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Toward a Microgravity Research Strategy (Appendix D) Toward a Microgravity Research Strategy D Fluids, Interfaces, and Transport STATUS Fluid and interfacial transport processes are ubiquitous throughout materials and biological systems. Gravitational effects influence these processes through the sedimentation of particles, buoyancy-driven convection, and the distortion of liquid and/or fluid interfaces by hydrostatic pressure. In many cases, gravitational effects limit the operation of materials-processing systems, add complexity, and potentially mask other physicochemical reactions that are under way. Experiments in a microgravity environment provide conditions for unraveling these complex transport processes in many systems. The new fundamental understanding gained from these studies has led to improved system performance on Earth, to new applications of the microgravity laboratory, and to the better operation of fluids-handling systems in space. REPORT MENU These experiments and the supporting analyses of fluid and interfacial NOTICE processes have played a dominant role in past research programs, making this MEMBERSHIP area one of the best-developed and best-documented fields of microgravity SUMMARY research. Many review papers have appeared on aspects of this subject. The CHAPTER 1 books by Feuerbacher et al.1 and Myshkis et al.2 are very good references to this CHAPTER 2 material. An extended summary of many of the ideas presented here is available CHAPTER 3 in the report of the discipline working group of NASA that focuses on this CHAPTER 4 research area (Rosenberger3). APPENDIX A APPENDIX B APPENDIX C Microgravity research in fluids and interfaces is usually classified either by APPENDIX D the fundamental processes under study or by the area of application. Important APPENDIX E fundamental processes include (1) diffusive phenomena, (2) body force-driven APPENDIX F convection and surface-tension-driven convection in fluids, (3) capillary phenomena, (4) nucleation and growth, (5) solidification and microstructure formation, and (6) flow of multiphase fluids. Applications include transport processes in materials-processing systems, the measurement of thermophysical properties, and fluid and energy management in space. Classification according to fundamental processes is used here to review the accomplishments and future potential for microgravity research in fluid and file:///C|/SSB_old_web/cmgr92appendd.htm (1 of 8) [6/18/2004 11:10:00 AM]
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Toward a Microgravity Research Strategy (Appendix D) interfacial systems. Applications of understanding fundamental transport and interfacial processes to particular systems are examined in the context of the connections between this section and other portions of the report. The discussion here focuses on the current level of understanding of physical phenomena and on the role played by past microgravity research in achieving this understanding. Potential applications of these fundamental results, both on Earth and in space, are highlighted and key issues deserving future research are addressed. Fundamental studies in transport processes can be divided into two segments: research aimed at the analysis of the effects of coupled transport processes where each process is well understood individually, and research focused on understanding of fluxes of mass, heat, and species in complex systems, where present theory gives a less than adequate description. Many of the problems in the classical theory of fluid mechanics and interface dynamics fall in the first category, in which mathematical descriptions in terms of conservation laws and constitutive equations are known. Given the availability of a database for the relevant transport coefficients (e.g., thermal conductivities, diffusivities, and fluid viscosities) and the dependence of the coefficients on temperature and concentration, these transport models can be solved by numerical methods to give insight into coupled processes and to design experimental systems. This approach is particularly appealing because of the incredibly high cost of microgravity experiments and the increasing capability of simulation methods to handle ever more complex problems. There are two major difficulties to the numerical simulation approach. First, the large uncertainty in a typical database for thermophysical properties poses a major limitation. For most technologically interesting materials, these properties are not known with any accuracy and, hence, this uncertainty is transferred to the calculation of the transport processes through the simulation. In fact, many microgravity experimental systems to measure these properties are proposed. Second, the environment of microgravity offers a less symmetric and less predictable laboratory than is typical on Earth. For example, the very constant and unidirectional gravitational acceleration on Earth is replaced in space by a jittering acceleration vector with direction and magnitude that depend on time over a wide range of frequencies. The use of symmetry restrictions in the calculations—a common mechanism used by analysts to cut the cost of the simulations—is not justified, and, in many cases, totally predictive simulations must be threedimensional and time-dependent. The difficulties of making these calculations and the uncertainty of thermophysical data dictate continued reliance on low-gravity experimentation. MAJOR RESEARCH ACCOMPLISHMENTS The major research challenges and accomplishments for each of the six areas of interest listed above are described below. file:///C|/SSB_old_web/cmgr92appendd.htm (2 of 8) [6/18/2004 11:10:00 AM]
