THE CONCEPT of an engine that would transform chemical energy directly into mechanical work has been pursued by experimenters for many years. Such a machine could be expected to be much more efficient in terms of fuel consumption than are the conventional systems that first convert chemical energy into heat or electricity and then into mechanical work. A one-step conversion could also be expected to produce less waste product.

Engines of this type exist in the form of muscle. An animal's muscles convert almost half of the energy from food into useful work, whereas the best modern steam-turbine systems have an efficiency of about 40 percent. A promising step toward the development of an engine based on synthetic muscle was taken about 20 years ago by a group of investigators at the Weizmann Institute of Science in Israel. The group found that a certain gel resembles muscle in that it contracts and lifts a weight when it is immersed in a solution of appropriate chemicals. Immersion in a different solution causes the gel to relax [see "Muscle as a Machine," by A. Katchalsky and S. Lifson; SCIENTIFIC AMERICAN, March, 1954]. A comparable material is now manufactured in the form of collagen tape. With this material Martin V. Sussman of the Tufts University department of chemical engineering made, while he was a guest of the Weizmann Institute, a mechanochemical turbine that rotates as smoothly as an electric clock. The output power can be taken from a pulley on one of the rotating shafts. Sussman writes:

"The new engine is actuated by a collagen fiber in the form of a loop, a portion of which is wrapped helically around a pair of contoured spindles that are canted with respect to each other [see illustration at left]. The spindles are partly submerged in a tank of salt solution and are free to rotate. The brine serves as the fuel. The remaining portion of the fiber passes through a tank of fresh water below the fuel.

"The salt solution causes the collagen to shrink and thus to develop a tensile force that is relieved as the fiber passes around the rotating tapered spindles. After spiraling through the brine the fiber is pulled from the solution by a belt-driven roller and lowered into the water bath. The diffusion of salt from the collagen into the water restores the material to its original length. Power developed by the turbine pulls the relaxed fiber from the water through a glass ring that guides the material to the top of the spindles for the next cycle.

"In its present state of development the turbine generates mechanical power measured in milliwatts at the output shaft. Even so, it demonstrates a basic thermodynamic cycle of some interest. The cycle is outlined by the accompanying graph [right], which shows how the length and the tension of one centimeter of collagen fiber vary in pure water and in brine.

"In this experiment the brine consisted of an eight-molar aqueous solution of lithium bromide. (A liter of solution at this concentration contains 695 grams of the salt.) The fiber shrinks to about 60 percent of its normal length when it is transferred from a bath of fresh water to a lithium bromide solution of this concentration. Collagen retains its flexibility when it is saturated with salt; it can be stretched to its salt-free length by exerting on its ends a tension of 52 kilograms per square centimeter of cross-sectional area. It is this property that the machine exploits to generate power.

"As shown by the graph, the cycle begins at point A, proceeds through points B and C and returns to A. At point A salt has been washed from the collagen. The fiber is at its normal length and in zero tension. In this state it is carried to the top of the spindles, which are cylindrical. The cylindrical shapes constrain the fiber so that it cannot shrink. The fiber makes two turns around the cylinders to establish solid frictional contact between the slippery collagen and the surface of the spindles.

"The material is then carried into the salt solution [point B]. Tension increases as the fiber absorbs salt. Tension is relieved as the shrinking material spirals through the brine and exerts torque on the tapered part of the rotating spindles [from point B to point C]. The taper of the spindles enables the force against which the fiber acts to decrease gradually as tension in the fiber decreases. The arrangement allows efficient release of the tensile energy in the fiber. The cycle is closed by washing salt from the collagen, which restores the fiber to its normal length [from point C to point A]. The cycle can then be repeated, although some degeneration of the collagen becomes evident after about 100 cycles.

