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

这是一项与微重力条件下的材料加工有关的基础性研究，例如在空间站上。这项研究的目的是开发一种利用表面张力或电力从原料熔体中去除气泡(或不混溶的液滴)的技术。在存在正常重力的地球表面上，熔体中的气泡在浮力的作用下向上上升，最终被移出熔体。然而，在微重力条件下，气泡在熔体中保持不变。它们在凝固后在材料中形成缺陷。需要一些来自浮力的其他力来消除微重力场中的气泡。使用表面张力差是其中一种替代方法。这种方法利用了液体的表面张力随温度或浓度变化的现象(仅在多组分溶液的情况下)。如果在熔体中建立了温度或浓度的梯度，每个气泡都会向表面张力较低的区域移动，因为表面张力梯度会在气泡表面诱导流动。本文对气泡的这种运动进行了研究，但由于驱动力较弱，由表面张力差引起的气泡运动在很多情况下显得有些太慢。因此，在本报告中，电场的应用被认为是替代方案。研究了电液动力(EHD)力对液体和气相的作用，探讨了将EHD力应用于气泡去除的可能性。

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.