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Toward a Microgravity Research Strategy (Appendix D) Diffusive Phenomena The potential for convection-free experimentation in a microgravity environment was a key motivation for many of the original experiments in space. Myriad experiments, ranging from the solidification of metals and semiconductors to electrophoretic separation of proteins, were performed with the hypothesis that, in microgravity, the absence of bulk fluid motion driven by buoyancy forces on Earth would lead to unique material properties and better separation efficiency. These experiments were partially successful. Electrophoresis experiments in space led to improvement in separation efficiency for model systems. Experiments aimed at achieving diffusion-controlled crystal growth showed the expected behavior for axial solute transport, but displayed the radial nonuniformity that was shown computationally to be characteristic of weak convection. These observations spawned both theoretical studies and Earth- based experiments that have demonstrated the importance of controlling very weak convective flows to obtain highly uniform solute concentrations in microgravity solidification systems. Removing the influence of convection also has uncovered the importance of secondary mechanisms for diffusive transport. Such mechanisms as the Soret effect for mass transport by a temperature gradient and the complementary Dufour flux for heat transfer driven by a concentration gradient have been shown to be important in many microgravity systems, where the effect would be masked by convection on Earth. Theoretical understanding of these effects in anything but very idealized model systems is severely hampered by the lack of experimental data for transport coefficients. Experimental measurement of diffusivities and other transport coefficients is an exciting research application of the microgravity environment. Measurement of diffusivities in space is particularly appealing, especially for liquids, because the diffusivity is so low that fluid velocities of less than 1 m/s, hardly avoidable on Earth, can lead to huge errors in the measurement. Recent measurements of liquid diffusivities in space claim an accuracy of 2 percent. Convection in Fluids More complex transport processes arise when convection is driven by multiple mechanisms. An example is double-diffusive convection, caused by simultaneous temperature and concentration gradients. In many materials- processing systems, these driving forces are affected differently by a low-gravity environment. Other driving forces for convection are independent of gravity and will become dominant in microgravity. Electrodynamic and magnetodynamic forces both fall in this category and have been studied in relation to convection during electrophoresis and to motion induced during magnetic levitation, file:///C|/SSB_old_web/cmgr92appendd.htm (3 of 8) [6/18/2004 11:10:00 AM]
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Toward a Microgravity Research Strategy (Appendix D) respectively. Alternatively, magnetic fields have been applied to suppress convection in electrically conducting fluids. In small-scale systems of only a few centimeters on a side, the fields generated by conventional magnetic technology are effective enough to alleviate the need for space experiments. In large experimental systems, magnetic fields used in conjunction with a microgravity experiment might be used to suppress residual convection that would occur otherwise if only microgravity or magnetic fields were used. The possibility of enhancing the microgravity environment through the use of magnetic fields deserves more study. Perhaps the most important and well-studied form of convection in microgravity is caused by differences in surface tension that arise because of temperature or concentration gradients along a liquid and/or fluid surface-so- called thermocapillary or Marangoni convection. This convection is unavoidable when a clean liquid interface exists in a nonisothermal system and, depending on the geometry of the liquid, can be more intense than motion caused by buoyancy. Moreover, thermocapillary flows show nonlinear transitions to time-periodic and, finally, chaotic convection as the temperature or concentration gradient is increased. Under these conditions, the bulk thermocapillary motion dominates heat and solute transport, and there is little advantage to a space experiment. Although much progress has been made in understanding thermocapillary convection, it is impossible to eliminate it in many systems, and its presence severely limits the possible configurations of low-gravity systems for performing convectionless experiments. Capillary Phenomena Without the hydrostatic pressure of gravity, many fluids systems can exist in space with only surface tension affecting a liquid surface. This class of capillary phenomena includes liquid films, foams, drops, bubbles, and other configurations of liquid and/or fluid surfaces that separate bulk phases. Statically levitated liquid drops and suspended bubbles are excellent examples for which fundamentally important experiments are possible only in space. Accordingly, microgravity studies of drop dynamics and coalescence have flourished and have found application as the basis for new measurement methods for liquid viscosity and surface tension. Experimental observations of drop dynamics made early in the NASA microgravity program and later results from Space Shuttle experiments spawned many high-quality, theoretical studies of the fluid mechanics of drops and bubbles and advanced this science considerably. Because these levitation experiments are containerless (i.e., the droplet does not contact a container wall), these techniques will be very important for measurements at high temperature and of highly reactive materials once experimental systems are available for precise measurement and environmental control. file:///C|/SSB_old_web/cmgr92appendd.htm (4 of 8) [6/18/2004 11:10:00 AM]
