SP-401 Skylab, Classroom in Space

 [82] Part II - Student Experiments   Chapter 7: Fluids in Zero Gravity.

[83] The behavior of fluids on Earth, where there is gravity, is well known. Water seeks its own level, often described as the hydrostatic paradox. For example, if a number of containers of different shapes, heights, and volumes are interconnected and a liquid is poured into them, the liquid will stand at the same level in each. This phenomenon is caused by pressure. On Earth, liquid conforms to the shape of its container and at any given level, exerts equal pressure in all directions. This pressure is the product of density and liquid depth plus atmospheric pressure. This equality of force is evidenced by the shape of a freely falling drop of liquid. In an ideal situation, such a drop assumes the shape of a sphere. A famous landmark in Baltimore, Md., is the 234-foot-tall Shot Tower, once used for the manufacture of cannonballs. Molten metal dropped from the top of this tower reached the bottom in near-spherical form.

Liquid in a container presents the problem of three boundary surfaces. There is a solid-liquid film, a liquid-vapor film, and a solid-vapor film. They are only a few molecules thick and exhibit the effects of both cohesion forces (the mutual attraction of like molecules) and adhesion forces (the attraction between unlike molecules). The interaction of these two forces results in the curvature where a liquid and a solid meet. The cohesion between molecules in a liquid surface is called surface tension. This is the force that draws a drop into a spherical shape. The adhesion between a liquid and a solid surface determines the wetting characteristics of the two, that is, whether the liquid beads up on the surface like water on wax or spreads out like oil on water.

Certain forces are also present on Earth that cause water and other liquids to flow in directions other than downward. These include the circular motion of water within waves (or as it goes down a drain), the random motion within the body of a liquid, called Brownian motion, and the movement of a liquid by means of capillary attraction. All these forces have major effects on Earth, from the supply of moisture to plant roots to erosion of sea shores, as well as in certain various manufacturing processes. But how do these forces act and react in zero gravity? The answer is important to space science because of the effect of these forces on the behavior of liquid rocket fuel, potential space....

The hydrostatic paradox is illustrated by the fact that the top of the liquid stands at the same level in each container when the liquid is in a gravity environment.

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The three boundary surfaces of liquid in a container. The surface of the liquid near the solid is curved if the solid-vapor surface tension is different from the solid-liquid surface tension.

....manufacturing systems, and crew comfort and health.

Skylab student investigators developed five experiments related to fluid behavior under conditions of weightlessness. These experiments concerned capillarity, liquid motion, the colloidal state of matter, Brownian motion, and very fine powder flow.

Capillary Study

The behavior of fluids, particularly their flow properties, in a low-gravity environment first became important when Robert H. Goddard launched his liquid-fueled rockets in the 1920's. He used gas-pressurized liquid flow as a substitute for gravity. In the more sophisticated space systems of today, it is sometimes necessary to design fluidflow systems that will not only overcome the lack of gravity but also overcome or take advantage of the effects of surface tension. On Earth, capillary effects are observed when fluid adhesion to a solid surface produces a force that is sufficient to support the weight of the column of fluid. If the fluid wets the surface of a small capillary tube, it is drawn up until the weight of the column equals the adhesion force. Similarly, the fluid is depressed if it does not wet the surface of the tube.

Roger G. Johnston of Ramsey High School, St. Paul, Minn., theorized that in Skylab's zero gravity, the capillary rise might continue to infinity, since there would be no gravitational forces. His experiment consisted of two capillary tube units and a capillary-wick device. Each capillary tube unit contained identical sets of three tubes of graduated sizes. Each also had a fluid reservoir, one holding water and the other oil. The capillary-wick device had three columns of twill and mesh screen with a reservoir of a water solution that simulated the....

Roger G. Johnston theorized that a weightless column of fluid might rise to infinity in the zero gravity of Skylab. The experiment was later continued aboard the joint Soviet and American Apollo-Soyuz Mission in 1975. At right [picture below], he is shown discussing his experiment with Gene Vacca of NASA Headquarters.

[85] ...properties of liquid hydrogen, a contemporary rocket fuel. During the experiment, the mouths of the capillary instruments were to be kept in contact with the reservoir fluid, but the capillary action was to be prohibited by a special valve until the experiment was activated by a Skylab crewman.

