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Fluid Physics Research Program INTRODUCTION AND BACKGROUND Fluid physics, while having an identity entirely its own, also serves as the underpinning of a large portion of the physical sciences research program of NASA's Office of Biological and Physical Re- search (OBPR). Indeed, it is the absence of buoyancy-driven fluid (liquid or gas) convection (that on Earth is caused by density variations coupled with gravitational acceleration) that gives rise to the curious phenomena observed in weightless environments. Hence, in microgravity, observations of spherical flame fronts, symmetric dendrite formation during solidification processes, unusual colloidal structures, and the growth of some living tissues and macromolecular protein crystals that differ from their terrestrial counterparts are all attributable to the lack of buoyant convection. Thus, the Physical Sciences Division's programs in combustion, materials. fundamental physics. and biotechnology all share an intersection with fluid physics. The motivation for investigating fluid behavior under the unique conditions afforded by NASA's microgravity facilities is the desire to further the understanding of the complex behavior of fluids by taking advantage of near-weightless conditions to make measurements and observations that are not possible in terrestrial laboratories and, thereby, study physical phenomena typically overwhelmed by buoyant convection. Many problems that occur due to the effects of buoyancy, sedimentation, hydro- static pressure gradients, or limitations due to the small length scales of interracial processes under normal gravity conditions can be avoided in microgravity. Indeed, some of the earliest problems associated with spaceflight that required solutions also are of a fluid-physics origin. For example, the problem of liquid management in space is exacerbated by the absence of gravity that on Earth keeps the liquid at the container "bottom." Solutions suitable for short-duration missions or longer missions with resupply capability will need to be rethought for future long-duration manned missions to Mars owing to more stringent mass limitations. Likewise, both life-support and heat-transfer systems rely on the transport of multiphase flows, knowledge of which is far from complete. The fluid physics program of NASA's OBPR supports both flight- and ground-based research. Since 1992 five major research thrust areas have emerged: (1) dynamics and instabilities, (2) complex 15

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16 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA fluids, (3) multiphase flow and heat transfer, (4) interracial phenomena, and (5) biofluid dynamics. Some of these areas (for example, areas 1, 3, and 4) have a richer history in terms of program support than others (for example, areas 2 and 5) and have therefore yielded longer "threads" of related research. A few of these threads are highlighted in the following section to provide a picture (rather than an extensive program review) of the program, and their impact is then discussed. FLUID PHYSICS RESEARCH: SELECTED EXAMPLES Thermocapillary Phenomena Thermocapillarity is the variation of a liquid's surface tension (or of the interracial tension between two immiscible liquids) with temperature. Thus, the existence of an interfacial-temperature gradient produces a force that drives interracial, and hence bulk, fluid motion. (Surface tension-driven flows can also occur when there are surface gradients in composition.) When the OBPR's physical sciences research program was known as the materials processing in space program, one early endeavor focused on the use of the containerless float-zone crystal-growth process to improve the size and quality of crystals of semiconductor materials in space, in the absence of the apparently detrimental effects of weight and buoyant convection. However, it was found that thermocapillary convection, which nor- mally plays a secondary role to buoyant convection on Earth, becomes dominant in microgravity environments, and the detrimental "striations" observed in Earth-grown material were also observed in some space-grown crystals. Eyer et al. (1984) demonstrated that surface-temperature fluctuations (due to unstable themocapillary convection) at free melt surfaces cause these striations. Smith and Davis (1983a,b), in their related theoretical studies, discovered a new type of instability, the hydrothermal wave, that is relevant for some range of the liquid's Prandtl number. The Smith and Davis theory was later conclusively confirmed in the laboratory (Riley and Neitzel, 1998~. Other research has been aimed at eliminating or suppressing the instability, for example, by using open-loop, feed-forward control. Terrestrial and spaceflight experiments (Kamotani et al., 2000) have also examined transitions to oscil- latory flow in geometries other than liquid bridges (captive drops held between solid supports) and thin layers. In addition to studying the instability of thermocapillary