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Toward a Microgravity Research Strategy (Appendix D) Nucleation and Growth The homogeneous nucleation and growth of a new phase, either liquid or solid, from a parent phase, either gas or liquid, constitute a classical area of physics and chemistry and are generic to many varying technologies, including crystallization from liquid solutions, solidification from undercooled melts, and aerosol formation from the vapor phase. Definitive experiments are extremely difficult on Earth because of the complexities introduced by the need for confining surfaces, which are sites for heterogeneous nucleation, and because of gravitationally induced particle settling. The prolonged availability of a well- controlled microgravity environment will make feasible the first experiments free of these effects. To date, these experiments are limited to either minute samples, which can be levitated effectively using acoustic or magnetic levitation devices, or to very-short-duration experiments in NASA free-fall facilities. Solidification and Microstructure Formation Prediction of the microstructure of a solidifying interface is a key problem in materials processing because of the crucial influence that this structure has on the extrinsic properties of the solid. The dynamic behavior of the solidification interfaces depends on heat and solute transport in the liquid phase as well as on interfacial properties of the melt and/or solid system. In the last decade, the problem of prediction of these microstructures has received intense theoretical attention. The emphasis has been on the search for precise descriptions for the morphogenesis of the microstructure patterns, such as solidification cells and dendrites, that form as a function of changes in macroscopic operating conditions—temperature gradient, growth rate, and alloy composition. The formation of these small-scale microstructures during phase change is ubiquitous, occurring not only in crystal growth from melts and solutions, but also in electroplating and phase changes in liquid crystal systems. Significant progress has been made on a number of problems, much of the research having been sponsored by NASA. For example, several recent, intricate theories have appeared to predict the dependence on growth rate and undercooling of the shape of a single dendrite growing in an undercooled melt. Microgravity experiments may be the only means of making distinctions between these theories because convection in the melt has a strong effect on experiments, as has been carefully documented. Similarly, the experimental growth of a rod-like, eutectic material on Earth and in microgravity has demonstrated the influence of convection on microstructure formation. On Earth, only thin-film solidification experiments are relatively free of convection, but these experiments may be altered significantly by the presence of the confining sidewalls. Theories exist that are beginning to link the nonlinear transitions that describe microstructure evolution during phase change with the flow in the adjacent liquid. These theories have identified new types of interface dynamics, which have not yet been confirmed experimentally. file:///C|/SSB_old_web/cmgr92appendd.htm (5 of 8) [6/18/2004 11:10:00 AM]
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Toward a Microgravity Research Strategy (Appendix D) Flow of Multiphase Fluids Multiphase fluids, such as bubbly gases and solid-laden slurries, form the class of perhaps the most industrially important liquids. Not only are many materials processed as multiphase liquids, but gas-liquid systems also appear in heat transfer systems that employ nucleate boiling and in many gas separation technologies. These fluids are not well understood in the sense that constitutive descriptions do not exist that give accurate predictions of the average stress in the multiphase liquid and of segregation of the components in nonhomogeneous flows. The development of these descriptions is one of the outstanding problems in fluid theology. Low-gravity experiments for isothermal gas-liquid flow will be very important in separating the effect of gas buoyancy for viscous and inertial interactions in the prediction of the rheological behavior of liquids containing many gas bubbles. Studies of mass and heat transfer in two-phase flow will also be important in understanding the effects of microgravity on the operation of classical heat transfer and separation technologies in a space environment. RESEARCH PROSPECTS AND OPPORTUNITIES Further progress in the fundamental understanding of transport processes and interfacial systems in microgravity will rely on the continued growth of theoretical and computational research and on expansion of the experimental database. The needs are not equal among these components; both the ground- based and microgravity experimental programs are much less developed than theory and should receive special attention. Experiments should aim at probing new phenomena and at clarifying the link between theory and experiment. In these experiments, emphasis should be placed on using well-characterized materials; when the characterization is not complete, appropriate measurements of the properties needed should be sponsored. Because many of the experiments will rely on similar systems, for example, directional solidification and drop levitation, integrated common equipment continues to be essential and should be emphasized, but the equipment should be developed in close collaboration with the principal investigators. Platforms for these experiments should include short-duration drop tower tests, sounding rocket flights, Shuttle bay experiments, and systems on the Space Station. The microgravity needs for a specific experiment should be considered carefully in order to leverage the most cost-effective environment for the measurements. Continued development of the ground-based research and analysis program is essential to increasing the vitality of this program. This is particularly true because of the important roles that theory and simulation play in unraveling file:///C|/SSB_old_web/cmgr92appendd.htm (6 of 8) [6/18/2004 11:10:00 AM]
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Toward a Microgravity Research Strategy (Appendix D) complex transport processes in microgravity. REFERENCES 1. Feuerbacher, N., H. Hamacher, and R.J. Naumann. 1986. Material Science in Space. Springer-Verlag, Berlin. 2. Myshkis, A.D., V.G. Babskii, N.D. Kopachevski, L.A. Slobozhanin, and A.D. Tyuptsov. 1976. Low-Gravity Fluid Mechanics. Springer-Verlag, New York. 3. Rosenberger, F. 1989. Report of the Fluids, Interfaces, and Transport Discipline Working Group. Microgravity Science and Applications Division, National Aeronautics and Space Administration. Last update 7/13/00 at 3:51 pm file:///C|/SSB_old_web/cmgr92appendd.htm (7 of 8) [6/18/2004 11:10:00 AM]