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View of a surface tension demonstration using water and food coloring held together with a metal loop.
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Don Pettit Space Chronicles

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 To my amazement, when the loop was withdrawn, a thin water film clung tenaciously to the loop.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.

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