"The maximum power that an ideal turbine of this type can generate is proportional to the area enclosed by points A, B, C and A of the graph. For a collagen fiber with a normal dry length of 270 centimeters per gram and a density of 1.3 grams per centimeter the area enclosed by points A, B, C and A of the graph indicates an energy conversion of 7.85 X 10 ergs of work per gram of fiber. The turbine that was used in the experiments has a collagen throughput of .09 gram per second, which corresponds to an ideal power output of 70 milliwatts.

"The machine delivers about 30 milliwatts at its output shaft, which corresponds to a mechanical efficiency of about 40 percent. A substantial portion of the difference between the ideal energy conversion and the actual output of the machine can be traced to power spent in overcoming friction in the bearings of the machine and in pulling the fiber through the liquids. Another factor that impairs efficiency arises from the transfer of liquid between the tanks of brine and wash water, both of which tend to cling to the collagen. Liquids so mixed do not contribute to the output. Doubtless efficiency could be increased by installing wipers at the points where the fiber is withdrawn from the tanks.

"The output of work per cycle can be increased somewhat by pretensioning the fiber before it is immersed in the brine. Pretensioning can be accomplished by feeding the washed collagen onto short inverted cones at the top of the cylindrical sections of the spindles. Salts other than lithium chloride can be used as fuel, including calcium chloride, magnesium chloride and potassium thiocyanate. I have even operated the turbine on brine from the Dead Sea!

"Any substance that shrinks reversibly or that develops tensile force when it is transferred from one environment to another might be substituted for collagen, although I have not found any other material that works as well. Fibers of crystalline polyethylene or of rubber might be used as the working fiber. In that case the environmental change would have to be thermal rather than chemical.

"The fact that stretched rubber tends to shrink when it is warmed was first observed by the 19th-century physicist James Prescott Joule. A number of fascinating engines have been based on this effect [see "The Amateur Scientist"; SCIENTIFIC AMERICAN, April, 1971]. My turbine can be operated on a heat cycle by substituting a fiber of rubber for the collagen and replacing the salt solution with hot water. Output power will vary with the difference in temperature between the beginning and the end of the cycle. In theory it is possible to operate the contraction cycle in reverse; cranking the output shaft should cause the turbine to operate as a chemical pump, which would remove salt from water, or as a refrigeration machine.

"The principal dimensions of the turbine appear in the accompanying illustration [left]. The working cones of the spindles taper at an angle of 10 degrees. The inverted cones for pretensioning the fiber taper at an angle of 15 degrees. A suitable angle for the axes of the spindles (the cant) is approximately seven degrees. The angle can be altered to adjust the pitch of the fiber helix, which will in turn control the extent of useful contraction of the fiber. The fiber will spontaneously follow a stable helical path when the spindles are rotated. Adjust the angle of the canted spindle so that the fiber contracts completely just before it leaves the helix.

"The collagen I have used is supplied in the form of tape normally employed for making absorbable sutures. The material is untanned. I tan, or cross-link, the molecules by soaking the fiber for 24 hours in a 1 percent solution of formaldehyde at room temperature. The treatment shrinks the collagen slightly.

"In the machine with which I have experimented most I use 32 feet of size 3/0 collagen tape in the form of a loop. The loop is closed by tying the ends with a square knot just before the material is soaked in formaldehyde. The slippery ends can be tied after soaking, but the tying is then best done by someone with surgical skills.

"The spindles were machined from plastic bar stock. They are supported by shafts that turn in high-precision ball bearings of stainless steel. The bearings seat in flanges of stainless steel. The supporting plate, the flywheel and most of the other parts are made from corrosion-resistant metal. It is best to make the tanks of clear plastic so that all parts of the mechanism are visible.

"At present the turbine does not hold great promise as a prime mover, but its deficiency in this respect is partly offset by its attractions as a demonstration apparatus. I am grateful to Ethicon, Inc., of Somerville, N.J., for its contribution of the collagen tape."