Astronaut Pogue initiated the capillary wicking device as specified, but there was little or no wicking action for more than 2 1/2 hours. At that point, it was necessary to move it, and the resulting agitation apparently initiated some minor wicking action. Several days later, the capillary tube units were activated, but no capillary action occurred. In an effort to find the cause for such a lack of action, the crew discovered evidence of fluid leakage from the reservoirs that had occurred prior to the operation of the experiment. The loss could have been caused by vibrations of the Saturn during launch or by the excessive temperatures and reduced pressures during the early days of the near-abortive Skylab mission.

Not dismayed by this discouraging turn of events, Johnston completed a theoretical study of the capillary behavior in which he derived rise-time characteristics. He also analyzed films taken during the mission for the oscillation frequency of freefloating water globules, obtaining computed results within 2 percent of theoretical values estimated before the flight of Skylab.

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Equipment used in Johnston's capillary action experiment. The two tube devices are shown left and center, while the wicking unit is on the right. Colored water and oil were used in the tube devices, but the wicking unit held only water to simulate liquid hydrogen, a common rocket fuel. The handles seen on the devices in the center and on the right operated special actuating valves.

Liquid Motion

A stone falling into a quiet pool produces concentric rings of disturbed water moving radially outward toward the shore. Lightning is seen as it occurs, but thunder is not heard for nearly 5 seconds. A newscast informs us that the city of Anchorage, Alaska, was severely damaged by an earthquake. Each of these events is an example of wave motion-water waves, light waves, sound waves, radio waves, and seismic or shock waves. All of these waves can be defined as physical disturbances transmitted from one point to another point in a fluid, solid, or vacuum. The disturbance may be a very simple one such as a rock falling into water, or it may be a very complex one like that caused by the seismic shock wave following an earthquake.

All waves have certain well-defined properties and are based on the concept of simple, harmonic motions. There are two basic types of waves, distinguished by the motion of the particles in the medium as the wave passes. In a transverse wave, the particles vibrate at right angles to the direction in which the wave propagates. In a longitudinal wave, they vibrate in the direction of travel. In either case, the wave is characterized by its amplitude, which is the displacement of the particles from the rest position, and its period, which is the time for one complete oscillation. The frequency of the wave is the reciprocal of the period, or the number of waves or cycles per unit of time [87] (i.e., cycles per second). Waves that occur in nature frequently are combinations of transverse and longitudinal movement and have very complex shapes.

W. Brian Dunlap of Austintown Fitch High School in Youngstown, Ohio? proposed a study of wave motion in a liquid. He was particularly interested in comparing surface waves over a liquid in zero gravity with those occurring on Earth. In space, with the absence of gravity, a liquid does not necessarily take the shape of its container as it does on Earth. Adhesion forces may hold the liquid in contact with its container, but the liquid can also assume a free-floating condition. It was in this latter state that Dunlap wished to examine the behavior of surface waves.

The analysis of surface waves in water is an extremely complicated mathematical problem. Their motion is neither transverse, as in the case of electromagnetic waves (light, radio, etc.), nor is it longitudinal, as for sound. Although the water is, in general, standing still, it is alternately a crest or trough. Water is a basically incompressible fluid, and as a wave passes, a crest transforms to a trough. This causes the water particles near the surface to roll as a series of circles. Thus, the wave motion is a combination of longitudinal and transverse motions of particles. Progressively deeper beneath the surface, the particles produce ever smaller circles until their motion disappears completely.

Dunlap proposed containing water within a large, transparent, rectangular vessel, partially filled. His hypothesis was that the water would float freely within the vessel with virtually no contact with the sides of the container. He proposed using a vibrating crystal oscillator to produce waves in the water.

In analyzing the implementation of Dunlap's concept, two problems immediately arose. The first was how to insure in zero gravity that the water would truly float free of the sides of the container and at the same time maintain physical contact with the vibrator drive mechanism without having the water break into droplets. The second and more decisive problem was that the mass and...

 . W. Brian Dunlap saw in Skylab a means of studying wave motion in a liquid that was free of the pull of gravity. He is shown below discussing the equipment used in performing his experiment with his science adviser Robert Head. After completing high school, he enrolled in engineering studies at Carnegie Mellon University.

