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Expedition Six
Space Chronicles #18

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By: ISS Science Officer Don Pettit

Caldrons Bubble

Processes involving the contact of gasses and liquids with a solid wall are ripe for discovery in the reduced gravity environment of orbit. Such processes, although commonly experienced in everyday life, can quickly become complicated once the details are scrutinized, although it is often the understanding of the details that leads to new insights.

One such process we wanted to observe was boiling; however, we could not figure out how to do it safely until last week. We were setting up to make some test solder joints using our soldering iron inside a maintenance glovebox designed especially for such operations. The idea struck me, why not use the soldering iron to boil water in thin film geometry while inside the glovebox? By keeping the volume of water small, perhaps a few milliliters, this operation could safely be done with no fear of errant droplets of hot water causing damage.

A video camera was set up with an old tee shirt for a backdrop. It is important to record the observations with a background that enhances the view of the bubbles. A standard 50-millimeter diameter water film was made about 300 micrometers thick. A new solder tip was placed on the soldering iron. We did not want rosin residues from prior soldering to coat the water surface and change the surface tension dynamics.

Watch the video.

Having set all this up, we proceeded to slowly place the hot tip into the film. It popped. Again and again it was repeated with the same results. The hot tip would pop the film as soon as it made contact. We made thicker films and finally had success when the film was 2 to 4 millimeters thick. Three millimeters seemed like a good compromise with all subsequent work done at this thickness.

When the tip was inserted into the film, intense Marangoni convection was seen. A trail of small bubbles produced by the hot tip acted as tracer particles and made the convective flow visible. This Marangoni convection was significantly more violent than the mild eddies that were driven by my flashlight some weeks ago. It looked like the invisible spoon of Marangoni had been exchanged for an eggbeater.

The soldering iron tip did not seem to transfer sufficient heat to make the water boil. Small bubbles formed locally at the tip, however, a true boil was not produced. I was also dissatisfied with the geometry.

The tip was cone-shaped with two small flats creating a screwdriver-type blade. A rounded lip transitioned from the solder iron's conical tip to its cylindrical-shaped barrel. The water film wanted to crawl up the tip and hang at the transition zone. This created a rather complicated geometry for the water-wall contact zone. When you design an experiment, it is best to create a geometry that is simple and readily tractable with mathematics. Nature is complicated enough as it is, and it does not help matters to inflict more complexities into the analysis.

Since we had only a soldering iron for a heat source, it was decided to penetrate the film well past the tip so the cylindrical barrel was perpendicular to the film. Now we had a nice simple geometry consisting of a heated cylindrical wall, 8.5 millimeters in diameter, concentric and perpendicular to a film of about 3 millimeters thick. Not only did this create a simpler geometry, but it also created a greater contact area for heat transfer.

When the barrel was placed into the film, surface tension forces caused the water to crawl up and down the barrel thickening the heated zone to perhaps 5 millimeters next to the wall, so the flat nature of the film was somewhat distorted. Within minutes, there was a full rolling or nucleate boil in the film. We were observing another "wow" moment.

Small gas bubbles formed at the wall and grew to perhaps 1 to 3 millimeters in diameter before they were whisked away by convection. Once surrounded by cooler water, the bubbles shrank to less than a millimeter and moved in circular convection patterns that often brought them back by the wall, although they would not re-attach. They aided in knocking loose newly formed bubbles by colliding with them.

The convection appeared to be driven by two components. Marangoni convection began soon after heating and created well-developed, fixed convection patterns just as small bubbles of less than a millimeter were beginning to form. Some of the small bubbles stayed attached to the wall while others popped off and "convected" away. Once the nucleate boil formed, the expansion of the bubbles at the wall created a mass flow away from the surface and added to the Marangoni convection. This resulted in bubbles forming at the wall, growing to a few millimeters, and then popping off and moving away.

It appeared like the convective flow generated from the bubble expansion dominated over Marangoni convection. To the first order, it looked like boiling seen on the bottom of any kitchen pot in the process of making dinner. I was both amazed and perhaps a little disappointed that boiling in the absence of significant gravity was not more exotic. I was hoping for at least an oscillating system where the water in contact with the hot wall would be pushed away by an expanding gas film until it burst and allowed water to once again contact the hot wall. The universe obviously did not first consult me on how this should work.

These observations did incite some new thoughts. Nucleate boiling under natural convection is a well-studied industrial process, and as classically developed on Earth, has gravity-driven buoyancy terms in the equations used for calculating heat transfer coefficients. Gravity supplies the force needed to drive the bubbles away from the heated surface so more water can move in, thus creating a steady state boiling process. At the small scale of bubbles forming on a hot surface, maybe there are other forces present as witnessed here on orbit, that are initially more significant than gravitational forces. Perhaps these same forces are what cause bubbles to pop off the surface and then move outward where the grip of gravity-driven buoyancy takes over. However, maybe Earth's gravity is the dominating force all along, and the observations made here are simply caused by small tertiary forces kept at bay by gravity and only allowed to exercise some authority in its absence.

Such questions, spawned from the delights of dancing on the edge of the unknown, take one right back to where you first started, except this time, with a wiser smile.