SUSSMAN has also devised a scheme for illustrating what Maxwell's demon was supposed to do. The demon was invoked by James Clerk Maxwell in one of his more puckish moments for a hypothetical experiment relating to apparent differences in the statistical behavior between a gas in bulk and a gas as an assemblage of molecules. The experiment required a pair of adjacent rooms at equal temperature. The partition between the rooms was to contain a small hole. Maxwell proposed to station his demon at the hole with instructions to let all randomly moving fast molecules (hot molecules) pass through freely and collect in one room and to allow all randomly moving slow molecules (cold molecules) to collect in the other room. Without expending physical work the demon would thus cause one room to become hot and the other one to become cold-in clear violation of the second law of thermodynamics, which stipulates that heat always flows from hot regions to cold regions and never in the opposite direction.

Maxwell never succeeded in laying hands on the elusive demon. Sussman, however, offers a trick for imprisoning the demon in a bottle that illustrates various aspects of thermodynamics. Sussman describes the arrangement as follows:

"My demonstrator of Maxwell's demon involves a relatively simple variation of an apparatus long used by teachers to illustrate the nature of irreversible processes. Essentially the apparatus consists of a flat-bottomed flask with a long neck that contains five white spheres and five black spheres [see illustration at left]. The neck can be sealed shut with a flame or with a stopper. A good stopper is a cork secured with sealing wax.

"Suppose the flask is inverted and the spheres have fallen into the neck so that the five black spheres are on the bottom and the five white spheres are above them. An 'irreversible' process can be illustrated by tilting the flask upright so that the spheres run into the bulb and mix, whereupon the flask is again inverted so that the mixed spheres fall into the neck. The mixing will doubtless alter the initial sequence of the spheres, because there are 252 ways in which five black spheres and five white spheres can be arranged in a row. (The number of possible permutations can be found by expressing the total number of spheres as a factorial number and dividing it by the product of the number of black spheres and white spheres, which are also expressed as factorial numbers: 10!/5! x 5!. The factorial symbol ! indicates the product of all integers from 1 through the number of interest. For example, 5! = 1 x 2 x 3 x 4 x 5 = 120. Similarly 10! = 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 x 10=3,628,800.The number of ways in which two groups of five objects each can be arranged in a row is therefore equal to 3,628,800/120 x 120 = 3,628,800/14,400 = 252.)

"When the number of objects increases to several thousand, the possible permutations become so many that the chance of a recurrence of an observed sequence is vanishingly small. That is why it is easy to mix cream into a cup of coffee but difficult to unmix the two. In such a case at least 10 particles of fluid have been intermingled. The number of possible permutations is astronomically large.

"The apparatus can also illustrate the fine structure of energy in a system, showing that the energy carried by myriad bits of matter can change only in discrete steps. Assume that each of the black spheres has unit mass and that the white spheres symbolize massless spacers. Support the flask neck down with the black spheres at the bottom.

"An energy level can now be assigned to each sphere according to its elevation above the closed end of the neck. The bottom sphere cannot fall below its present position. Hence it can do no work. It occupies the zero energy level, which is termed the ground state.

"An energy level can be similarly assigned to the four remaining black spheres. For example, the second sphere in the column could fall a distance equal to its diameter if the bottom sphere were removed. It could do work. One unit of energy is assigned to it. Two units of energy are assigned to the sphere in the next higher position, and so on. This scheme results in a total energy carried by the five black spheres of 0 + 1 + 2 + 3 + 4 = 10 units.

"Assume now that the spheres run into the body of the flask, mix and return to the neck in a different array [see illustration at left]. In this array black spheres occupy the ground level and the fourth, fifth, seventh and eighth energy levels, so that the new array has a total energy of 24 units. It is of course possible that the white spheres will find their way to the bottom of the neck and be surmounted by the black spheres. This pattern would represent the maximum energy configuration that the system could reach: 5 + 6 + 7 + 8 + 9 = 35 units.

"Engaging now in a flight of fancy nomenclature, let each array be called a 'state.' As I have noted, a system of five black spheres and five white spheres can have 252 states. The energies of the states can range from 10 to 35 units. In the absence of factors prejudicial to one state or another one would expect the average energy of the flask to be 22.5 units.

