In keeping with the charge that NASA's space activities shall contribute to the knowledge of phenomena in space, a series of science demonstrations, enhanced by Skylab's weightless environment, was scheduled to illustrate certain scientific principles. They ranged from the mere satisfaction of curiosity to providing significant scientific results.
The demonstrations utilized either onboard equipment or extremely simple implements that could be easily taken into orbit aboard the space station. Small kits were launched for the second and third visits to supply the few items not originally on board.
However, it was really the interest and ingenuity of the crew, and their desire to contribute time designated for rest and recreation that made them possible. Fifteen science demonstrations were devised for the second crew to perform. Two of these were added after the crew, at mid-mission, requested additional activities. Twenty-one demonstrations were provided for the third crew, but not all were performed, since the science demonstrations could be done only on a "time available" basis.
The demonstrations involved mechanics, magnetic effects, particle physics, fluid phenomena, crystal growth, and life sciences. All the demonstrations were recorded on the videotape; photographs were later made from conversion of the tapes to film. Because of this, the photographs were not always of the best quality. (For available instructional material based on these demonstrations, see p. 178.)
 On Earth, gravity has a strong influence on the behavior of liquids. Because of this force, a liquid settles in the bottom of its container. Another force acts upon any liquid that has surface exposed to a gas. The mutual attraction of the liquid molecules at the surface produces a tension force that causes the surface to behave as if it were covered with an elastic membrane. Gravity usually dominates the surface tension force on Earth, flattening the "membrane" so the presence of the force is not readily detected. However, in a small volume of free-falling water, surface tension can be observed to form the water into a uniform, nearly spherical shape, as it does in a raindrop.
In Skylab, the effect of gravity was negated and surface tension became the dominant force acting upon a liquid. Because of surface tension, the behavior of a liquid was considerably different in zero gravity.
A number of fluid-mechanics experiments were performed in Skylab to demonstrate and to evaluate the behavior of liquids under conditions where surface tension forces dominated. In certain cases, phenomena were demonstrated that are not possible on Earth; in others a comparison between the phenomena in space and on Earth was possible.
A knowledge of zero-gravity fluid mechanics is essential to the design of any fluid system that operates in space, from rockets with liquid propellants to the water used in a life-support system. In the future, space may become the best place for processing of materials and pharmaceuticals, thus eliminating detrimental convection and sedimentation effects caused by gravity, and permitting containerless handling of fluids where contamination from the container occurs. In order to perform such processes, the basic fluid phenomena must be understood. Even Earth-based phenomena, such as the falling raindrop mentioned above, can be studied in a more basic form when the effect of gravity is eliminated.
A number of free-floating water globules were formed as part of the Skylab fluid-mechanics experiments. An undisturbed drop of liquid assumes a spherical shape in zero gravity. The "membrane" effect of surface tension causes the liquid to be drawn into a shape that has a uniform pressure over its entire surface, i.e., a sphere.
The interaction of a free-floating drop with various solid surfaces was also demonstrated. A drop was placed on a string to prevent it from drifting out of the view of the camera. The "membrane" of surface tension must attach itself to any solid surface that the liquid contacts. Surface tension caused the drop to be strongly centered on the string. In this position, the pressures on the drop were in equilibrium. Since the "membrane" was distorted by the string, the shape of the drop was elongated in the direction of the string. As the size of the solid surface contacting the drop was increased, the distortion of the....
....surface was increased. While the string produced nearly imperceptible distortions, a drop on a straw was noticeably distorted.
A drop of water placed on a flat plastic surface had a hemispherical shape. In this demonstration the liquid surface was perpendicular to the solid surface at their intersection. In maneuvering the drops of water for the experiments, the astronauts found that wires or strings were the most effective instruments for applying small forces to the floating water globules. If objects with a larger surface were used, such as a straw, the drop merely stuck to the object and was difficult to detach.
The nature of the surface also influenced the shape of a drop on the surface, a fact that was demonstrated by placing water on each of three materials: plastic, metal, and paper. How readily a liquid wets a surface depends upon their relative energies. Surface energy is a quantitative measure of the energy that produces the forces of adhesion and surface tension. Metals have high surface energies in comparison to liquids, so the liquids readily spread over them. Plastics have lower surface energies than metals, so water did not wet plastic as it did metal. Some of the water was absorbed into paper, influencing wetting.
