[Marangoni Exp]
Chaos, Turbulence and its Transition Process in Marangoni Convection

RESEARCH OVERVIEW

• The objective of scientific reaserch on Marangoni convection utilizing microgravity is to make clear the flow transition phenomena from steady to osicillatory, chaotic, and finally turburent flows. Therefore, it is important to understand an underlying principal of Marangoni convection. The finding and knowledge obtained through space experiment is applied to industrial processes as well as fluid physics.
• JAXA has been carrying out four Marangoni experiments to fully understand Marangoni convection in microgravity on board the ISS. Fundamental questions regarding Marangoni Convection are as follows; (1) when and how are the onset of unsteady (or oscillatory) convection determined, (2) What are the characteristics of unsteady and three-dimensional flow, and temperature fields? (3) What are the mechanisms that are responsible for the formation of particle accumulation structures (PASs)? Answering these questions through space experiment should contribute to the better understanding of Marangoni convection. It will complete in 2015.
• On the ground, we can see the only several millimeters liquid bridge because surface tension cannot support its self weight due to gravity. On the other hand, microgravity conditions provide us several advantages as follows; (1) Large and long liquid bridges can be formed, (2) Pure and ideal Marangoni convection can be observed. So, space experiment are expected to be solved the whole picture of Marangoni convection. This contributes to advance of basic science of a fluid physics directly. Moreover, the knowledge of Marangoni convection must be useful to improve the industrial process such as semiconductors, optical materials, bio materials, welding and micro/nano technologies and to increase the efficiency of thermal devices (i.e. heat pipe, evaporators/condensers).

DESCRIPTION

Marangoni flow is categorized in the natural convection same as buoyancy convection caused by density difference. A trait of Marangoni convection is a surface-tension-driven flow which driving force is localized at the only surface. Surface tension is the characteristic of a liquid in which it forms a layer at its surface so that this surface covers as small an area as possible. One can see the coin floating on the water. Surface tension is the force to be keeping the heavier coin on. In general, surface tension becomes strong with decreasing temperature. When a temperature difference exists along surface, the surface is pulled toward low temperature region. The surface tension difference is also produced under existing concentration distribution. May have heard or seen "tears of wine". It can be caused by Marangoni effect under concentration difference near the meniscus. Its effect was named after Italian physicist Calro Marangoni who mainly studied surface phenomena in 19th century.

Such a phenomenon is often observed in everyday life. For example, oil in a pan heated from center moves to peripheral side. Oil floating on water immediately moves when a surfactant (e.g. detergent) drops onto a part of the oil because of the imbalance in the surface tension. The detergent caused the center to have a lower surface tension. On the other hand, the outside has a higher surface tension, so the center and the oil were pulled out in all directions to equalize the surface tension. These phenomena are resulting from Marangoni effect.

Moreover, Marangoni convection affects the quality of grown crystal such as semiconductors, optical materials or bio materials. Therefore, it is important to understand an underlying principal and nature of Marangoni convection. The finding and knowledge obtained through space experiment is applied to industrial progression as well as advance of fluid dynamics. A liquid bridge configuration is often employed to investigate Marangoni convection because it is simulated a floating-zone method which is one of the crystal growth technique.

A liquid bridge (cylindrical liquid column) of silicone oil is formed into a pair of supporting solid disks. The convection is induced by imposing the temperature difference between disks, one end heating and other end cooling. Due to the convective instability, flow transits from laminar to oscillatory, chaos, and turbulence flows one by ones as the driving force increases. Scientists will observe the flow and temperature fields in each stage and investigate the flow transition conditions and processes. Fundamental questions regarding to Marangoni Convection are as follows;

• What are the conditions that determine the onset of unsteady (or oscillatory) convection in liquid bridge?
• What are the characteristics of unsteady, three-dimensional flow and temperature fields?
• What are the mechanisms that are responsible for the formation of dynamic particle accumulation structures (PASs)?

Answering these questions should contribute to the better understanding of the instability mechanisms of Marangoni convection.

