A Study on Bubble Dynamics in a Liquid under Temperature, Concentration or Electric Potential Gradient

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This is a fundamental study related to materials processing under microgravity condition as available, for instance, on a space station. The objective of the study is to develop a technique of removing gas bubbles (or immiscible liquid drops) from the melt of a raw material utilizing surface tension or electric force.On the earth surface, where the normal gravity exists, the bubbles in the melt rise upwards by the action of buoyancy force, finally being removed out of the melt. However, under the microgravity condition bubbles remain stationary in the melt. They form defects in the material after solidification.Some other forces from buoyancy are needed to remove bubbles in the microgravity field. Use of the surface tension difference is one of the substitutions. This method makes use of a phenomenon that the surface tension of a liquid changes with temperature or concentration (only in the case of multicomponent solution). If the gradient of temperature or concentration is established in the melt, every bubble moves toward the region where the surface tension is lower, because the surface tension gradient induces a flow at the surface of the gas bubble. Such a motion of the gas bubble is investigated in the present study.However, the bubble motion induced by the surface tension difference is, in many cases, a little too slow because of the weak driving force. Thus, application of an electric field is considered in this report as the alternative. The electrohydrodynamic (EHD) force electric acting on both the liquid and the gas phases are investigated, and the possibility of application of the force to the removal of gas bubbles are discussed.