http://www.sciam.com/article.cfm?articleID=000510B0-4B92-1C6D-84A9809EC588EF21 SCIENCE AND THE CITIZEN February 1999 issue Taming Maxwell's Demon Random molecular motions can be put to good use By George Musser Building a miniature machine is not as simple as scaling down the parts. For one, the inherent chaos of the microworld tends to overwhelm any concerted motion. But what if a motor could work with the disorder, rather than against it? The recent fabrication of nanometer-size wheels brings this vision even closer to fruition. On the face of it, seeking useful power in random molecular motions seems to repeat the mistake of Maxwell's demon, a little device or hypothetical creature that tries to wring regularity out of the randomness by picking and choosing among the motions. One incarnation of the demon, devised by the late Richard Feynman, is a ratcheted gear attached to a microscopic propeller. As fluid molecules buffet the propeller, some push it clockwise, others counterclockwise--a jittering known as Brownian motion. Yet the ratchet allows, say, only clockwise motion. Voila, , a perpetual-motion machine: the heat represented by molecular tumult is turned into consistent clockwise rotation without any loss. (Feynman proposed to use it to lift fleas.) Advertisement IAN WORPOLE NANOSCALE BROWNIAN MOTOR, recently built as a molecule, applies power to a ratchet and lets random molecular motions turn the rotor. But no demon or mortal has ever challenged the second law of thermodynamics and won. According to the law, one of the most subtle in physics, any increase in the order of the system--as would occur if the gear turned only one way--must be overcompensated by a decrease in the order of the demon. In the case of the ratcheted gear, the catch is the catch. As Feynman argued, the ratchet mechanism itself is subject to thermal vibrations. Some push up the spring and allow the gear to jiggle out of its locked position. Because the gear teeth are skewed, it takes only a tiny jiggle to go counterclockwise by one tooth, and a larger (and less probable) jiggle to go clockwise. So when the pawl clicks back into place, the wheel is more likely to have shifted counterclockwise. Meanwhile the sudden jerk of the propeller as the ratchet reengages dumps heat back into the fluid. The upshot: no net motion or heat extraction. In 1997 T. Ross Kelly, JosŽ PŽrez Sestelo and Imanol Tellitu of Boston College synthesized the first molecular ratchet. The propeller has three blades, each a benzene ring, that also act as the gear teeth. A row of four benzene rings--the pawl--sits in between two of the blades, and the propeller cannot turn without pushing it aside. Because of a twist in the pawl, that is easier to do in the clockwise direction than counterclockwise. For another minipropeller, fashioned by James K. Gimzewski of the IBM Zurich Research Laboratory and his colleagues, the asymmetry is provided by the arrangement of neighboring molecules. Yet the researchers see their wheels spinning equally in both directions, as Feynman's analysis predicted. Nevertheless, the basic idea suggests to theorists a new kind of engine. Instead of directly driving a rotor, why not let it jiggle and instead apply power to a ratchet? For example, imagine using tweezers to engage and disengage the microscopic ratchet manually at certain intervals. Then there would be net motion counterclockwise. The second law stays happy because the tweezers must exert energy to push the pawl back into place. In so doing, they restore heat to the fluid. In practice, the ratchet could take the form of an asymmetric electric field turned on or off by light beams or chemical reactions. There is no need to coordinate the moving parts or to exert a net force, as with ordinary motors. (A simulation is at monet.physik.unibas.ch/~ elmer/bm on the World Wide Web.) Researchers have increasingly found that nature loves a Brownian motor. In the case of ion pumps, which push charged particles through the membranes of cells, the ratchet may be a protein whose internal electric field is switched on and off by reactions with ATP, the fuel supply of cells. The movement of materials along microtubules in cells, the flailing of bacterial flagella, the contraction of muscle fibers and the transcription of RNA also exploit Brownian motion. To turn his rotor into a motor, Kelly is trying to attach extra atoms to the propeller blades in order to provoke chemical reactions and thereby jam the ratchet at the appropriate points in the cycle. Gimzewski, meanwhile, is using a scanning tunneling microscope to feed in an electric current. Because internal friction is negligible, these motors could use energy with