Free-floating drops of liquid were rotated to demonstrate the effect of centrifugal force on their shape. Centrifugal forces and surface tension forces balanced one another to establish the equilibrium shape. Strings were used to apply a force on the edge of a drop, causing it to rotate. Beyond a certain rotation rate, the drop assumed the shape of a peanut. As the rotation rate increased, the neck of the center of the drop became thinner. At some higher rotation rate, surface tension could no....
 ...longer hold the drop together, and it separated into two drops of equal size. Raindrops are believed to break up within clouds in this manner.
Another interesting demonstration of the influence of the liquid surface in zero gravity was the melting of ordinary ice. An ice cylinder was photographed as it melted. As water formed, it collected on the ice cylinder to give the ice/water combination a more spherical shape. On Earth, gravity would drain the liquid away from the ice. The time required for the ice to melt is increased in zero gravity because the layer of liquid insulates the ice from the surrounding warm air. It took approximately 190 minutes for the ice to melt in Skylab and only 130 minutes for it to melt in a duplicate experiment on Earth. The melting of ice simulated some of the fluid aspects of processing metals in zero gravity.
Oscillation of Liquid Drops
Free-floating drops of water were caused to oscillate by stretching and releasing them. Oscillations were induced by placing the flat ends of plungers removed from syringes against opposite sides of the drop, and then rapidly pulling them away from the drop. The surface of the drop was stretched until it broke away from the plungers and the drop oscillated at a constant frequency. Surface tension pulled the distorted drop back to the spherical shape, but the inertia of the liquid mass continued the motion of the liquid beyond the equilibrium spherical shape, and the drop distorted in a direction perpendicular to the initial perturbation. Again, surface tension opposed the distortion. The shape passed through the equilibrium condition again and then distorted along the axis of the initial disturbance. Oscillation occurred at the harmonic frequency of the drop. Since water is viscous, energy was dissipated with each oscillation cycle, and the drop eventually came to rest in a spherical shape. A water drop on a flat surface was also oscillated and behaved in a similar manner.
The demonstrations were significant because they were the largest drops (6 cubic inches) that have ever been observed in oscillation. Their size allowed detailed observation of the oscillations.
Drops of water were also impacted with one another to observe their coalescence. The sizes of....
....the drops impacting and being impacted and their relative speed and angle of impact were varied. The initial joining of the drops was dependent upon the rate at which the air between the drops was moved out of the way. It is possible, and this was demonstrated in Skylab, for two drops to bounce off one another if a film of air remained between the drops. After the drops combined, it was also....
....possible for them to separate again. If their momentum before impact was sufficiently large, surface tension could not hold the combined drop together. Coalescence occurred at relatively low velocities in the Skylab demonstrations, so that the resulting drop formed from the impacting drops remained intact. Coalescence is another phenomenon important in the formation of raindrops.
Two liquids that will not mix, such as oil and water, are said to be immiscible. When vigorously shaken, the two liquids can become intermingled, but one does not dissolve in the other. Eventually, the force of gravity will cause the heavier liquid to separate from the lighter, producing two distinct layers. Experiments were made in Skylab to determine what happens when immiscible liquids are mixed in zero gravity.
Oil and water were placed in transparent plastic vials. By swinging the vials on the end of a string, the two liquids were separated by centrifugal force. The vials were then shaken to disperse the liquids and observed to see whether separation took place. While a gravity force was not present to separate the liquids, some separation by coalescence was possible. Small drops of the same liquid joined as they came into contact, eventually into significant amounts.
On Earth, a dispersion of the two liquids separated completely in 10 seconds. In Skylab, the dispersions were observed for a period of 10 hours, during which time only a very small amount of coalescence occurred. Low gravity thus provided an opportunity to form a dispersion of immiscible liquids which could be solidified in that form. The demonstration showed that composite materials with unique properties could be manufactured by such a process.
Because of surface tension, a liquid can be stretched into a very thin film. After it is formed, the liquid continues to drain from it, because of surface tension and gravity, until the film becomes so thin that it ruptures. Certain additives to water, such as soap, strengthen the surface of a film, and fairly large films can be formed.
Experiments were performed aboard Skylab to determine how the behavior of films would change....