"The energy of the system can be altered by performing 'work' on the system as a whole. For example, loosen the ring supporting the bottle and slide it and the bottle upward on the stand. A11 black spheres, since they have mass, gain energy when the bottle is lifted. The effect suggests a tenet of thermodynamics: The energy of a system can be increased by performing work on the system.

"Just as the energy of the black spheres is increased with respect to the rest of the universe by lifting the bottle, so the work spent in compressing a gas adds to the energy of its constituent molecules. The similarity goes even further, because the work of compressing the gas requires a change in the geometric coordinates (volume) of the gas, just as the work done on the bottle requires a change in the geometric coordinates of the bottle. Continuing the analogy, the effect of increasing the energy of molecules by adding heat to the system can be simulated by altering the probability that a given state of the black spheres will appear. Simply add or remove white spheres.

"In reality the addition of white spheres has two effects. Their presence allows individual black spheres to climb higher into the neck of the flask and so reach higher levels of energy. The white spheres also introduce indeterminacy into the system. Each white sphere that is added increases the number of ways that black spheres and corresponding energy levels can be permuted. Hence the addition of white spheres to the bottle increases both the freedom of the black spheres to move through the accessible energy levels and the difficulty or uncertainty of describing the exact state of the system at any instant.

"In thermodynamics the measure of that uncertainty is known as entropy. With no white spheres in the bottle there is only one possible state for the black spheres. There is no uncertainty and hence zero entropy-a condition analogous to achieving a temperature of absolute zero in a perfect crystal. The second law of thermodynamics implies that any physical system left to itself can move only in the direction of increased randomness. In other words, the 'white spheres' of nature permeate and spread, destroying organization, structure and uniqueness. In short, entropy increases with time.

"Having thus illustrated by analogy the irreversible processes of nature, the granular character of energy and the classical concept of entropy, the bottled particles can be returned to their initial state of minimum energy for another demonstration by invoking Maxwell's demon. Grasp the flask at the base of the neck in the neck-up position and swirl the contents with a circular motion of the wrist. The rotating spheres will climb to the middle of the bulb, where they will be held by centrifugal force.

"While continuing the swirling movement slowly turn the flask upside down. Gradually decrease the amplitude of the swirling movement. As the balls lose velocity the black spheres will spiral into the neck, followed by the white spheres. Why? The explanation lies in an inoffensive deception. The black spheres are built so as to lose momentum rapidly. They are hollow and contain a small ball bearing. Friction between the inner surfaces of the sphere and the steel ball dissipates some momentum as heat. The white spheres are solid.

"For the bottle I use a two-liter flask with a long neck; it is available from distributors of chemical glassware. The cork spheres are 3/4 inch in diameter. They are stocked by novelty stores.

"The black spheres are altered by cutting the cork in half with a razor blade and removing material from the center. Improvise a semicircular template of 7/32-inch radius from a piece of sheet metal. With a draftsman's compass inscribe a carefully centered circle on the flat face of each hemisphere. Material within the circle can be removed smoothly and quickly with a high-speed burr 3/8 inch or less in diameter. The inscribed circle defines the limit of the cut. The depth of the cut can be gauged with the template. The high-speed cutter and the motor-driven chuck are available from dealers in craftsmen's supplies. The hollow center can also be cut with a high-speed lathe. Chucks of various kinds for supporting the hemispheres can be improvised from either wood or metal. The complete Maxwell's demon bottle is also available commercially from the Sargent-Welch Scientific Company (7300 North Linder Avenue, Skokie, Ill. 60076)."

Bibliography

MECHANOCHEMICAL TURBINE: A NEW POWER CYCEE. M. V. Sussman and A. Katchalsky in Science, Vol. 167, No. 3914, pages 45-47; January a, 1970.

ELEMENTARY GENERAL THERMODYNAMICS. M. V. Sussman. Addison-Wesley Publishing Co., 1972.

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