The Microgravity Research Experiments (MICREX) Database

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This STS Spacelab D1 experiment was the sixth in a series of investigations designed by Schwabe et al. to examine oscillatory, thermocapillary Marangoni convection (see Schwabe, TEXUS 3, TEXUS 3b, TEXUS 5, TEXUS 8, and STS-007).

The D1 experiment was different from all of Schwabe's earlier efforts in that it employed an open cavity (often referred to as an open boat) instead of a floating zone system. An open cavity has one free surface along the top of the container. When the walls of the cavity are differentially heated (thus creating a thermal gradient along the free surface) thermocapillary convection occurs at the surface. "The main advantage of...[the open cavity] configuration compared to a float zone is the visibility of the region near the free surface." (1, p. 92) The specific objective of the experiment was to study low-gravity thermocapillary-driven flow in the cavity.

The experiment hardware consisted (in part) of a cuvette fashioned from quartz glass (20 mm X 20 mm X 20 mm). Two aluminum heaters were placed at opposite ends of the cuvette. Prior to launch, the small cavity between the heaters was filled with liquid paraffine tetracosane (C24H50). The liquid, which has a melting point of 50.9 ĄC, was in a solid state during the STS launch and was heated to a liquid state during the low-gravity mission. Thus, the cavity did not have to be filled during the mission (which, reportedly, would have been a difficult task).

"All rims which were in contact with the fluid were made as sharp as possible and coated with NUFLON (trademark of Fluomicron), which is not wetted by the paraffine. The NUFLON acted as antispreading barrier against capillary outflow." (4, p. 122) Further, a double-walled heatshield made from plates of quartz glass, surrounded the fluid sample and guaranteed a temperature gradient mainly parallel to the free surface.

A special visualization system, which could illuminate a central vertical section of the cuvette with a light band, made it possible to document the position of ceramic tracer particles in the fluid with a 16 mm motion picture camera.

During the mission, the paraffine was melted and then the temperatures of both heaters were held at 60 ĄC for approximately 1 hour. While the right heater remained at 60 ĄC, the temperature of the left heater was raised in 5 ĄC steps to 120 ĄC. Then, the temperature of the right heater was raised to 90 ĄC, stabilized, and then raised in 10 ĄC steps to 110 ĄC.

After each temperature increase, there was a "thermalisation" time of approximately 15 minutes. After this thermalisation, the camera was turned on for 45 seconds and the fluid motion was documented by the camera. "Thus, the experiment was programmed to observe the convection for...[a temperature difference of] 0 ĄC, 5 ĄC,...55 ĄC, 60 ĄC with the right heater at 60 ĄC, and for...[a temperature difference of] 30 ĄC, 20 ĄC, and 10 ĄC with the left heater at 120 ĄC." (4, p. 123) The Principal Investigator reported the heat transfer was determined via thermocouples.

Post-flight analysis of the film indicated that the sample melted as expected. When both heaters were set at 60 ĄC, the fluid was clear, without bubbles, and the tracer particles were well illuminated. However, the tracers did not move "...towards the middle of the liquid surface which is the coldest spot when both heaters are at 60 ĄC. Some tracers... [stuck] to the liquid surface." (4, p. 123)

Further, during the time when the temperature difference between the two heaters was increased (from 5 ĄC to 55 ĄC), the expected motion of the tracers from hot to cold along the free surface was not observed. Strong motion in the expected direction occurred only for a temperature difference of 60 ĄC.

The suppression of the expected Marangoni convection was attributed to an unclean free surface: dirt present on the free surface acted like a solid layer, hindering the thermocapillary flow. At a temperature difference of 60 ĄC, the dirt layer ruptured, thermocapillary flow was realized "...and the flow compressed the dirt in front of the cold wall (and the dirt mixed with the bulk liquid.)" (4, p. 124)

"Such a stable solid skin on the free surface has never been observed in experiments on [the] ground. On [the] ground there is always some fluid motion due to buoyancy which hinders the formation of a 'solid' and complete skin of dirt." (4, p. 123) In contrast, in the low-gravity environment, the dirt was allowed to diffuse from the bulk liquid to the surface during the hour that both heaters were at 60 ĄC.