....in zero gravity. Films were formed by expanding wire hoops, a circular one in the form of a lasso and a rectangular one that had one sliding side. The sliding rectangle was shown to be a more controlled method of forming a film. It was demonstrated that large films (3-inch-diameter loop) could be formed from plain water, something that is not possible on Earth. The largest films, formed  from a soap solution (6-inch-diameter loop) were about the same size as could be formed on Earth, but films formed on Earth ruptured sooner than those formed in Skylab. With gravity absent, the rate at which liquid drained to the edge of the film was reduced.
Wire was formed to the shape of a tetrahedron and a cube, so that three-dimensional films could be made. When the frame was slowly pulled from the soap and water solution, it was full of liquid. The liquid surface adhered to the wires, allowing the frames to act as containers. When they were shaken, most of the liquid was emptied from them, and the three-dimensional films remained. These films went from the frame to the center of the cube or tetrahedron. As with the loops, the three-dimensional films formed in space lasted longer than those formed on Earth, about I minute in Skylab compared to only a few seconds on Earth.
Diffusion in Liquids
Diffusion is a process by which fluids can become uniformly mixed as a result of the random motions of their molecules. Differences in temperature within a fluid will be equalized by diffusion. When two different fluids in solution are present, diffusion will uniformly mix the two. Diffusion by itself is difficult to observe on Earth because gravity produces convection or circulation within a fluid due to differences in density of its constituents. For this reason, a diffusion experiment was performed in the zero gravity of Skylab.
A tube was partially filled with water, and a concentrated solution of tea and water was carefully placed on top of the water. The diffusion of the tea into the water was observed for a period of 3 days. During this period the tea diffused a distance of approximately 0.8 inch into the water (measured at the center of the tube). Theory predicted the same value for the rate of diffusion.
However, an unexpected result occurred when the tea diffused very little along the wall of the tube, apparently due to some effect of its wall. It is possible that electrostatic repulsion between the wall and the tea particles may have produced the bullet-shaped diffusion front.
This experiment was a specific application of zero-gravity fluid mechanics to the processing of....
...materials in space. John Carruthers of Bell Laboratories, Murray Hill, N.J., suggested a demonstration to investigate a means by which metals can be melted and solidified under controlled conditions without using a container. It is difficult to provide a container for certain molten metals that does not introduce contamination into them and degrade their quality.
A liquid-floating zone is a mass of molten metal suspended between two solid rods of the same metal. In zero gravity, with the proper alinement and spacing of the solid rods, the surface tension force will form the molten metal into a uniform cylindrical shape. The molten metal will then solidify without the undesirable circulation within the metal that would exist in a gravity environment. Thus the liquid-floating zone provides a containerless method of obtaining uniform solidification of a metal in zero gravity. Large single crystals of a metal can be made using this method.
A Skylab science demonstration investigated the behavior of a liquid-floating zone, simulated with water suspended between two parallel circular disks. Zones of various volumes were formed by placing liquid on each disk with a syringe and bringing the disks together to form a single column. The zone was oscillated and rotated to determine its stability. (It is desirable to rotate a zone so that the molten metal will be uniformly heated and mixed.)
When the zone was rotated, it was found that the liquid could swing out from between the disks and rotate like a jump rope. A further increase in rotation rate would cause the zone to break apart. By adding soap to the water, forming a more viscous liquid, the stability of the zone was increased, and the "jump rope" did not form. As the zone was rotated, it remained alined with the disks. At a large enough rotation rate, the zone constricted in the center and finally broke apart. The effect of rotating one disk, both disks in the same direction, and both disks in opposite directions was also evaluated.
The tests demonstrated that the liquid-floating zone method of processing metals in space is feasible. The size, oscillation, and rotation of the zone must be properly controlled to obtain uniform solidification of the molten metal.
Charged Particle Mobility
One of the more promising applications of processing materials in space is the purification of biological compounds by electrophoresis.  Electrophoresis is defined as the movement of suspended charged particles through a fluid under the influence of an electrical potential. When dispersed in a water solution, practically all substances acquire an electric charge because they tend to exchange hydrogen ions with their surroundings, and such ions are usually present in water.
