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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over and then withdrawn from a 50 millimeter diameter wire loop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withdrawn, a fat film of perhaps 4 millimeters thick was produced.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.
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