convection, considerable research has been done that utilizes thermocapillary convection to control the position and motion of liquids and gases in microgravity. In addition to their utility in microgravity, thermocapillary processes are useful in remov- ing air bubbles from glasses during their manufacture. Recent extensions of these ideas are finding applications to problems encountered in moving liquids or gases through small channels in micro- electromechanical systems (MEMS) devices. Other NASA-sponsored work dealing with thermocapillarity has either developed independently of the above research or been spawned by it. For example, recent studies (Dell'Aversana and Neitzel, 1998) showed that noncoalescence can be sustained by thermocapillary-driven flow in a thin region separating two drops of the same liquid phase. Current work on this subject is investigating the possibility of using noncoalescing nonwetting systems as nearly frictionless bearings for low-load applications. Capillary Phenomena Interfacial or capillary phenomena are those features of liquid-gas or liquid-liquid interfaces other than the thermocapillary phenomena discussed above. These phenomena are of particular interest to

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FLUID PHYSICS RESEARCH PROGRAM 17 NASA because of the need to manage liquids in weightless environments, but they also pertain to problems encountered in terrestrial environments. Capillary effects such as wicking in heat pipes (Faghri, 1995; Peterson et al., 1998), capillary pumped loops (Westbye et al., 1995), and vane structures in cryogenic storage (Dodge, 1990) can be used to manage the disposition and transport of liquids under weightless conditions. The dynamics of moving contact lines is an important but poorly understood aspect of wetting that is the subject of investigation in around and flight experiments (Decker and Garoff, 1997; Weislogel and Lichter, 1998) and is associated with thin films, coating flows, and drying processes such as the removal of rinse water from the surface of a silicon wafer during wet processing. Contact-line dynamics affects the behavior of vapor bubbles in boiling, where to adequately model nucleation of bubbles on the heater surface requires knowledge of the dynamic contact angle behavior. The study of capillary surface equilibrium shapes and their stability is a well-established area of research. Configurations of interest to NASA researchers have ranged from liquids partly contained in angular (Concus and Finn, 1990) and smooth-walled containers (Slobozhanin and Alexander, 2001) to captive drop or liquid bridges (Lowry and Steen, 1995~. Interest in the latter was motivated by materi- als-processing techniques such as float-zone crystallization and zone refining. In addition to analyzing the stability of such configurations, recent research has focused on using forced flow (Lowry and Steen, 1997) and acoustic (Morse et al., 1996) and electric fields (Burcham and Saville, 2000; Marr-Lyon et al., 2000) to stabilize liquid bridge configurations that would otherwise be unstable. The oscillations, dynamics, and break-up of drops, jets, and other free surfaces have been and continue to be studied (e.g., Agrawal et al., 2000; Eggleton et al., 2001; McKinley and Trip athi, 2000~. While there is a great deal of classical and current literature on capillary dynamics, many problems remain unsolved, for example, those of sloshing or other motions that require knowledge of contact-line behavior and await improve- ments in our understanding of contact-line dynamics. Complex Fluids Research on the rheology and thermodynamical behavior of complex fluids (colloids, granular materials, plasmas, and foams) has emerged as a prominent part of the ground-based and flight pro- grams following the 1991 NASA Research Announcement. That there are similarities between these apparently different systems was illustrated recently (Trappe et al., 2001~. For example, a number of systems including colloids, granular media, foams, and molecular systems can undergo nonequilibrium transitions between different fluidlike states and from fluidlike states to solidlike states. Colloids, granular media, and molecular systems can exhibit "jamming," where crowding of the compo- nent particles prevents them from further exploration of the phase space. Recent results suggest that attractive interactions in colloidal systems may have the same jamming effect as confining pressure in granular media and that a jamming phase diagram for attractive colloidal particles provides a unifying link between the glass transition, "elation, and aggregation. Microgravity research on colloids is focused on disorder-order transitions in hard-sphere colloidal dispersions (Cheng et al., 1999; Zhu et al., 1997) and on binary colloidal structures (Hiddessen et al., 2000~. Microgravity experiments on colloids were motivated by the emerging field of colloid engineer- ing and directed self-assembly of mesoscopic structures. Colloidal phase diagrams, growth kinetics, and physical properties obtained from flight experiments and supporting ground-based research will yield information that will facilitate the use of colloidal precursors to fabricate novel materials. Flight experiments flown between 1996 and 1998 involved monodisperse hard-sphere colloids, binary colloi- dal alloys, and colloid-polymer mixtures; they have produced rich and in some cases unexpected results, such as coarsening during crystallization (Cheng et al., 2002~. Ground-based research has included the

