Liquid Materials and Flow Processes in Reduced Gravity
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MATERIALS PROCESSING IN THE REDUCED GRAVITY ENVIRONMENT OF SPACE Guy E. Rindone, editor
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LIQUID MATERIALS AND FLOW PROCESSES IN REDUCED GRAVITY
L. E. SCRIVEN Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, MN 55455
Institute of Technology,
ABSTRACT Fluid states are central to melt and vapor processing. Fluids flow. As they do they convect heat, solutes, and inTheir imporclusions. Liquid/gas interfaces have tension. tance rises when scale is small or effective gravity low. The forces that move fluids and mold fluid interfaces are recounted here. Fundamentals of viscous flow, convective transport, and capillary phenomena are summarized with special attention to the roles of gravity in liquid processing. Research needs are highlighted.
INTRODUCTION The processing of metals, semiconductors, and ceramics involves melts very The processing of polymerics involves frequently and the vapor phase sometimes. melts or liquid solutions as the rule. Materials processing - by which is gentypically involves the liquid state erally meant solid materials, ultimately or the vapor state along with highly nonisothermal solidification processes. Yet the fluid mechanics of many if not most such materials processing is not adequately understood. In particular, the origins and the consequences of convective transport are poorly understood. Of all the types of convection, the most familiar is no doubt convection that is forced by a pressure difference, as in But more talked about in materials processing is buoyancy-driven natpumping. ural convection, or free convection, which depends on the action of gravity. Because of this, gravity is frequently regarded as a culprit by materials scientists - as a bad thing one would like to eliminate, or at least reduce whenever possible. It can be inconGravity is far from the only source of convective motions. sequential. It can be but a minor influence, particularly in the smaller reAnd gions that may matter most at solidification fronts and at fluid menisci. For materials processing research, especially any it can be a major factor. that involves fluid states and is contemplated for the reduced gravity environment of space, it is important to understand the relevant fluid mechanics and convective diffusion transport mechanisms. There are scientific principles that apply, principles that are fundamental These enable one to learn a lot from theoretical analyses and and universal. transparent ones and from experiments on more convenient liquid materials low-melting ones - and to apply the knowledge to opaque, high-temperature melts. Information from experiments with gallium can be applied to processing of germanium, say. This is not to deny the necessity of studying the real sysThe point is that there is a tapestry of fluid mechtem insofar as possible. anics and transport phenomena and materials science, into which Nature has woven germanium so tightly that only if one will look hard, one can be beautifully guided by the warp and woof of the wh
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