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IMAGE: View of surface tension demonstration using water that is being held in place by a metal loop.
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Don Pettit Space Chronicles

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.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 05/14/2003
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