Now, why do we need to conduct Marangoni experiment on board the International Space Station (ISS)? On the ground, we can see the only several millimeters liquid bridge because surface tension cannot support its tare weight due to gravity. On the other hand, microgravity conditions provide us strong advantages as follows;

• Large and long liquid bridges can be formed.
• Therefore, high Marangoni numbers can be realized.
• No density-driven convection exists.
• No gravity-induced deformation of liquid bridge exists.
• Very long period for experiment can be allotted utilizing the ISS.
• Quite precise data with a wide range of parameters can be obtained by utilizing these merits in space.

At the same time, a liquid bridge is very sensitive against even week vibration (called g-jitter) in the ISS because the liquid is not contained and is sustained by the only surface tension between supporting disks. Therefore, Marangoni Experiment is performed during a crew sleeping time (21:30-06:00 GMT) when the g-jitter becomes slightly calm.

In Marangoni Exp, Marangoni convection occurred in a liquid bridge is observed to make clear the flow transition phenomena resulting from a fluid instability. A silicone oil with a viscosity of 10 cSt(10 mm2/s), which is about ten times higher one of water, is employed as working fluid and is suspended between a pair of solid disks (50mm in diameter). Small amount of fine particles is mixed into liquid bridge for flow visualization. One of the disks is heated and another cooled to impose temperature difference on both end of the liquid bridge. The temperature difference is gradually enlarged in order to increase the driving force of a thermocapillary flow (Marangoni flow). The flow transits from steady to oscillatory flow at the certain critical temperature difference. With increasing the temperature difference, the convection becomes more complicated toward turbulent via chaotic flows . These transition processes are observed in detail.

We employ Fluid Physics Experiment Facility (FPEF) mounted in Ryutai Rack inside KIBO Pressurized Module. Experiment is conducted in combining FPEF and an experiment unique hardware which is exchangeable according to the purpose of investigation and is called "Experiment Cell". FPEF equips several cameras and Infrared Imager for flow and temperature visualizations. Three units of black and white CCD cameras are mounted near the heating disk to observe three dimensional flow pattern through the transparent sapphire disk. This system allows constructing 3-dimensional visualization of flow field using 3D Particle Tracking Velocimetry (3D-PTV) technique. A color CCD camera takes side view of liquid bridge to check the flow pattern and liquid bridge shape. An infrared imager is used to observe dynamic temperature distribution on the liquid bridge surface. Marangoni experiment also uses Image Processing Unit (IPU) and Microgravity Measurement Apparatus (MMA) with accelerometer to measure microgravity environment near the FPEF.

SPACE APPLICATIONS

The valuable knowledge from Marangoni space experiment is also applicable to the high performance heat exchanger and heat pipe both in the space and on the earth. For future space development, it should be necessary to more efficient and compact thermal management system, no doubt to help its development.

EARTH APPLICATIONS

The obtained knowledge on the Marangoni convection is vital for the production of high-quality crystal growth such as semiconductors, optical crystal so on. Since the surface tension is dominant not only under the microgravity but also in the micro-scale, the results obtained on the nature of the Marangoni convection will significantly contribute to various micro-fluid handling techniques in micro-TAS (Micro total analysis system) such as DNA examination and clinical diagnostics.

OPERATIONAL REQUIREMENTS AND PROTOCOLS

Number of experiment: 30 runs

Downlink of data: (Real-time)

• Five channel video images from observation cameras
• Telemetry concerning health & status of equipment and experimental data (e.g. temperatures, pressure)
  (Non real time)
  ・Five channel video play back
  ・Acceleration data file

Return of samples Hard disk drive units recorded high resolution video images As preparation for experiment by an ISS crew, the experiment cell is assembled and installed in FPEF. The experiment is operated from Space Station Integration and Promotion Center, Tsukuba Space Center by ground staffs.

The experiment procedure is as follows;

(1) Preparation of experiment

• Starting up FPEF, EC and Image Processing Unit (IPU)
• Setting of FPEF, observation systems
• Start of acceleration measurement

(2) Experiment

• Liquid bridge formation
• Adjustment of Volume ratio of liquid bridge
• Imposing the temperature difference between hot and cold disks to induce convection
• Video Recording
• Bubble removal if liquid bridge contains bubble

(3) Post experiment operation

• Liquid bridge formation
• Video play back down link and acceleration data files transfer
• Shut down FPEF, EC and Image Processing Unit (IPU)