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18 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA examination of fractal colloidal aggregation and the behavior of colloid polymer gels. This research has been productive, yielding the first detailed information about the consequences of scale-invariant struc- ture on the properties of colloids (Cipelletti et al., 2000~. The entropically driven growth of colloids in low volume-fraction systems has also been investi- gated (Crocker et al., 1999~. Crystallization in low volume-fraction suspensions is driven by attractive particle interactions caused by the entropic depletion. Entropic depletion creates a condition not unlike supersaturation. This drives the ordered aggregation of the particles. These interactions create growth conditions similar to those associated with atoms and molecules; this is in direct contrast to the "space- filling" mode of colloidal crystal growth that is driven by packing constraints. In the low volume- fraction limit, nucleation of colloidal crystals can occur on a surface in the absence of bulk phase separation. The use of surface templates offers further options for controlling the growing structures (Crocker et al., 1999~. Aspects of crystallization are being investigated through experiments on plasma dust crystallization. In these experiments, spheres interact through a shielded Coulomb potential, causing them to arrange in liquidlike structures or solidlike structures. The "condensed" (liquid and crystalline) states of colloidal plasma systems were studied under microgravity conditions (Morfill et al., 1999~. The observed states represented new forms of matter: quasi-neutral, self-organized plasmas. In contrast to states observed in terrestrial measurements, the systems under microgravity are three-dimensional and exhibit stable vor- tex flows, sometimes adjacent to crystalline regions, and a central "void" free of microspheres. Related ground-based research on plasma dusts has also yielded fruitful results for example, Pieper and Goree (1996) examined the applicability of fluid-based dispersion relations to strongly coupled dusty plasmas. They measured real and imaginary parts of the complex wave number for low-frequency compressional waves in dusty krypton plasma. Their results agreed with a theoretical model of damped dust acoustic waves, ignoring strong coupling, but not with a strongly coupled dust-lattice wave model. Complex fluid rheology is an emerging research area that promises to take advantage of low-gravity conditions to isolate particular aspects of fluid rheology. Magnetorheological fluids are composed of magnetically soft particles dispersed in liquids. Applied magnetic fields can then be used to alter their properties rapidly and reversibly. Ground-based experiments by Furst and Gast (1999) on magneto- rheological fluids have made some advances. They investigated the micromechanical properties of dipolar chains and columns in a magnetorheological suspension. Using optical tweezers, they directly measured the deformation of dipolar chains parallel and perpendicular to the applied magnetic field and observed the field dependence of mechanical properties such as resistance to deformation, chain reorga- nization, and rupturing of the chains. These forms of energy dissipation are important for understanding and tuning the yield stress and the theological behavior of magnetorheological suspensions. Foams have unique theological properties that, depending on the stress-strain conditions, range For example, at small-amplitude strains, foams can deform and recover their shape elastically; at larger strains, viscoelastic behavior occurs (manifested by a hysteretic strain-energy curve), and if the strain exceeds a critical value, the foam flows. Foam rheology has thus far been studied only on the ground (Gopal and Durian, 1999~. Granular dynamics has emerged as a new area in the fluid physics program. Results to date are ground-based. For example, Howell et al. (1999) carried out experiments on a slowly sheared two- dimensional granular material and found a continuous transition as the packing fraction (the ratio of solid [granular] and total volumes) passed through a value equal to 0.776. As the critical packing fraction is approached from above, the compressibility becomes large, the mean velocity slows, the force distributions change, and the network of stress chains changes from a tangled dense network to intermittent long radial chains near the critical value. Other planned space experiments involve the from solidlike to fluidlike.

