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Fluid Physics


CONTENTS
Complex Fluids Dynamics and Stability
Multiphase Flow and Heat Transfer Fundamental Physics
Interfacial Phenomena  

A fluid is any material that flows in response to an applied force; thus, both liquids and gases are fluids. Some arrangements of solids can also exhibit fluid-like behaviors; granular systems (such as soil) can respond to forces, like those induced by earthquakes or floods, with a flow-like shift in the arrangement of solid particles and the air or liquids that fill the spaces between them. Fluid physicists seek to better understand the physical principles governing fluids, including how fluids flow under the influence of energy, such as heat or electricity; how particles and gas bubbles suspended in a fluid interact with and change the properties of the fluid; how fluids interact with solid boundaries; and how fluids change phase, either from fluid to solid or from one fluid phase to another. Fluid phenomena studied range in scale from microscopic to atmospheric and include everything from the transport of cells in the human body to changes in the composition of the atmosphere.   description under photo

Side views of water and airflowing through a clearpipe. At 1g, the air stays on top. In microgravity, the air canform a core down the center of the pipe.

The universal nature of fluid phenomena makes their study fundamental to science and engineering. Understanding the fluid-like behavior of soils under stress will help civil engineers design safe buildings in earthquake-prone areas. Materials engineers can benefit from a better grasp of how the structure and properties of a solid metal are determined by fluid behavior during its formation. And knowledge of the flow characteristics of vapor-liquid mixtures is useful in designing power plants to ensure maximum stability and performance. The work of fluid physics researchers often applies to the work of other microgravity scientists.

Complex Fluids
This research area focuses on the unique properties of complex fluids, which include colloids, gels, magneto-rheological fluids, foams, and granular systems.

Colloids are suspensions of finely divided solids or liquids in fluids. Some examples of colloidal dispersions are aerosols (liquid droplets in gas), smoke (solid particles in gas), and paint (solid in liquid). Gels are colloidal mixtures of liquids and solids in which the solids have linked together to form a continuous network, becoming very viscous (resistant to flow). Magneto-rheological fluids consist of suspensions of colloidal particles. Each particle contains many tiny, randomly oriented magnetic grains and an externally applied magnetic field can orient the magnetic grains into chains. These chains may further coalesce into larger-scale structures in the suspension, thereby dramatically increasing the viscosity of the suspension. This increase, however, is totally reversed when the magnetic field is turned off.

A foam is a nonuniform dispersion of gas bubbles in a relatively small volume of liquid that contains surface-active macromolecules, or surfactants (agents that reduce the surface tension of liquids). Foams have striking properties in that they are neither solid, liquid, nor vapor, yet they exhibit features of all three. Important uses for custom designed foams include detergents, cosmetics, foods, fire extinguishing, oil recovery, and many physical and chemical separation techniques. Unintentional generation of foam, on the other hand, is a common problem affecting the efficiency and speed of a vast number of industrial processes involving the mixing or agitation of multicomponent liquids. It also occurs in polluted natural waters and in the treatment of wastewater. In all cases, control of foam rheology and stability is required.

Examples of granular systems include soil and polystyrene beads, which are often used as packing material. Granular systems are made up of a series of similar objects that can be as small as a grain of sand or as large as a boulder. Although granular systems are primarily composed of solid particles, their behavior can be fluid-like. The strength of a granular system is based upon the friction between and geometric interlocking of individual particles, but under certain forces or stresses, such as those induced by earthquakes, these systems exhibit fluidic behavior.

Studying complex fluids in microgravity allows for the analysis of fluid phenomena often masked by the effects of gravity. For example, researchers are particularly interested in the phase transitions of colloids, such as when a liquid changes to a solid. These transitions are easier to observe in microgravity. Foams, which are particularly sensitive to gravity, are more stable (and can therefore be more closely studied for longer periods of time) in microgravity. In magneto-rheological fluids, controlling rheology induced by a magnetic field has many potential applications, from shock absorbers and clutch controls for cars to robotic

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USML-2 Payload Commander Kathryn C. Thomton works at the
Drop Physics Module, used to investigate liquid drop behavior in microgravity.
joint controls.Under the force of Earth's gravity, the magnetic particles in these fluids often fall out of suspension due to sedimentation, but in microgravity this problem is eliminated. Investigations of the behavior of granular systems, which have previously been hampered by Earth's gravity, are more feasible in microgravity because they do not settle as they do on Earth.

Multiphase Flow and Heat Transfer
This research area, which has applications in the engineering of heat transfer systems and gas purification systems, focuses on complex problems of fluid flow in varying conditions. Scientists are seeking to add to their currently limited knowledge of how gravity-dependent processes, such as boiling and steam condensation, occur in microgravity. Boiling is known to be an efficient way to transfer large amounts of heat, and as such, it is often used for cooling and for energy conversion systems. In space applications, boiling is preferable to other types of energy conversion systems because it is efficient and the apparatus needed to generatepower is smaller.

