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IMAGE: View of surface tension demonstration using water that is being held in place by a metal loop.
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 #17

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By: ISS Science Officer Don Pettit

Symphony of Spheres

Once again, I set out to explore one thing and discovered another. And what was discovered was more interesting than the prosaic task I had planned. What I had intended was to make a sphere of water about 50 millimeters in diameter and blow a bubble inside of increasing size until a thin spherical shell was made. As the bubble grew in diameter, would it center itself inside the sphere? I did not know and intended to find out so our shower-hygiene area was once again turned into a wet chemistry laboratory.

A new and improved technique was used to make the water sphere. First, about 25 milliliters of water were squirted into a small plastic bag. This was placed over and then withdrawn from a 50 millimeter diameter wire loop leaving a thin film. By squeezing extra water out as the bag was withdrawn, a fat film of perhaps 4 millimeters thick was produced. Water was then added to this film inflating it into an undulating sphere. Small occluded air bubbles were sucked out with a coated cannula attached to a syringe. After about five to ten minutes, the sphere had settled down into a nice crystal ball. I marveled at how simple a lens this made and how tiny details in my fingerprints were seen with a somewhat aberrant view.

Next an air bubble was injected inside using the cannula and syringe. I expected a single bubble to grow on the end of the cannula as air was slowly injected into the sphere. What happened was different. A bubble formed on the cannula tip and after it grew somewhere between 15 to 20 millimeters in diameter, it moved from the tip and attached to the side of the cannula, crawling towards the syringe until it broke away from the tip. A new bubble immediately formed, grew and detached from the tip. This process repeated leaving a number of 15 to 20 millimeter diameter bubbles. It seemed that a single large air bubble did not want to form.

With some effort, a single bubble of about 30 millimeters in diameter was finally made by coaxing the smaller bubbles to coalesce. This bubble did not want to remain centered. It preferred to be attached to the outer surface. It could be centered with the cannula but then small fluid motions within the sphere would bring it back to the surface. After some experimenting, it was found that the best way to make a single bubble and have it remain centered was to first stir the sphere into a slow rotation and then inject the bubble along the axis of rotation. The fluid dynamic forces associated with the rotation allowed for a single bubble to cling to the cannula tip and grow to the desired diameter while remaining nearly centered. After about ten minutes the rotation stopped and the bubble stayed approximately in the center of the sphere.

The next step was to inflate the bubble to larger diameters. Submerging the cannula tip in the water but holding it about 5 millimeters from the bubble, air injected at a rate of about 50 milliliters per minute formed small bubbles that quickly contacted and collapsed into the large bubble. This process drove steady state oscillations in the large bubble creating a series of standing waves with a wavelength of about 6 to 10 millimeters. The bubble became warped into a football where each end became a node point for the oscillations. When it oscillated, it reminded me of squirming bark beetle grubs kicked out from a rotting Douglas fir stump.

But the most amazing phenomena appeared after the air injection stopped and the oscillations died down. Created inside the air bubble were a half-dozen or so small spherical droplets of water, one to four millimeters in diameter, orbiting around like a miniature solar system. They ricocheted off of the inner surface of the bubble and continued to swirl around inside their little world. Perhaps once out of every six to eight collisions with the interface, a small amount of water was absorbed from the drop, resulting in a dramatic increase in velocity and an abrupt change in direction of the remaining droplet. It was as if a small rocket thruster propelled the droplet with an impulsive force perpendicular to the interface. As the droplets became smaller and smaller, their velocity continued to increase until the subsequent collision caused complete absorption. This motion appeared almost life-like so that for a minute I thought we were looking through a magnifier at some new form of creatures zooming around inside of a three-dimensional petri dish. I let out such a loud shout of exclamation that a crewmate from the next module came over to see if I was all right.

Soon we were all marveling over the show inside this fluid crystal ball. Of all the things on orbit I have seen to date, this is by far the most amazing. If one were to postulate that this phenomenon would occur, your colleagues would shake their heads and quickly find some excuse to dash off to another laboratory. It is one more example that demonstrates how nature has such a vivid imagination, more so than human beings will ever possess, and the only way to discover what nature has to offer is to seek it ourselves in the wilderness of the unknown.

We found that the easiest way to produce these droplets was to directly inject water into the bubble using a coated cannula. It was easy to penetrate both the water sphere and the bubble without significantly disrupting things. The droplets could readily be injected with various diameters at different velocities and directions. If given a velocity tangential to the bubble interface, the droplets moved around in circles, pressed against the inside by the forces from the radial acceleration of circular motion. Sometimes this radial force was sufficient to create a small flat spot at the point of contact, particularly for larger droplets of 6 to 8 millimeters in diameter. They either rolled, or perhaps scooted - I did not know which at this time - for several minutes before becoming absorbed. After the first absorption event, the remaining portion of the droplet was propelled away from the surface and ricocheted around inside of the bubble as if it were a ball in a three-dimensional game of billiards.

