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Fluid Physics
CONTENTS
| 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. |
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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.
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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. |
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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.
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.
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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.
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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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