Miscible Fluids in Microgravity (MFMG
In March 2003, NASA called for zero-upmass experiments that could be performed on the ISS with materials already on board the station. We proposed an experiment called Miscible Fluids in Microgravity (MFMG).
We had two scientific objectives:
1) To determine if isothermal miscible fluids can exhibit transient interfacial phenomena similar to that observed with immiscible fluids. Specifically, we proposed that a stream of a miscible fluid would breakup into drops as a stream of oil would in water from the Rayleigh-Tomotika instability. We also proposed that a blob of water in honey could spontaneously become spherical as happens with immiscible fluids. Of course, the drops and streams eventually would dissolve but during the slow diffusion process, these transient phenomena might occur.
2) To determine if miscible fluids in a thermal gradient can exhibit transient interfacial phenomena similar to that observed with immiscible fluids. Specifically, we proposed that a stream of honey in water could migrate and/or breakup in a temperature gradient and blob of water in honey could also migrate for it is well known that immiscible drops and bubbles migrate in thermal gradients.
Bessonov et al. showed that a drop can migrate in a temperature gradient if the square gradient parameter is temperature dependent . However, even if the square gradient parameter is temperature independent, convection can occur if the diffusion coefficient is temperature dependent . A gradient in temperature will mean a gradient in the diffusion coefficient. As time passes, this will cause a gradient in the width of the transition zone between the honey and water, which will cause a gradient in the effective interfacial tension. This should lead to migration of a drop or stream toward the high temperature region.
We then had to identify potential fluids and develop a means to perform the experiments, including creating a temperature gradient. We determined that one fluid would be water, and so we evaluated several other water-soluble fluids that are on the ISS, including rinseless shampoo, Edge shaving gel, ultrasound gel, and honey. We determined that honey and water would be the best miscible fluids because the astronauts had ready access to Russian honey for their tea and the properties were closest to a Newtonian fluid (although it is not one).
The next challenge was to determine the vessel for performing the experiments. We created a vessel from urine collection syringes and crew drinking straws (Fig. 2). The syringes had to be modified by using a Leatherman tool to make the plunger able to be moved the entire length of the syringe. Also, the plunger had a large void, which would have allowed a large bubble to be present. Duxseal, a ventilation duct repair compound, was used to fill this void.
Fig. 2 Left: A modified urine collection syringe. The marks are 1 cm apart. Right: The plunger of the syringe was filled with Duxseal.
The diameter of the syringe is 1.4 cm.
We also needed a means to create a temperature gradient across the syringe.
With the help of Bioserve Space Technologies, we decided to use the Commercial
Generic Bioprocessing Apparatus (CGBA), which can maintain a temperature
of 37 ęC, although it cools down once it is disconnected and moved.
Video was taken of the experiments. And some cases, we were able to watch the experiment in real time. The crewmember also took digital still images at regular intervals or used the intervalometer on the camera.
There were several serious limitations to performing the experiments under these constraints. We were neither able to choose optimal fluids nor able to characterize the fluids used. We did obtain a can of honey similar to the ones on the ISS but the viscosity we measured can only be taken as a reasonable estimate of the viscosity of the honey on the ISS. Also, we had no precise equipment to measure weak fluid motion and small displacements.
Because we were not able to train the crewmembers and four different crewmembers performed experiments, each crew member had to learn how to do the experiment based on our written procedures and in real-time discussions.
Honey can also crystallize and so astronaut Dr. Michael Foale heated the honey in the crew food warmer in January 2004 to make sure the honey would be liquid. Mike Foale modified our procedures by using a large syringe to store all the honey from the can. He also developed an elegant procedure for eliminating bubbles in the syringes by slinging the syringes on a string to use centrifugal force to move the bubbles. Still, it was very difficult to introduce one fluid into another without air being entrained. Bubbles pose a serious complication because of the occurrence of surface-tension induced convection . Bubbles can migrate in a temperature and/or concentration gradient [27-29].