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FLUID PHYSICS RESEARCH PROGRAM 19 influence of inertia on segregation in granular systems with two particle sizes (Louge et al., 2001), and flight experiments are planned for the ISS. Multiphase Flow and Heat Transfer Although it has been of interest to the microgravity program for some time, microgravity research in . . . ~ . . . . . . . . . ~ . . . . .. . multlphase mult~phase flow has not been pursued vigorously within the fluid physics program "however, flow research is pursued outside the program by other NASA divisions). As discussed in previous NRC reports (NRC, 1995, 2000), NASA is well aware that designers of future space systems must face a number of issues and concerns related to multiphase flow and heat-transfer processes in weightless and reduced-gravity environments. Applications involving such processes include gas-liquid and liquid- liquid systems for advanced life support operations (evaporators, condensers, thermal buses, and elec- trolysis units) and particulate-fluid systems that are encountered in association with planetary explora- tion (dust control in human habitats, in situ processing of planetary materials for power, and so on). Preliminary low-gravity experimentation (mostly on KC-135 aircraft) has identified low-gravity flow regimes and phase distribution in isothermal gas-liquid flows. Boiling heat transfer has been studied extensively outside NASA's program over the last 50 years, and there has been some interest in determining boiling heat-transfer regimes under low-gravity condi- tions. However, results obtained in low-gravity drop facilities and aircraft have been contradictory, with some data showing that pool boiling heat fluxes were insensitive to changes in gravity level, and other data suggesting that heat-transfer rates are enhanced in low-gravity conditions. A significant unknown in the prediction and application of flow boiling heat transfer in microgravity is the upper limit of the heat flux for the onset of dryout (or critical heat flux) for given conditions at fluid-heater surfaces, including geometry, system pressure, and bulk liquid subcooling. Furthermore, the dependence of the critical heat flux on gravity has yet to be fully explained. As a result, there is still no rational basis for predicting pool boiling heat transfer under microgravity conditions (NRC, 1995, 2000~. Current re- search in the program is focused on these and related issues, but future progress requires access to longer-duration low-gravity conditions than can be provided by drop facilities or aircraft. Biofluid Dynamics Although fluids clearly play a role in many biological processes, biofluid dynamics has only re- cently emerged as a thrust area within the fluid physics program. To date, ground-based studies in this area have focused on two themes: microgravity effects on transport across endothelial cell membranes (Chang et al., 2000a,b) and capillary-elastic instabilities in the closure and reopening of small airways in lungs in microgravity (Howell et al., 2000~. IMPACT OF THE FLUID PHYSICS RESEARCH PROGRAM The need to understand the behavior of liquid propellants under weightless conditions was recog- nized in the early days of NASA's space program, and it can be argued that the roots of microgravity fluids research extend back to early experimental and theoretical work in this area. The fluid physics program became established in its own right following the 1991 NASA Research Announcement (NRA). Until then, fluids research had played only a secondary role in that most of it was motivated by or directly related to materials research. Research in thermocapillarity, however, has been dominated by NASA-sponsored investigations for

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20 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA the last couple of decades. For example, a simple search in the Institute for Scientific Information (ISI) science citation index for 1980, 1985, 1990, and 1995 yielded 9, 7, 38, and 80 articles, respectively, showing the expansion of the field in just over 15 years. NASA-sponsored investigators were and continue to be leaders in thermocapillary flow research. The importance of thermocapillary flows in low gravity was discussed by Ostrach (1982~. Early work that established the foundations for subse- quent thermocapillary flow research was performed by Sen and Davis (1982), who obtained the first solutions for thermocapillary-driven convection in bounded geometries. Smith and Davis (1983a,b) proposed the existence of a hydrothermal wave mechanism for thermocapillary flow instability. Oscil- latory thermocapillary flows were later discussed by Ostrach et al. (1985~. VanHook et al. (1997) recently resolved a long-standing disagreement between theory and experiment in the formation of hexagonal patterns during Marangoni instability of a thin liquid layer heated from below. (Marangoni instability of a static fluid state occurs when flow arises due to surface tension gradients caused when an initially flat isothermal surface deforms to a non-planar non-isothermal surface.) Thermocapillary flow research, originally motivated by problems in crystal growth techniques, has been undertaken outside NASA's program and adapted to other technologies. For example, recent innovations involving thermocapillarity include liquid positioning in MEMS devices (APS, 2000; Gwynne, 2000; Mazouchi and Homsy, 2001) and microchip thermocapillary pumps for DNA analysis (Sammarco and Burns, 2000; Kataoka and Troian, 1999~. Some research themes (complex fluids, multiphase flow, biofluids) are still developing. Neverthe- less, ground-based research has already yielded new results. For example, the work of Furst and Gast (1999) involving magnetorheological fluids is an important step in the understanding of the effect of magnetic fields on fluid behavior. Such fluids are used in advanced vibration technology (ranging from loudspeakers to automobile-braking systems) and