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Expedition
Six Space Chronicles #8
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By:
ISS Science Officer Don Pettit
Studying
Water Films
Quite by accident,
we have made a most surprising observation. We planned to use the
castile soap and glycerin from my shaving kit to make thin films.
We wanted to see what thin films might have to offer in a weightless
environment and felt that it was a topic ripe for discovery. We
also have a copy of C. V. Boys book, Soap Bubbles, first published
in 1911, which is still a wonderful treatise on thin films and figured
that one should not be on space station without a copy. A water
solution with 2.5% castile soap and 15%glycerin was prepared. The
details of how this was stirred up in 0g without making a mess will
be the subject of another chronicle entry. For now, let us assume
there is 200 ml of this solution in a 0g beaker (what works well
for a beaker is also the subject of another chronicle entry). Stainless
steel safety wire 0.025" in diameter was made as an expandable wire
loop that could be continuously re-sized from 35mm to over 150mm
in diameter. The diameter was adjusted by pulling on a control wire
sort of like how a puppeteer controls his marionette, only instead
of seeing the mouth move and eyebrows wiggle you see the loop expand.
This served two purposes. First, it was difficult to work with the
size of a liquid-filled container needed to insert a large wire
loop, so this approach would let you start small and end big. Second,
by stretching a film, one could observe a number of delightful optical
interference effects as the film went through the process of thinning.
So with all
this in mind we prepared to work with the solution. However, we
never got to the soap solution due to being diverted with simply
water. To minimize the potential mess with the soap solution, a
"dry run" was made with only water. A bare wire loop 53mm in diameter
was submerged in the 0g beaker containing our onboard de-ionized
water. To my amazement, when the loop was withdrawn, a thin water
film clung tenaciously to the loop. I have never before witnessed
such a large-scale thin film of water. The film was thick by thin
film standards so perhaps it would be better called a macro-film.
It appeared to be about half the thickness of the loop wire diameter
placing it at about 300 micrometers.
These films
were surprisingly robust and could withstand numerous mechanical
tortures without breaking. Blowing on the film created ripples that
quickly dampened when the perturbations ceased. Oscillating the
loop through tens of centimeters with a period of about 2 seconds
distorted the film with patterns like seen in a soft rubber membrane
when driven by a sound oscillator. The displacement at the center
was several centimeters. These films proved to last over 12 hours
if left undisturbed.
The wire loop
could be expanded to 115mm before it broke. As the film expanded
and stretched, it reached a stage at about 70 to 80 millimeters
in diameter where ripples were no longer seen. Apparently, the film
thickness was thinning and surface tension forces were sufficiently
strong to prevent ripples. Closely spaced interference fringes were
seen in the stretched film demonstrating that the sides were becoming
flat and parallel. Based on the initial film diameter and thickness,
the film volume was about 660 cubic millimeters. Geometric arguments
gave the 115mm diameter stretched film a thickness of about 60 micrometers
or 100 times the wavelength of light. Thus by a simple measurement
of the initial and final diameters, an estimate on the final film
thickness was obtained. Even the stretched film was about 50 times
thicker than films made with soap solutions.
A water solution
of tracer particles previously prepared from 5 micrometer diameter
mica flakes was used to map out inter-film flow fields by selectively
placing a drop of this solution on a film with a coated canella
and observing the patterns of dispersion. Convective flows perpendicular
to the film plane were effectively excluded by the smallness of
that dimension. Without external perturbations, convection within
the film plane was not observable, leaving only diffusion to slowly
disperse the particles. Tracer particle patterns lasted for well
over 4 hours. As if viewed through a lens with aberrations, the
edges would become increasingly fuzzy from the slow effects of diffusion
as time progressed. Blowing air at an oblique angle to the film
through a canella would force film flow in the direction of the
air stream but the flow would dampen within a second when the perturbation
was removed. The canella would pass through the film causing minimum
distortion and allowed a means to impart an in-plane rotation. Once
stirred with an initial velocity of about one centimeter per second,
the resulting motion slowly dissipated over 10 minutes. The rotation
induced a small inward directed flow. When the motion stopped, a
spiral swirl of tracer particles was visible as if they were frozen
in ice.
A drop of red
food coloring, (left over from frosting our Christmas cake) was
placed in the center of a 300 micrometer thick film. Since the food
coloring is an alcohol-based solution, I expected the resulting
pattern to exhibit the effects of an in-plane concentration driven
convection, perhaps showing fingering patterns like seen on the
meniscus edge in a fine glass of wine. Instead, the red color hung
in a circular spot with edges that slowly became increasingly dispersed
as hours passed. A canella was used to gently blow on the red spot,
and it was discovered that the food coloring could be moved around
within the confines of the film much like finger-paint can be spread
by the fingers of a child. Small wisps of color with edges becoming
fuzzy with time would stay visible for several hours. Green, blue
and yellow food coloring were added making the film look like an
abstract canvas. I wonder what someone like Matisse could do with
this ephemeral medium? Eventually, all the colors blended together
yielding a rather dull looking green, perhaps the true color of
the universe?
Experiments
were made where water was added to and removed from a film. A syringe
was used to gently introduce water to the film. The film bulged
to perhaps 2 millimeters where the syringe tip was introducing water
and would quickly fan out within the film in a series of ripples.
Above a thickness of about two millimeters, the film sides were
notably curved instead of parallel and made literally a "meniscus
lens" with optical power. The focal length could be changed simply
by adding additional water. If a towel was carefully placed against
the outside of the wire loop it would withdraw water via capillary
action. Water was drawn off the film leaving it at its original
thickness of about 300 micrometers. It was difficult to make the
film thinner using a towel without causing breakage. Water was added
and withdrawn at the same time from opposite edges and if the flows
were balanced, a steady state was observed where the resultant in-plane
laminar flow rippled as it moved across the film. Film thickness
could be adjusted from about 300 micrometers to about 2 millimeters
by regulating the flows. Air bubbles present in the syringe water
were excluded from the film or would pop if incorporated in the
film.
When a new
film was drawn from the 0g beaker, air bubbles would also be largely
excluded. Sometimes a small bubble would become trapped in the film
or a bubble would be intentionally placed on the film. If the bubbles
were of the order of about two times the film thickness or larger,
they would pop after some tens of seconds. Apparently, capillary
forces caused the bubble wall that was exposed above the surface
of the film to drain into the film until it thinned to the breaking
point.
Diverted by
water films, our original intent to study soap films will have to
wait for another time. Observations of nature, no matter how seemingly
arcane, are like peeling off one more layer on the great onion of
knowledge, tickling your imagination with what you have found but
always revealing yet another tantalizing layer underneath. I hope
we never get to the core.
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