temperature gradients can also be more civilized than
the high temperatures of a welder's torch. Thus Marangoni convection
is a choice topic for study in the microgravity environment of orbit.
Being aware
of Marangoni convection but not particularly wanting to study it
at the moment, I was forced into considering its effects while observing
diffusion in a stagnant thin film of water. After creating about
a 300 micrometer thick water film on a wire loop 50mm in diameter,
tracer particles were dispersed so fluid flow could be observed.
This makes a simple two-dimensional experiment in which to gather
observations. After waiting some minutes for all visible fluid motion
to cease, detailed observations were started through a magnifying
lens. There was no observed fluid motion, just as expected. I shined
the light from a flashlight onto the film so the reflected glint
from the free surface could be seen. Within a few seconds motion
in the film was observed. It looked like someone was stirring it
with an invisible spoon. Withdrawing the flashlight stopped all
motion within seconds. After repeating this process a number of
times the conclusion was drawn that the flashlight was driving convection
within the film. This was not exactly what I had planned, however,
nature goes about its business in spite of that. Could it be some
type of photophoretic pressure on the trace particles? That seemed
unlikely. I inspected the flashlight. It was a small two AA cell
type with a beam that can be focused down into a smallish spot.
Directing the spot onto the back of my hand yielded the feeling
of warmth. The explanation was Marangoni convection. Heat from the
flashlight was producing a warm spot in the liquid that in turn
produced a local change in surface tension. This created an imbalance
in forces and caused the fluid to move. By removing the forces due
to gravity and allowing large length scales in which to observe,
this rather obscure and unplanned phenomenon converted my diffusion
experiment into one of convection.
I had not planned
to study Marangoni convection, however, it was so fascinating that
I spent the next few days of spare time making observations. Discoveries
made while exploring someplace unknown can fall in the realm of
something useful and worth more studies of something seemingly not
so practical although fascinating. However, even the obscure is
worth knowing. Imagine if you had planned a detailed series of fluid
experiments where you directed lights into an apparatus so that
video could be recorded. By not understanding the subtle effects
of Marangoni convection, you may unknowingly change the dynamics
of your experiment and thus record data riddled with unintended
effects. By investigating these effects, one is equipped with the
knowledge to design useful experiments for studying other things
without such unwanted behavior.
You never know
when a seemingly obscure and insignificant phenomenon may affect
something useful like the quality obtained when joining two pieces
of metal through the application of heat. | 