as cooling fluids in transformers and are an attractive option for controlling fluids in weightless environments. As noted previously, the colloid experiments flown between 1996 and 1998 have produced rich and in some cases unexpected results, such as coarsening during crystallization (Cheng et al., 1999~. Work by Crocker et al. (1999) established surface templates as another option for controlling the growing structures. These preliminary results from experiments on colloids show every indication that future work in complex fluids will produce significant results. Indeed, a recent article (Anderson and Lekkerkerker, 2002) on the insights into phase-transition kinetics that can be obtained from colloid science attests to this. Other work on complex fluids has also begun to have an impact. Experiments by Howell et al. (1999) on granular flow have established a continuous order-disorder transition in stress distribution as a function of the granular-packing fraction. In addition to its scientific value, this work is an important step toward understanding the flow dynamics of granular media. The processing of granular media is important in industries ranging from food storage and packaging to pharmaceuticals. Approximately 50 percent of the chemical industry products and at least 75 percent of the raw materials are in granular form (Nedderman, 1992) amounting to $61 billion annually. It is estimated (Jaeger et al., 1996) that 60 percent of the capacity of many U.S. industrial plants is wasted because of problems related to the transport of granular materials. Thus, the impact on industry of even small gains in understanding the dynamics of granular media should be profound. Flight investigations into the liquid and crystalline states of colloidal or "dusty" plasma systems (Morfill et al., 1999; Thomas et al., 1994) revealed new forms of matter: quasi-neutral, self-organized plasmas. Pieper and Goree (1996) resolved a long-standing controversy over the applicability of fluid- based dispersion relations to strongly coupled dusty plasmas. The pioneering work of these investiga- tors has resulted in a rapid increase in published research on dusty plasmas over the last 10 years.

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FLUID PHYSICS RESEARCH PROGRAM 2 While NASA-supported (through the fluid physics program) multiphase-flow research has pro- duced some significant advances (for example Dukler et al., 1988), this work is cited infrequently, possibly because it is mostly relevant to secondary and tertiary oil recovery in the petroleum industry and to NASA fluid system designers. In a more general context, interest in the link between small-scale fluids processes and larger-scale continuum hydrodynamics led to significant work by Koplick and Banavar (1995) that clearly demonstrated the link between specific fluid processes at the molecular scale and the large continuum scale. This work is significant because it quantifies the extent to which continuum models can be expected to give reliable predictions at very small length-scales. Aside from fundamental contributions to specific topic areas, the overall impact of the fluid physics research program can be put into perspective by considering the following: In 2001 there were 110 PIs in the program. Between 1998 and 2000, the research sponsored by the program produced several hundred papers that were published in internationally recognized journals (NASA, 1998-2000~. Of these papers, over 120 were published in the Journal of Fluid Mechanics and Physics of Fluids, two prominent journals for fluid dynamics; 44 in Physical Review Letters, a leading physics journal; 8 in Nature; and 7 in Science. (The last two are recognized as the two premier journals covering all of science.) Furthermore, 4 of the fluid physics program's investigators are members of the National Academy of Sciences, 8 are National Academy of Engineering members, and there were 37 fellows of the American Physical Society, 5 fellows of the American Society of Mechanical Engineers, and 12 fellows of the American Institute of Aeronautics and Astronautics. 1 a, FUTURE DIRECTIONS IN FLUID PHYSICS RESEARCH Fluid physics should continue to serve a dual purpose in NASA's physical sciences research pro- gram. For scientists in general, it provides access to a unique laboratory that permits the isolation and study of the effects of nongravitational forces on fluid behavior. For NASA, the program facilitates the acquisition of knowledge necessary for the next generation of mission-enabling technologies essential to NASA's human exploration and development of space. Indeed, the need for improvements in the understanding and application of fluid phenomena (e.g., multiphase-flow processes) has already been recognized as one of the primary opportunities for future fluids research (NRC, 2000~. In what follows, the committee outlines areas of research that should be pursued, their significance, and the expected benefits of the results. In some cases, these recommendations are similar to those of an earlier NRC report on the role of microgravity research in support of technologies for the human exploration and development of space (NRC, 2000), and more details can be found in that report. In other instances, the recommendations are based on the promise of advances in fundamental knowledge or innovation in terrestrial technologies. Research motivated entirely by NASA's mission must be made visible across all organizations within NASA. This is essential if the work is to enter into the conceptual stages of mission and mission systems design. Furthermore, it is essential that OBPR personnel keep the research community (outside and inside NASA) apprised of design issues that could be resolved through research within the OBPR. Research that is solely related to NASA's space exploration mission can be assigned a high priority only if OBPR meets this obligation. 1J. Sherwood, principal research scientist, Schlumberger, letter dated December 10, 2001.