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A small cylinder containing water, an air bubble, and a window for photographing the waves on the fluid in zero gravity permitted Dunlap to achieve meaningful data from his Skylab experiment.

....volume of the required experiment were excessive. Also, the time required to develop and test it was too great for the Skylab schedule.

However, since NASA considered the behavior of liquids to be of such great importance to future space programs, alternate procedures were developed. While not meeting Dunlap's objectives in their simplest form, a method of exciting oscillations in a small amount of water was developed. A small cylinder of water containing a trapped air bubble was utilized. An expandable membrane or diaphragm closed one end of the cylinder, and a clear window was provided to enable photography of the observed motion. The diaphragm was held in place by a piston which could be released and rapidly moved away. This action allowed the diaphragm to move, producing a sudden expansion of the gas bubble and creating waves at the surface of the water. The oscillations were photographed as they became smaller and gradually disappeared. Analysis of these waves would give insight into the behavior of the water in zero gravity.

The cylinder with entrapped air was filled at Earth's atmospheric pressure. When the experiment was to be performed, the trapped gas bubble could expand against the Skylab atmosphere at approximately 5 pounds per square inch (absolute), so that an expansive pressure of about 10 pounds per square inch was available to initiate the experiment.

The experiment was launched into orbit in the unmanned Skylab and exposed to high temperature and low pressure following the loss of the meteoroid shield. During the second manned period, Scientist Pilot Garriott tried to operate it, but release of the piston failed to impart any motion to the liquid-gas interface. Part of the diaphragm was reported to be protruding into the liquid. This indicated the possibility that some of the air and liquid could have leaked out, resulting in little or no differential pressure across the diaphragm. The exact cause of the malfunction could not be determined, since this equipment was not returned to Earth at the end of the mission.

Interest in such behavior prompted Scientist Pilot Kerwin during the first manned period and Garriott during the second manned period to demonstrate that small quantities of water could be easily handled in the form of free-floating globules. Consequently, a science demonstration was performed during the third manned period [89] that showed oscillations on the surfaces of freefloating water globules and globules attached to a flat surface. Data were recorded on videotape and subsequently converted to 1 6-mm film. Dunlap analyzed these data to determine periods of oscillation of free-floating globules and found agreement with the theory to be much better than expected.

Colloidal State

The chemistry of colloidal materials is important to many disciplines of science and industry. This branch of chemistry deals primarily with the study of particulate materials of one phase of matter (i.e., solid, liquid, or gas) dispersed in another. It is important to the understanding of many ordinary materials, including glass, rubber, celluloid and other plastics, ore and minerals, beer, most foods, smoke, and pharmaceuticals.

Colloids are substances in a state of fine dispersion. They are, chemically speaking, mixtures as opposed to solutions. For example, maple sirup is a simple solution of sugar and water. The sugar dissolves in the water to form a homogeneous substance. In contrast, milk is a colloidal suspension of fat, protein, lactose, minerals, and vitamins in water.

Colloidal systems consist of submicroscopic particles which are distributed throughout another substance. The dispersed phase (usually of lesser relative concentration) is surrounded by the suspending solid, liquid, or gas called the dispersion medium or external phase. For the three phases of matter, there would obviously be nine possible types of dispersions, as shown in the accompanying table. However, only eight are colloidal, because "gas in gas" is considered to be a true solution.

Keith McGee of South Garland High School, Garland, Tex., conceived a series of investigations to study the colloidal state of matter. He proposed an experiment composed of four rectangular chambers, with transparent viewing and photography ports, heaters to control temperatures, power supply to establish electric fields, and the necessary controls. By this means he intended four specialized investigations in colloidal chemistry.

However, the time required to develop McGee's experiment was excessive. McGee then became associated with R. S. Snyder, the principal investigator for electrophoresis experiments in the Apollo program.

 Dispersed phase Dispersion medium In solid In liquid In Gas . Solid Solid sols Alloys Paper Gem stones Suspension or sol Paint Milk of magnesia Some inks Solid aerosol Smoke Dust Liquid Solid emulsion Jellies Glue Gels Emulsions Milk Protoplasm Butter Liquid Aerosol Mists Sprays Steam Fog Gas - Pumice Hydrogen in platinum metal Foams Whipped cream Solution Not a colloidal dispersion

Electrophoresis is an analytic technique used by chemists to separate the ingredients of a mixture, especially organic compounds, by means of an imposed electric field. An electrophoretic experiment was carried out aboard Apollo 16, the second mission to the Moon. The results showed that in the zero gravity of space, with reduced sedimentation and convective mixing, a much sharper separation of constituents could be obtained. A similar experiment was later performed on the Apollo-Soyuz Test Project in 1975. Such experiments suggest that it may be possible to produce extremely pure pharmaceuticals and other materials in space.