Photographs of the low-gravity fluid motion (for a temperature difference of 60 ĄC) further illustrated that the free surface was bent, and that the cuvette was no longer filled to the rim. The anti-wetting barrier failed at the hot block and some of the fluid leaked out at that location. One large convection cell, which occupied almost the whole volume, moved from hot to cold at the free surface driving the flow in the bulk fluid from cold to hot. This was the motion expected for thermocapillary convection (in the absence of strong gravity-driven forces). The convection cell was asymmetric because of the dirt skin compressed at the colder heating block ("...the flow dives down before reaching the cold wall." (4, p. 124))

A 1-g reference experiment coupled with an analytical simulation of the flow fields (which included the bending of the free surface and a dirt-skin in front of the cold heating block) illustrated that "...the flow under normal gravity is much more complex than the flow under microgravity. The fluid volume is occupied by more than one convection roll. In the upper part we have two vortex centres near the free surface.... Both vortex centres are nearer to the free surface than the one under microgravity because it is hot fluid circulating near the free surface which cannot dive down under normal gravity. The fluid circulation near the free surface is mainly driven by surface tension forces. The flow very near the hot wall and very near the cold wall circulating along the bottom of the cuvette is due to buoyancy forces." (4, p. 124)

An analysis of the nondimensional numbers governing the fluid system indicated that thermocapillary forces dominate in the low-gravity experiment. This was confirmed by the low-gravity observations. Further, velocity profiles of an element at the free surface going from hot to cold indicated that "...under microgravity the maximum flow speed in the free surface reaches 20...[mm/s] whereas it is only 10...[mm/s] under normal gravity." (4, p. 126) Other interesting velocity profile results are presented in Reference (4).

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(2) Schwabe, D. and Lamprecht, R.: Marangoni-Konvection im offenen Boot (MKB). Naturwissenschaften, 73.Jahrgang Heft 7, July 1986, pp. 350-351. (abstract in English, text in German; post-flight)

(3) Schwabe, D., Lamprecht, R., and Scharmann, A.: Marangoni Convection in an Open Boat. In Scientific Goals of the German Spacelab Mission D1, WPF, 1985, pp. 55-56. (preflight)

(4) Schwabe, D., Lamprecht, R., and Scharmann, A.: Marangoni Convection In an Open Boat. In Proceedings of the Norderney Symposium on Scientific Results of the German Spacelab Mission, D1, Norderney, Germany, August 27-29, 1986, pp. 121-126. (post-flight)

(5) Hamacher, H., Merbold, U., and Jilg, R.: Analysis of Microgravity Measurements Performed During D1. In Proceedings of the Norderney Symposium on Scientific Results of the German Spacelab Mission D1, Norderney, Germany, August 27-29, 1986, pp. 48-56. (post-flight; acceleration measurements)

(6) Schwabe, D., Lamprecht, R., and Scharmann, A.: Experiments on Steady or Oscillatory Thermocapillary Convection in Space with Application to Crystal Growth. In Physicochemical Hydrodynamics-

Interfacial Phenomenon, Ed. M. J. Verlade, NATO ASi Series B: Physics, Vol. 174, pp. 291-310, Plenum Press, New York, 1988.

(7) Schwabe, D. and Scharmann, A.: Marangoni and Buoyant Convection in an Open Cavity Under Reduced and Under Normal Gravity. Adv. Space Res., Vol. 8, No. 12, pp. 175-185, 1988.

(8) Schwabe, D., Lamprecht, R., Mibelcic, M., and Scharmann, A.: Thermocapillary and Buoyant Convection in an Open Cavity Under Normal And Reduced Gravity. Submitted to Journal of Fluid Mechanics.

(9) Schwabe, D.: Surface Tension Driven Flow in Crystal Growth Melts. In Crystal Growth Properties and Applications, Vol. 11, pp. 75-112, Springer, Berlin.

(10) Input received from Principal Investigator D. Schwabe, August 1989 and August 1993.

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