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22 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Multiphase Flow and Heat Transfer Multiphase-flow and heat-transfer technology is a critical technology for space exploration and a sustained human presence in space (NRC, 2000) and has relevance to numerous terrestrial technologies. NASA has often avoided using multiphase systems and processes in spacecraft because their behavior under low-gravity conditions is not well understood. Without a quantitative understanding of such systems, the design of revolutionary mission-enabling technology will be either severely impeded or forestalled altogether. Research on multiphase flow and interracial processes is essential to providing a knowledge base for the development of mission-enabling technologies with the potential to bring about revolutionary changes in spacecraft hardware (NRC, 2000~. Phase-change systems for power, propulsion, and life support will be required for reliable long-term operation and improved efficiency. For example, two- phase liquid-vapor heat rejection systems lead to significant reductions in vehicle size, volume, and weight. The dynamics of miscible and immiscible interfaces in two-phase flows has relevance to advanced life support systems and to terrestrial applications such as secondary oil recovery. Droplet dynamics and liquid atomization (in, for example, sprays) occur in power and propulsion systems and in thermal control systems. Bubbly flows, such as those that occur in thermohydraulic loops, also need improved understanding so that problems anticipated under microcravitY conditions can be addressed. T T ~ . . ~ ~ . . ~~ ~ . . ~ ~ ~ ~ ~ . ~ ~ . ~ under terrestrial conclltlons, gravity usually Dominates the behavior of many of these multlpuase sys- tems, affecting such important parameters as heat transfer, pressure drop, interracial area in multiphase liquids, flow stability, and transitions. There is much to be learned about the behavior of these systems under low-gravity conditions. Specific examples of research topics that should be pursued are (1) the identification of low-gravity flow regimes, the mechanisms that govern the effects of gravity, and interracial and bulk constitutive laws for specific flow regimes through experiments and the synergistic development of computer- modeling capabilities; (2) assessment of the effects of gravity on forced convective boiling, two-phase forced convective heat transfer, and convective condensation heat-transfer; (3) investigation of schemes for active and passive single- and two-phase heat transfer and pressure drop reduction; and (4) assess- ment of the effects of gravity on flow regimes and the stability of adiabatic and two-phase boiling flows in norous media. and on flows in norous media used for Plant or cron growth for food sources. ~ ' ~ 1 1 ~ ~1 1, r ,1 1 11 1 r. , , IT ~ ~ ~ ~ 1 , - lhe results ot this research will have slgmilcant impact on NAbA'S space exploration program, and the increased knowledge of constitutive laws for multiphase-flow systems will undoubtedly impact industry here on Earth (e.g., thermal systems, power generation, waste treament, and mineral separa- tions technology). Complex Fluids Self-assembly and Crystallization Recent advances using new imaging techniques that allow direct observation of individual colloidal particles undergoing phase transitions have elucidated some of the details underlying transitions be- tween gas, liquid, solid, and liquid crystalline phases. These transitions, while ubiquitous in nature, are not always accessible to experiment. Preliminary microgravity experiments have demonstrated the value of conducting such experiments in a weightless environment and have already produced surpris- ing results. Colloidal research planned for the ISS and in complementary ground-based programs will provide a knowledge base for self-assembly in the fluid phase. Self-assembly of colloids offers a direct

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FLUID PHYSICS RESEARCH PROGRAM 23 route to the fabrication of micro- and nanoscale devices with controllable structure and properties. Such research is also expected to advance fundamental knowledge and lead to innovation in terrestrial technologies for example, the fabrication of novel materials such as photonic crystals. Complex Fluid Rheology The fluid physics program has already initiated research on the theological behavior of other complex fluids, such as the particle dynamics and segregation flows of dry granular materials, or magnetorheological fluids. Preliminary results are promising, and these studies should be continued. Improved understanding of granular flows will also be beneficial to in situ resource utilization (ISRU) (on planetary exploration missions) and to the industrial processing and packaging of granular materials (pharmaceuticals, food, building materials, and so on). The ability to tailor theological response to rapidly changing conditions using magnetorheological fluids has already led to their incorporation into active