Another of the mechanisms by which energy and matter move through liquids and gases is diffusive transport. The way atoms and molecules diffuse, or move slowly, through a liquid or gas is due primarily to differences in concentration or temperature. Researchers use microgravity to study diffusion in complex systems, a process that would normally be eclipsed by the force of gravity.

Understanding the physics of multiphase flow and heat transfer will enable scientists to extend the range of human capabilities in space and will enhance the ability of engineers to solve problems on Earth as well. Applications of this research may include more effective air conditioning and refrigeration systems and improvements in power plants that could reduce the cost of generating electricity.

Interfacial Phenomena
Research in this area focuses on how an interface, like the boundary between a solid and a liquid, acquires and maintains its shape. Interface dynamics relate to the interaction of surfaces in response to heating, cooling, and chemical influences. A better understanding of this topic will contribute to improved materials processing and other applications.

Interfacial phenomena, such as the wetting and spreading of two immiscible liquids or the spreading of fluid across a solid surface, are ubiquitous in nature and technology. Duck feathers and waterproof tents repel water because the wetting properties of the surfaces of their fibers prevent water from displacing the air in the gaps between the fibers. In contrast, water spontaneously displaces air in the gaps of a sponge or filter paper. Technologies that rely on dousing surfaces with fluids like agricultural insecticides, lubricants, or paints depend on the wetting behavior of liquids and solids. Wetting is also a dominant factor in materials processing techniques, including film and spray coating, liquid injection from an orifice, and crystal growth. Interfaces dominate the properties and behavior of advanced composite materials, where wetting of the constituent materials dictates the processing of such materials. Understanding and controlling wetting and spreading pose both scientific and technological challenges.

In reduced gravity, wetting determines the configuration and location of fluid interfaces, thus greatly influencing, if not dominating, the behavior of multiphase fluid systems. This environment provides scientists with an excellent opportunity to study wetting and surface tension forces that are normally masked by the force of Earth's gravity. This research also provides information that can help improve the design of space engineering systems strongly affected by wetting, including liquid-fuel supply tanks, two-phase heat transfer and/or storage loops, and fluids management devices for life support purposes
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Comparison of thermocapillary flows on Earth (top) and in micogravity (bottom). The flow pattern (indicated by the white areas) in the Earth-based experiments is only evident on the fluid's surface, while the flow pattern in microgravity encompasses the entire fluid.

Dynamics and Stability
This broad area of research includes drop dynamics, capillarity, and magneto/electro-hydrodynamics.

Drop dynamics research deals with the behavior of liquid drops and gas bubbles under the influence of external forces and chemical effects. Research in drop dynamics ranges from the study of rain in the atmosphere to the investigation of chemical processes. A potential application of these studies is in the realm of materials processing. In forming solid materials from liquids in space, it is usually important to create pure and/or uniform solids-gas bubbles and drops of foreign liquids are undesirable. Yet due to the microgravity environment, these bubbles and drops of substances of lower densities would not "rise to the top" the way they would if they were on the ground, which makes extraction of the bubbles difficult. Researchers are attempting to resolve this problem in order to facilitate better materials processing in space.

Scientists are also interested in studying single bubbles and drops as models for other natural systems. The perfect spheres formed by bubbles and drops in microgravity (due to the dominance of surface tension forces) are an easy fit to theoretical models of behavior-fewer adjustments need to be made for the shape of the model. Investigators can manipulate the spherical drops using sound and other impulses, creating an interactive model for processes such as atom fissioning.

Capillarity refers to a class of effects that depend on surface tension. The shape a liquid assumes in a liquid-liquid or liquid-gas system is controlled by surface tension forces at the interface. Small disturbances in the balance of molecular energies at these boundaries or within the bulk of the liquid can cause shifts in the liquid's position and shape within a container (such as a fuel tank) or in a containing material (such as soil). These changes, or capillary effects, often occur in liquids on Earth,but are to some degree masked or minimized by the stronger force of gravity. In microgravity, however, capillary effects become prominent. The study of capillary phenomena in microgravity will enable researchers to better understand and predict fluid configurational changes both on Earth and in low-gravity environments.

Microgravity fluid physics researchers also study the effects of magnetic and electric fields on fluid flows, or magneto/electrohydrodynamics. Promising microgravity research subjects in this area include weak fluid flows, such as those found in poorly conducting fluids in a magnetic field, and Joule heating. In Earth's gravity, Joule heating causes buoyancy-driven flows which, in turn, obscure its effects. In microgravity, however, buoyancy-driven flows are nearly eliminated, so researchers are not only able to study the effects of Joule heating, but they can also observe other processes involving applied electric fields, such as electrophoresis.