Why do some droplets undergo many collisions or roll on the interface for minutes and then exchange a fraction of their volume while others may become completely absorbed after one collision? It was almost as if there is some active repulsion force distributed over the droplet surface and when it wears thin absorption takes place. Perhaps there was some induced charge on the outer surface that keeps the interfaces at a distance sufficient to prevent coalescence. Like the fluid mechanical version of scuffing your feet across a carpet and sparking your dog on the nose, fluid friction at the wall of a non-conducting tube can cause the liquid to become charged. Friction mechanically separates electrons from their atomic hosts and a static charge of hundreds to thousands of volts can be produced. If you are pumping fuel into an airplane, this effect can create a spark and a fire, thus refueling operations require a grounding wire to shunt away the flow-induced static charge. This type of flow-induced charging may cause the droplets to become charged, especially when injected through a small diameter coated cannula (inside diameter of 200 micrometers). If present, this residual static charge could result in a repulsion force across the interface and keep the droplets at a distance sufficient to prevent immediate coalescence. When this charge wears thin, then the process of coalescence can begin.

A gas bubble or droplet has an internal pressure that arises inside due to surface tension forces manifesting themselves across the curved interface. This pressure differential is inversely proportional to the radius of curvature so the smaller the droplet, the greater the internal pressure. When the droplet becomes so small that the internal pressure matches the vapor pressure of the liquid, the droplet is no longer stable and evaporates in a flash. This critical radius was discovered by Lord Kelvin and now bears his name.

The pressure inside these droplets has to be greater than in the surrounding water across the bubble interface due to the droplet's smaller radius of curvature. Thus when the exchange of water occurs it must be from the droplet to the surrounding water. This exchange of mass from the droplet across the interface imparts momentum to the droplet like a miniature rocket thruster. The exchange of mass happens quickly; from examining individual video frames taken at a rate of about 30 per second, the exchange happens in less than one frame. This bounds the mass exchange to somewhere less than about 33 milliseconds. The exchange of mass thus creates a force impulse and results in a velocity component perpendicular to the interface. What remains of the droplet shoots across the inside of the bubble as if it were some sort of darting organism.

To observe the mixing in the water during the droplet mass exchange, food coloring was diluted at a ratio of about 100 to 1 and injected as a droplet into a bubble. It was obvious right away that the presence of the food coloring affected the process of coalescence. The food-colored droplets were completely absorbed after one to four collisions, making it difficult to observe the process since it was over after such a short time. Perhaps the addition of food coloring altered the magnitude of the droplet's static charge. The water used for this experiment came from our water reprocessor that makes high resistivity water through an ion exchange process. The components in food coloring may have added ions that increased the resistivity and thus prevented the static charge from forming.

As the food-colored droplet rattled around inside the bubble, my jaw dropped as we saw that each mass exchange shot a small vortex ring into the surrounding water. The vortex ring's size and velocity depended on how much water was transferred during a collision. Sometimes a series of little tiny rings shot out and stopped within the body of the surrounding water. Sometimes a large mushroom cloud formed that looked like a miniature nuclear explosion. This was most definitely another "wow" moment. Again, nature was tickling our imaginations.

Since the food coloring dramatically shortened the droplet lifetime, we next tried a suspension of tracer particles. This was a dilute solution of water with 5 micrometer diameter mica flakes. These flakes are made commercially by the ton for putting the sparkle in eye shadow and other cosmetics. It is well known among fluid dynamists that they also make great tracer particles for fluid experiments. Unfortunately, this knowledge forever changes your view of sparkling eyelids. Tracer-laden particle droplets lived through more collisions than those with food coloring, however their lifetime was still significantly shorter than for pure water. Each collision with mass exchange created a small glittering vortex ring in the surrounding water. Again, the effect was most definitely a "wow" moment as these vortex rings shot out from the bubble into the water leaving iridescent angelic-looking halos.

An observation on a drop about 5 millimeters in diameter answered the question of rolling or scooting when a droplet moves in circles. The tracer particles revealed that this droplet was definitely rolling with no observed slippage. It reminded me of a bowling ball rolling down a never-ending circular alley in search of pins. Up until this observation, I assumed the droplets were scooting on the interface more like a hockey puck on ice. For the droplet to be rolling, there must be some frictional force between the two bodies and this implies some level of interesting interactions across the interface. Perhaps this is a manifestation of the fluid dynamic no-slip boundary condition. However, this interaction cannot be so strong, else it causes coalescence.

Left in the wake of these observations were more questions than answers and a mind full of wonder. I have found that this is not unusual for someone engaged in exploration.

Will anything practical come from this? For my symphony of spheres, perhaps not. Will this discovery tickle our imaginations and enrich our minds? Most definitely, yes. Will it incite new ideas for future discoveries? Maybe. My personal reward for undertaking new explorations is simple and does not require a practical application.

When you enter the realm of the unknown, you see things in a truly naive state where prior knowledge is of little help; you can once again see the world through the wondrous eyes of a child.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 05/13/2003
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