Different types of particles or molecules acquire different charges and also demonstrate characteristic degrees of mobility-the ability to migrate- because they experience different drag forces due to their particular sizes and shapes. Assuming a common starting place, continued application of an electric field to a group of different particles suspended in a solution will result in stratification or sorting of the charged ones into separate and distinct zones.
Electrophoresis has contributed significantly to biological sciences and shows potential for large-scale purification of a wide range of medically important substances. It is a useful technique, for example, in the separation of complex biological mixtures of proteins. As a result, there has been an advance in electrophoretic techniques applied to various other substances as well. However, it has not yet been possible to develop electrophoresis to provide commercially significant quantities of materials with the necessary quality control. The scientific, biomedical, and economic implications of such a development could be of major significance. Large quantities of human blood proteins are now separated by standard methods involving alcohol and salt fractionation, or distillation, resulting in low purity. The same situation prevails with enzyme and protein hormones. All of these substances have widespread clinical and research applications.
Even more promising is the possibility of applying electrophoresis to the separation of living cells. It is necessary to isolate each type of cell of the human body to determine its function. Some cells are so much alike, or appear in such small quantities, that isolating them is almost impossible on Earth.
Pure cell populations have widespread potential uses in a variety of applications including preparation of vaccines. Why does the human body reject a transplanted heart but not a tumor? Today the answer is not yet known. Perhaps the answer is in the fact that cells called Iymphocytes have not been fully isolated nor is their relation to the body's immune response known with certainty. It may be that their isolation can be accomplished in space. Once the important cells and their behavior have been isolated, it may also be possible to trick the body into rejecting a tumor but not a transplanted heart.
The major problem encountered in this process on Earth is circulation within the transfer medium, primarily caused by convection and sedimentation. These are differential effects of gravitational force on layers of different temperatures and on various particle sizes. The obvious solution of using the space environment to stabilize liquid media for electrophoretic separation was proposed late in 1969. It was postulated that high separation resolution should be obtainable because convective mixing and sedimentation would be greatly reduced or eliminated.
An opportunity presented itself to include a simple electrophoresis experiment, called charged....
.....particle mobility, as part of the science demonstrations for the third Skylab mission. Electrophoresis experiments performed on Apollo 14 and 16 had indicated the possible advantages of zero-gravity electrophoresis, but they also demonstrated that absence of gravity did not eliminate all problems. Separation boundaries were parabolic, distorted by random fluid motions of undetermined origin. To overcome these difficulties, Milan Bier, a biophysicist at the Veterans Administration Hospital in Tucson, Ariz., proposed the use of isotachophoresis, an electrophoretic technique with self-sharpening boundaries, stabilized by electrical forces. The main limitations imposed in developing the Skylab demonstrations were the short time in which the experiment had to be prepared, the limited power available, and the requirement for the entire package to fit the available volume in an existing launch canister: a cylinder 3.5 inches in diameter and length.
A simple electrophoretic assembly was constructed, consisting of two Plexiglass modules. The observation channel was 0.2 inch in diameter and I inch long. One of the two modules contained a mixture of two colored proteins, ferritin and hemoglobin. The second module contained a suspension of human red blood cells. Because of the probability of cell sedimentation due to centrifugal effects during liftoff, the cathode compartment was completely filled with the cell suspension, and a small stirrer was incorporated to permit the astronauts to perform a resuspension of the cells after reaching orbit.
The results of the two experiments were limited. The main reason appeared to be significant leakage of the fluids from both modules, resulting in many air bubbles in the chambers, even though all fluids had been carefully degassed prior to filling with cells. The leakage probably occurred during launch as a result of acceleration and vibration. The protein experiment was a failure, as there was no observable migration of any colored proteins after the astronauts opened the sliding gate with the sample. An examination of the silver anode showed that at no time was there any current through the cell, though there was electrical continuity of all connections. The most likely explanation is that an air bubble completely prevented passage of the electrical current.
The results with the blood-cell suspension were better. Although the view was partially obstructed by air bubbles, the advancing front nevertheless showed little bowing. Upon completion of forward migration, the current polarity was reversed, and the astronauts cleared the air bubbles from the observation channel by mechanical agitation. The crew then repeated the frontal migration a second time. The photographs show an extremely sharp boundary, with a blunt parabolic profile. The sharpness and self-restoring properties of boundaries in isotachophoresis make it an attractive candidate for future space applications.