Thermocapillary Force using Acoustic Levitator

Research on thermocapillary force in a levitated droplet is under development in the Advanced Thermal Science Laboratory in MAE at WMU to conduct the research eventually using the microgravity facilities such as space shuttles and International Space Station (ISS). The idea here is to simulate droplets in weightless at Earth gravity using a commercialized instrument called Ultrasonic or Electostatic Levitator, which can levitate a 2-3 millimeter liquid droplet free in air. A sting heater then approaches to the proximity of the droplet so that temperature differences form along the droplet surface. As a result of that, thermocapillary force exerts along the droplet surface and induces an internal flow in the droplet.

Two liquid droplets in a commercialized Levitator

If fluorescent tracer particles are suspended in the droplet, the flow motion can be recorded by a high-speed video camera. This would allow to further study thermocapillary force in the droplets. This technique has been recently developed in materials science, e.g. on glass and metal solidification . The high-speed video camera is available for this purpose in MAE at WMU.

The previous research on the Microgravity Pool Boiling conducted for the last decade strongly suggests a further study on surface tension in conjunction with thermocapillary force and Marangoni convection. Surface tension on vapor bubbles with respect to temperature differences along the surface turned out to be a challenging problem although a number of studies on gas bubbles are available. Under a terrestrial gravity, it is known that buoyancy is a dominant driving mechanism, resulting in the enhanced heat transfer in boiling. Under absence of buoyancy in microgravity, such as in space, it is found, according to the recent research performed, that surface tension plays an important role in boiling, unexpectedly providing a possibility of enhancement in heat transfer under subcooling conditions.

After a heater power off during the Microgravity Pool Boiling, a small vapor bubble on the heater surface that appears/stays for a few seconds in the upper part of the following motion picture is suddenly forced toward the heater surface (right in the picture) and moved toward the large bubble along the heater surface and absorbed into the large bubble. This attributes to thermocapillary force acting on the bubble surface, showing in the motion picture as: Thermocapillary Force on  a Vapor Bubble during Pool Boiling Experiment (3.1 MB mpg file)

See Also: Vapor Bubble Migration

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