[Marangoni UVP]
Spatio-temporal Flow Structure in Marangoni Convection

RESEARCH OVERVIEW

The objective of the scientific research on Marangoni convection in microgravity is to better understand the flow transition phenomena from steady, to oscillatory, chaotic, and finally turbulent flows. Therefore, it is important to understand the underlying principal of Marangoni convection. The findings and knowledge obtained through this experiment can be applied to industrial processes, as well as fluid physics.

JAXA has been conducting four different Marangoni experiments to fully understand Marangoni convection in the microgravity environment 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 should contribute to the better understanding of Marangoni convection. This investigation is scheduled for completion in 2016.

On the ground, only several millimeters of the liquid bridge can be observed because the surface tension cannot support its weight due to gravity. However, microgravity conditions provides advantages: such as (1) larger and longer liquid bridges can be formed, and (2) pure and ideal Marangoni convection can be better observed. So, in this experiment, great insight into the process of Marangoni convection is expected to be learned, contributing to the advancement of basic fluid physics. Moreover, the knowledge of Marangoni convection can be useful in improving the industrial process used to make items such as semiconductors, optical materials, bio materials, welding, and micro/nano technologies. Also, the better understanding of Marangoni convection can be applied to increase the efficiency of thermal devices, such ad heat pipes, evaporators, and 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.

Many 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 simulates the "floating-zone method", which is one of the techniques for crystal growth. 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 is there a need to conduct Marangoni experiment on board the International Space Station (ISS)? On the ground, only several millimeters of the liquid bridge can be seen, because surface tension cannot support its tare weight due to gravity. On the other hand, microgravity conditions provide 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 UVP, Marangoni convection 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 than that of water, is employed as working fluid and is suspended between a pair of solid disks (50 mm in diameter). Small amounts of fine particles are mixed into the liquid bridge for flow visualization. One of the disks is heated, and another cooled, to impose a temperature difference on both ends 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 a certain critical temperature difference. With increasing temperature difference, the convection becomes more complicated toward turbulent via chaotic flows. These transition processes are observed in detail.

The Fluid Physics Experiment Facility (FPEF) is used and mounted in the Ryutai Rack inside the KIBO Pressurized Module. The experiment is conducted in combining FPEF and experiment unique hardware which is exchangeable according to the purpose of investigation and is called the "Experiment Cell". The FPEF is equipped with several cameras, and an 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 for constructing a 3-dimensional visualization of the flow field using the 3D Particle Tracking Velocimetry (3D-PTV) technique. A color CCD camera takes side views of the liquid bridge to check the flow pattern and liquid bridge shape. An infrared imager is used to observe the dynamic temperature distribution on the liquid bridge surface. Two Ultrasonic Velocity Profilers (UVPs) are embedded in the cooling disk. UVP is a method to measure an instantaneous velocity profile in a liquid flow by echography. This technique is expected to reveal spatio-temporal flow structure. The Marangoni experiment also uses the Image Processing Unit (IPU) and Microgravity Measurement Apparatus (MMA) with accelerometer to measure microgravity environment near the FPEF.

SPACE APPLICATIONS

The Marangoni investigation explores the effect of surface tension gradients on fluid flow in space. Understanding this type of fluid flow is applicable to heat exchangers and heat pipes used on the International Space Station. By studying Marangoni convection in microgravity, researchers can gain new insight into this fundamental property of mass flow and heat transfer. Results could inform future development of thermal management systems for future space missions.

EARTH APPLICATIONS

Marangoni convection occurs where a temperature difference exists along the surface of a liquid. This can be observed in any kitchen or wine glass: oil in the center of a hot pan moves to the side, for instance. The “tears of wine” viewed in the shadow of a wine glass results from the lower surface tension of alcohol as compared to water. In industrial processes, Marangoni convection is important for the production of high-quality crystals, such as those used in semiconductors and optics. The interface between liquid and gas is also relevant to a wide range of micro-fluidics applications, including DNA studies and clinical diagnostics.

OPERATIONAL REQUIREMENTS AND PROTOCOLS

Number of experiment: 25 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, velocity profile by UVP, pressure) (Non real time) - Five channel video play back - Acceleration data file Return of samples - None

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 - Measument of velocity profile by UVP - Bubble removal if liquid bridge contains bubble (3) Post experiment operation - Retrieving liquid bridge - Video play back down link and acceleration data files transfer - Shut down FPEF, EC and Image Processing Unit (IPU)