During the Skylab mission, a science demonstration utilizing a concept very similar to the one proposed by McGee was suggested by Milan Bier from the Veterans' Administration Hospital at Tucson, Ariz. This demonstration is described later. The conclusion drawn from the Skylab demonstrations of the electrophoretic process substantiated the earlier Apollo findings.

Brownian Motion

In 1827, Robert Brown, an English botanist, observed through a microscope that pollen grains suspended in water continuously vibrated. Brown....

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 . . Keith McGee proposed an ambitious experiment in the field of colloidal chemistry, which proved unattainable within the time available for student experiments aboard Skylab. However, he became associated with a scientist who had important experiments in a related field in the Apollo program and the later Apollo-Soyuz project.

[91] ....attributed this motion to the living matter. It was not until 1860, when the kinetic theory was proposed, that this hypothesis was disproved. It was then that nonliving particles in a similar suspended state were observed in the same type of motion. Today, it is well known that Brownian motion, named for its discoverer, is due to unbalanced molecular impacts on colloidal particles.

Gregory A. Merkel of Wilbraham and Monson Academy, Springfield, Mass., proposed a study of the effects of gravity on Brownian motion. His idea was to place a crystal of a colored salt in a graduated cylinder, held in place at the bottom of the cylinder. The cylinder would then be placed in a constant-temperature water bath. As the crystal dissolved, the solution near it would become colored. The apparatus, if left undisturbed, would allow the color to migrate throughout the cylinder in time due to the random Brownian motion of the solution's molecules.

Such an experiment is a classical high school demonstration of the kinetic theory of matter and....

In 1909, the French physicist J. Perrin examined fine particles in a glass container. The sketch at the left shows what he saw after a time interval in which he expected that the particles would all have settled to the bottom. The zigzag lines on the right show the paths taken by five individual particles as recorded on movie film in less than 0.3 second during a later experiment.

[92] ....requires several weeks of observations for the solution to take on a uniform color. Skylab, however, did not provide a perfectly stable platform for such an experiment. Maneuvers to maintain or change the orientation of the vehicle and vibration due to pumps, fans, and other machinery all produced small accelerations that would have precluded the satisfactory performance of Merkel's experiment. However, during the second mission a demonstration of the diffusion of tea into water was performed by the astronauts and is described later.

Powder Flow

The transport of fluids in a space vehicle has been an area of study and experiment since the early development of liquid-propellant rocketry. Liquid fuels, liquid coolants, lubricants, and water all have presented problems of one kind or another in the space environment. Kirk M. Sherhart of Berkley High School, Berkley, Mich., suggested an experiment to study the flow of powdered solids as contrasted with liquids in zero-gravity environment.

Several simple plastic models of various sizes.....

Gregory A. Merkel felt that the zero gravity of Skylab might significantly alter the movements of very small particles suspended in water, a phenomenon called Brownian movement.

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 . Kirk M. Sherhart suggested an experiment for Skylab that concerned the effects of weightlessness on the flow of very fine powders. Later, he became interested in computer engineering.

....were constructed with mechanical pistons used to force steel marbles, small plastic beads, and a fine powder similar to talcum through sized openings. Preliminary tests revealed that the packing together of the spheres prevented their flow. The consensus of the researchers was that Sherhart's idea was worthy of investigation. However, there was not sufficient time to perform the necessary basic research toward development of an experiment that had a reasonable probability of success. As a result, even though Kirk's experiment did not fly, he was affiliated with the NASA researchers in the area of materials handling in space at the Goddard Space Flight Center.

The lack of gravitational force and very low accelerations in Skylab furnished a challenge to several students who saw how such a condition could be used to study the behavior of fluids. Again, their scientific curiosity and imagination prompted them to take advantage of weightlessness to propose experiments that were at once stimulating and fruitful.