[Dynamic Surf]
Experimental Assessment of Dynamic Surface Deformation Effects in Transition to Oscillatory Thermo capillary Flow in Liquid Bridge of High Prandtl Number Fluid

RESEARCH OVERVIEW

The objective of scientific research on Marangoni convection utilizing microgravity is to make clear the flow transition phenomena from steady to oscillatory, chaotic, and finally turbulent flows. Therefore, it is important to understand an underlying principal of Marangoni convection. The findings and knowledge obtained through the 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 the space experiments should contribute to the better understanding of Marangoni convection. The investigation is to be completed in 2016.

On the ground, only several millimeters of the liquid bridge can be seen, because the surface tension cannot support its own weight due to gravity. On the other hand, microgravity conditions provide some advantages: (1) Large and long liquid bridges can be formed, and (2) Pure and ideal Marangoni convection can be observed. As result, space experiments are expected to solve the whole picture of Marangoni convection. This directly contributes to the advancement of knowledge in the basic science of fluid physics. Moreover, the knowledge of Marangoni convection is useful in improving industrial processes such as the production of semiconductors, optical materials, bio materials, welding and micro/nano technologies, and in increasing 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.

Why is the Marangoni experiment conducted 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 Dynamic Surf, 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 5 cSt(5 mm2/s), which is about five times higher one of water, is employed as working fluid and is suspended between a pair of solid disks (10mm 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.

The Fluid Physics Experiment Facility (FPEF) mounted in Ryutai Rack inside KIBO Pressurized Module is used. The 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". A white and black CCD camera is mounted on the experiment cell to observe the flow patterns. And, Dynamic surface deformation (DSD) is measured using a microscopic imaging displacement meter (MIDM), which is newly-developed by science team. The hardware for MIDM, consisting of a monochromatic CCD camera, a microscopic lens, and a back illumination light source, is installed in the custom-built EC. FPEF equips Infrared Imager for temperature visualizations. 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 the Marangoni space experiment is also applicable to the high performance heat exchangers and heat pipes both in space and on 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 microgravity conditions, but also in the micro-scale, the results obtained on the nature of the Marangoni convection can significantly contribute to various micro-fluid handling techniques in micro-TAS (Micro total analysis system), such as DNA examination and clinical diagnostics.

KAMOTANI Yasuhiro [Case Western Reserve University]