 

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This sequential photo shows a liquid bridge undergoing a series of shape changes. Liquid bridge investigations on the Shuttle have tested theories of electrohydrodynamics.

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In materials science research, float zone samples are sometimes used for crystal growth. For a float-zone sampler the surface tension of the melt keeps the sample suspended between two sample rods in a furnace. A thorough understanding of the capillarity and surface tension effects in a molten sample allows better experiment control and results prediction.


Fundamental Physics

Physics is a major part of fundamental science where the ultimate goal is to establish a unified description of the basic laws that govern our world. At present fundamental physics includes low temperature physics, condensed matter physics (the study of solids and liquids), laser cooling and atomic physics, and gravitational and relativistic physics. A unifying characteristic of these research areas is that they address fundamental issues which transcend the boundaries of a particular field of science.

The majority of experiments in fundamental physics are extensions of investigations in Earthbased laboratories. The microgravity experiment in these cases presents an opportunity to extend a set of measurements beyond what can be done on Earth, often by several orders of magnitude. This extension can lead either to a more precise confirmation of our previous understanding of a problem, or it can yield fundamentally new insight or discovery. The remainder of fundamental physics research involves tests of the fundamental laws which govern our universe. Investigations aim at enhancing our understanding of the most basic aspects of physical laws, and as such may well have the most profound and lasting longrange impact on mankind's existence on Earth and in space.
 

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Researchers observe thefloat package and data rack of a superfluid helium experiment on a parabolic aircraft fight.


There are many examples of how fundamental science has had an impact on the average person. Basic research in condensed matter physics to explain the behavior of semiconductors led to the development of transistors which are now used in communication devices, and which produce ever more prevalent and capable computer technology. Research in low temperature physics to explore the properties of fluids at very low temperatures led to advanced magnetic resonance techniques that have brought extremely detailed magnetic resonance imaging to the medical doctor, so today much exploratory surgery can be avoided. A less widely appreciated part played by fundamental science in today's world has been the need to communicate large quantities of data from physics experiments to collaborators at many locations around the world. Satisfying this need was instrumental in the development of the Internet and the World Wide Web.

Fundamental physics research benefits from both the reduction in gravity's effects in Earth-orbit and from the use of gravity as a variable parameter. In condensed matter physics, the physics of critical points has been studied under microgravity conditions. This field needs microgravity because the ability to approach a critical point in the Earthbound laboratory is limited by the uniformity of the sample which is spoiled by hydrostatic pressure variations. One of the important issues in condensed matter physics is the nature of the interface between solids and fluids. The boundary conditions at this interface have an influence on macroscopic phenomena, including wetting. The microscopic aspects of the system near the boundary are difficult to study. However, when the fluid is near a critical point, the boundary layer adjacent to the solid surface acquires a macroscopic thickness. Research under microgravity conditions permits the study of not only the influence of the boundaries on thermodynamic properties, but also transport properties such as heat and mass transport. One of the most dramatic advancements in atomic physics over the last decade has been the demonstration that laser light can be used to cool a dilute atomic sample to within micro- or even nano-degrees of absolute zero. At these low temperatures, the mean velocity of the atoms drops from several hundred m/s to cm/s or mm/s, a reduction by four to five orders of magnitude. When atoms are moving this slowly, measurements of atomic properties can be made more precisely because the atoms stay in a given point in space for a longer time. In this regime, the effects of gravity dominate atomic motion so experiments performed in a microgravity environment would allow even more precise measurements.

Among the most important goals of such research is the improvement of ultra-high precision clocks. These clocks not only provide the standard by which we tell time, but are crucial to the way we communicate and navigate on Earth, in the air, and in space. Laser cooled atoms have significantly improved the accuracy and precision of clocks because these atoms move very slowly and they remain in a given observation volume for very long times. However, observation times in these clocks are still affected by gravity. Because of the effects of gravity, the atoms used in these clocks ultimately fall out of the observation region due to their own weight. Increased observation times are possible in microgravity and can result in further improvements in precision of at least one or two orders of magnitude.

Indeed, clocks are central to the study of general relativity and in questions concerning the very nature of gravity itself. The motivation for space based clocks is not only tied to the improved performance expected in a microgravity environment but also these clocks will have access to different positions in space than are available on Earth. An important example of this physics is revealed in the comparison of an Earth-based clock with a space-based clock. This comparison provides a direct measurement of the gravitational redshift. Tests of Einstein's theories of relativity and of other theories of gravitation serve as a foundation for understanding how matter and space-time itself behave at large length scales and under extreme conditions. The freefall environment of orbit, the use of low temperature techniques, and the use of high precision frequency standards offer opportunities to perform improved tests of these theories. Direct tests of gravitation theories and other fundamental theories, including the Law of Universal Gravitation, can be performed in a microgravity environment.


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