damping control systems and braking systems and into cooling systems for electrical transform- ers. Their use in weightlessness has additional appeal since they could replace buoyancy as a means of controlling fluid motion. Furthermore, the manipulation of a small volume of liquid at microscales has clear overlap with research areas recommended in the emerging technology areas in Chapter 7. Interfacial Processes In low gravity, surface-tension-related phenomena can dominate liquid behavior. At small length scales, gravity is often not a controlling factor in determining the disposition of small liquid volumes, and surface forces predominate. Thus, research on interracial processes will be important for mission- related technologies and for terrestrial applications. The microgravity environment of a low-Earth-orbit laboratory allows for the isolation of interracial effects such as surface tension and offers experimental- ists expanded length scales on which to observe interracial phenomena and compare them with the same phenomena on Earth. Wetting and Spreading Dynamics Experimental and theoretical research in these areas is necessary for improved understanding of thin-film dynamics in a variety of applications that range from coating flows to boiling heat transfer. Contact-line dynamics can control the coating of solid surfaces, the cooling of hot surfaces, and the behavior of vapor bubbles in boiling. On a macroscopic scale, contact angles depend on contact-line speeds and, hence, on flow driven by gravity. Ultrathin liquid films can rupture, i.e., form dry spots, as a consequence of intermolecular attractions, creating new contact lines. Such considerations are impor- tant in the design of, for example, micro heat pipes. There is also overlap with research issues in microfluidics and nanotechnology, discussed in the next chapter. Capillary-Driven Flows and Equilibria Surface tension depends on the temperature of the interface and the concentration of impurities, regardless of whether they are intentional or accidental. When temperature and/or concentration vary along a fluid-fluid interface, stresses are created that drive motions in the fluid that enhance the transport of heat and mass. Steady motions can become unstable and lead to time-oscillatory behavior. Such

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24 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA variable surface-tension effects can control the migration of suspended droplets or the motion of drop- lets on solid surfaces and can be exploited to control droplet placement in low-gravity environments. Capillary-driven flows and transport regimes associated with evaporation and condensation are important for both terrestrial and space-based applications. Such flows occur in chip-based chemical assays, micro heat pipes, and larger-scale space-based systems, and they merit further investigation. Capillary equilibria associated with filling and maintaining liquid volumes are important for small- scale applications on Earth and for both small- and large-scale space applications. Under low-gravity conditions, surface tension can control the shapes and stability of liquid bodies. Small disturbances can dramatically shift the position of a liquid from one portion of the container volume to another, leading to configurational changes that can be important for the drainage of fuel tanks, fluids handling, and the storage of cryogenic fluids. While much work has already been done in this area, research on capillary equilibria needs to be extended to meet specific problems posed by spacecraft fluid system geometries and to applications involving microfluidic devices (see also Chapter 7~. Coalescence and Aggregation Phase separation involves the isolation of a solid, liquid, or gas or all three from the liquid or gas in which it or they are dispersed. Numerous fluid-fluid phase-separation processes rely on the coales- cence or aggregation of dispersed phases to form continuous phases, for example, droplet condensation, boiling, condensation, and foam drainage. Relative motions caused by gravity, thermocapillary forces (due to the temperature dependence of surface tension), and intermolecular forces all contribute to foam drainage and film rupture, which can be either advantageous or disadvantageous, depending on the application. Research on the effects of gravity or its absence on coalescence and aggregation is necessary for the human exploration and development of space (HEDS). These processes are important to power and life support systems for HEDS (e.g., the separation of liquid phases or the removal of bubbles or solid particles from a liquid in waste management systems) and to many related terrestrial applications. Biofluid Dynamics and Related Interdisciplinary Research New synergies gained from using the insight and techniques of fluid physics and transport phenom- ena in the world of biological sciences hold considerable promise. Future research directions that can evolve out of preexisting research themes in the fluid physics program are outlined below. New