The Microgravity Research Experiments (MICREX) Database

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Surface-tension-driven convection (also known as Marangoni convection) results because fluid elements on the liquid free surface with a low surface tension tend to flow toward fluid areas of higher surface tension. The surface tension of most fluids decreases as the fluid temperature increases. However, some fluids, such as aqueous alcohol solutions, have a surface tension minimum when plotted with temperature: surface tension decreases with increasing temperature down to a minimum temperature (and minimum surface tension) and then begins to experience an increase in surface tension with temperature.

On Earth, both surface-tension-driven convection and buoyancy-driven convection act on the fluid system. In a reduced-gravity environment, the Marangoni convective contributions should be more easily discerned. "Thus, when the motor of the convection is the thermocapillary effect alone, it is expected that at temperatures around the minimum, the convection will slow down and that the onset of oscillatory modes...will appear for higher thermal constraints." (1, p. 301) An imposed temperature difference, (delta T) = T1 and T2, parallel to a liquid-gas interface, will cause flows in the adjoining phases. It had been observed that, if T1 and T2 are lower than the Tmin then surface flow is from the hot side to the cold side. The flow is opposite if T1 and T2 are above Tmin.

This TEXUS 8 experiment was the first in a series of investigations designed by Legros et al. to study the effect of a surface tension minimum on thermocapillary convection. The specific objective of the experiment was to observe fluid motion under an imposed temperature difference such that T1 and T2 are above Tmin.

The experiment was performed in the TEXUS Experiment Module TEM 06-6. The module contained a rectangular stainless steel cell with two Pyrex observation windows. The cell was 3 x 10 -2 m long by 2 x 10 -2 m high by 10 -2 m wide. A n-heptanol solution (6.3 x 10 -3 molal, Tmin = 40 ¡C) containing 90-micron diameter Dow latex particles was used as the test fluid.

During the low-gravity phase of the mission, the cell was partially filled by activating a slider between the empty cell and a fluid reservoir. A pneumatic piston slowly pushed the fluid into the cell. "Small Teflon pieces in the metallic walls and grooves coated with Teflon in the Pyrex windows succeeded in stopping the creeping of the fluid. After the filling, the depth of the liquid in the middle of the surface was 6.6 x 10 -3 m and the radius of curvature of the surface along the meridian plane was 2.95 x 10 -2 m." (1, p. 305) Opposite sides of the cell were differentially heated imposing a temperature gradient parallel to the liquid-gas interface. Both side temperatures were above the Tmin temperature of 40 ¡C (placing them in the range that the surface tension increases with the temperature of the aqueous solution). A 16-mm camera was used to film (1) the filling of the cell and (2) the resultant convective motion (25 frames/sec).

Post-flight examination of the thermal data indicated the temperature of the hot side was a constant 66 ¡C (plus or minus 0.1 ¡C) and that of the cold side was 46 ¡C (plus or minus 0.1 ¡C).

Reportedly, the initial concentration of the latex particles was not constant. However, this allowed a good visualization of the fluid stream lines because of elongation of the high concentration regions.

Reportedly, the experiment was a success, leading to the following conclusions:

(1) The Teflon pieces in the stainless steel walls and Teflon coated grooves in the Pyrex prevented the fluid from creeping up the walls and resulted in a rather flat gas-liquid interface.

(2) The observed fluid flow was from the cold to the hot side, as expected.

(3) The convective flows were initiated at the interface near the hot side and developed slowly. After about 6 minutes, a single flow cell occupied the entire volume.

(4) At any particular time the velocities at the interface were dependent upon position with respect to the thermal gradient. The velocities were higher nearest the hot wall and decreased at a rate of -0.0136 x 10 -3 m/s 2 (velocity/time).

(5) The velocities near the interface were higher than those in the bulk liquid.

From these results (see Reference (1) for more details of the velocity fields), it was determined that (within the temperature range of the experiment) "...the surface tension is increasing with temperature, indirectly indicating that a surface tension minimum is present at a lower temperature, and thus is still existing under such non equilibrium convective conditions." (5, p. 135)

Reportedly, the amount of reduced-gravity time that the TEXUS rocket could produce was too short to achieve steady state conditions. Thus, the surface velocities were still increasing at the end of the low-gravity period.

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The specific reasons why n-heptanol was selected were not detailed in the available publications. It is believed that an n-heptanol aqueous solution was used because of its particular surface tension properties.

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(2) Legros, J. C., Limbourg-Fontaine, M. C., and PŽtrŽ, G.: Influence of Surface Tension Minimum as a Function of Temperature on the Marangoni Convection. Acta Astronautica, Vol. 11, Number 2, 1987, pp. 143-147. (preflight; experimental setup)

(3) Legros, J. C., PŽtrŽ, G., Limbourg-Fontaine, M. C., Villers, D., and Platten, J. K.: Thermocapillary Convection Around a Surface Tension Minimum: A Microgravity Experiment (TEXUS 8) and Numerical Simulation. 5th Elgra Meeting Proceedings in Elgra News, 6, 1984, pp. 41-49.

(4) Legros, J. C., PŽtrŽ, G., and Limbourg-Fontaine, M. C.: Study of Marangoni Convection Around a Surface Tension Minimum Under Microgravity Conditions. Advances in Space Research, Vol. 4, Number 5, pp. 37-41, 1984. (post-flight)

(5) Legros, J.C., Limbourg-Fontaine, M.C., and PŽtrŽ, G.: Surface Tension Induced Convection in Presence of a Surface Tension Minimum. In Scientific Results of the German Spacelab Mission D1, Norderney Symposium, Norderney, Germany, August 27-29, 1986, pp. 131-140. (post-flight)

(6) Influence of a Surface Tension Minimum on the Marangoni Effect. In Summary Review of Sounding Rocket Experiments in Fluid Science and Materials Sciences, TEXUS 1 to 20, MASER 1 and 2, ESA SP-1132, February, 1991, pp. 60-61. (post-flight)

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