research directions in biofluids, such as microfluidic systems for drug delivery, are discussed in Chapter 7. Cellular Biotechnology Growth of tissues and cells in bioreactors has been motivated by the engineering of human tissues for a variety of transplantation purposes from articular cartilage in the knees to pancreatic cells for the treatment of diabetes. Studies of tissues and cell cultures grown in terrestrial bioreactors and microgravity bioreactors have shown striking differences in morphology and structural properties. These differences are attributed to the presence or absence of gravity and the resulting differences in flow patterns within the bioreactor. The flow within the bioreactor is known to influence growth, morphology, and structure, by virtue of shear stress exerted on the tissue or cell and mass transfer. To design and operate bioreactors more effectively and efficiently requires a better understanding of these

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FLUID PHYSICS RESEARCH PROGRAM 25 effects. Advances in the understanding of transport processes in bioreactors will be of interest to NASA from the viewpoint of HEDS medical applications and will lead to significant advances in the biological sciences and the biotechnology industry by enabling better control of tissue and cell growth. Physiologic Flows The elimination of gravity is known to affect the human body through the modification of stresses and transport processes. In the lungs, air-liquid interface problems occur in relation to airway closure and reopening, and particle deposition and clearance are particularly important where dusty planetary environments are expected. Bone loss and regeneration experienced during long-term spaceflight are influenced by transport processes, as are other intercellular and intracellular functions. Fluids research in connection with biomedical applications (both terrestrial and space-related) will be necessary to better define paths to effective countermeasures. REFERENCES Agrawal, A.K., Parthasarathy, R., Pasumarthi, K., and Griffin, D. 2000. Gravitational effects on flow instability and transition in low-density jets. Pp. 1190-1192 in Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, Ohio, August 9-11, 2000. CP-2000-210470. National Aeronau- tics and Space Administration, Washington, D.C. American Physical Society (APS). 2000. Microfluidic technologies on the rise at DFD meeting. APS News 9~3~: 6. Anderson, V.J., and Lekkerkerker, H.N.W. 2002. Insights into phase transition kinetics from colloid science. Nature 416: 811- 815. Burcham, C.L., and Saville, D.A. 2000. The electrohydrodynamic stability of a liquid bridge: Microgravity experiments on a bridge suspended in a dielectric gas. J. Fluid Mech. 405: 37-56. Chang, Y., Yaccino, J., Lakshminarayanan, S., Frangos, J., and Tarbell, J.M. 2000a. Shear-induced increase in hydraulic conductivity in endothelial cells is mediated by a nitric oxide-dependent mechanism. Arterioscler. Thromb. Vasc. Biol. 20: 35-42. Chang, Y.S., Munn, L.L., Hillsley, M.V., Dull, R.O., Yuan, J., Lakshminarayanan, S., Gardner, T.W., Jain, R.K., and Tarbell, J.M. 2000b. Effect of vascular endothelial growth factor on cultured endothelial cell monolayer transport properties. Microvascular Res. 59: 265-277. Cheng, Z., Chaikin, P.M., Zhu, J., Russel, W.B., and Meyer, W.V. 2002. Crystallization kinetics of hard spheres in microgravity in the coexistence regime: Interactions between growing crystallites. Phys. Rev. Lett. 88: 015501. Cheng, Z., Russel, W.B., and Chaikin, P.M. 1999. Controlled growth of hard-sphere colloidal crystals. Nature 401: 893-895. Cipelletti, L., Manley, S., Ball, R.C., and Weitz, D.A. 2000. Universal aging features in the restructuring of fractal colloidal gels. Phys. Rev. Lett. 84: 2275-2278. Concus, P., and Finn, R. 1990. Capillary surfaces in microgravity. Pp. 183-206 in Low-Gravity Fluid Dynamics and Transport Phenomena (J.N. Koster and R.L. Sani, eds.~. American Institute of Aeronautics and Astronautics, New York. Crocker, J.C., Matteo, J.A., Dinsmore, A.D., and Yodh, A.G. 1999. Entropic attraction and repulsion in binary colloids probed with a line optical tweezer. Phys. Rev. Lett. 82: 4352-4355. Decker, E., and Garoff, S. 1997. Contact angle hysteresis on ambient surfaces: The need for new experimental and theoretical models. J. Adhes. 63: 159. Dell'Aversana, P., and Neitzel, G.P. 1998. When liquids stay dry. Physics Today 51: 38-41. Dodge, F.T. 1990. Fluid management in low gravity. Pp. 3-13 in Low-Gravity Fluid Dynamics and Transport Phenomena (J.N. Koster and R.L. Sani, eds.~. American Institute of Aeronautics and Astronautics, New York. Dukler, A.E., Fabre, J.A., McQuillen, S.B., and Vernon, R. 1988. Gas liquid flow at microgravity conditions: Flow patterns and their transitions. International Journal of Multiphase Flow 14: 389-400. Eggleton, C.D., Tsai, T.M., and Stebe, K.J. 2001. Tip streaming from a drop in an extensional flow in the presence of surfactants. Phys. Rev. Lett. 87: 048302.

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