nearly 100 percent efficiency. Unfortunately, that is not as good as it sounds: most of the output is squandered by external friction with the fluid. One potential application is fine sifting, made possible because particles of different sizes are affected by Brownian motion to different degrees. In principle, a system could sort a continuous stream of particles, whereas current methods such as centrifuges or electrophoresis are restricted to discrete batches. Nanoforklifts are also possible: a particle--the forklift--would wriggle forward, encounter a desired molecule and latch onto it. The composite, being bigger, would experience a different balance of forces and be pushed backward. Brownian motion could even be the basis for a computer, as Charles H. Bennett of IBM argued in the early 1980s. Such a computer would use jiggling to drive signals through--reducing voltages and heat dissipation. Brownian motors are one more example of how scientists and engineers have come to see noise as a friend rather than merely a foe. © 1996-2003 Scientific American, Inc. All rights reserved. Reproduction in whole or in part without permission is prohibited. popular descriptions: http://monet.physik.unibas.ch/~elmer/bm/ In his famous lectures Richard Feynman discussed the impossibility to violate the second law of thermodynamicsby a ratchet mechanism. The simplest model for a ratchet is an overdamped Brownian particle in an asymmetric but spatially periodic potential (with asymmetry and period L). Due to the fluctuating force caused by the pushing molecules of the surrounding fluid or gas the Brownian particle may overcome the potential barrier moving to the left or to the right. The probabilities for both directions are equal. Thus on average the particle does not move. Hence building a motor which turns thermal energy into mechanical work from a single heat bath is impossible. Where does the energy come from leading to a drift against the external force? The energy does not come from the heat bath but from the ratchet potential when it is switched on. At that moment the potential energy of the particle will be suddenly increased. In the simulation this can be seen by a sudden increase of the energy bar. But most of the energy pushed into the system will be just dissipated into the heat bath due to the relaxation of the particle into a potential minima. Only a tiny portion will be used for doing work. Thus a Brownian motor does not violate any law of thermodynamics it only turns one type of work into another one. Nevertheless the fluctuating force due to the heat bath is essential for a Brownian motor. _________ Hence at first it would appear that above Latched Walker DNA construction can realize latched movement just by use of Heat Energy. In fact, the latching of random movement is well known not to succeed: since total energy would not be conserved, it would violate the laws of thermodynamics. Hence by the laws of thermodynamics, this latched random autonomous motion will eventually reverse itself. The popular physics literature is full of schemes -- violating the laws of thermodynamics, and so theoretically infeasible - for harnessing heat energy to fuel nanomechanical motion. Feynman in his Lectures on Physics [F51] gave a detailed critique of a proposed method (known as the Thermal Ratchet) for ratcheted movement using heat energy. However, Astumian [A97,A01] discusses how latched or ratcheted molecular movement is feasible in many cases (and used by many biological motors) if there is a source of external potential energy fueling the movement. It generally results in a Brownian movement with a biased drift in a given direction. An example of a well studied protein motor using this scheme is the movement of myosin along an actin filaments [KTI+99], and related methods for quantum ratcheted movement have also been investigated [HLN01]. For a brief overview of the use of such methods for the design of molecular motors with latched molecular movement using an external energy source, see [M99]. Brownian ratchet \newline \noindent[F51] R. Feynman, Feynman Lectures on Physics, Vol I, Chpt. 46, The Thermal Ratchet, Addison-Wesley Publishing, Reading, MAÊ (1969). \newline \noindent[HLN01] T. Humphrey, H. Linke, and R. Newbury, Pumping Heat with Quantum Ratchets, to appear in Physica E Physica E 11/2-3,(2001), cond-mat/0103552. \newline \newline \noindent[A97] R. D. Astumian, Therodynamics and Kinetics of a Brownian Motor, Science, Vol. 276, p 917-922, (1997). \newline \noindent[A01] R. D. Astumian, Making Molecules into Motors, Scientific American, Vol 285, p 57-64, (July 2001). \newline \noindent[KTI+99] K. Kitamura, M Tokunaga, A. H. Iwane and T. Yanagida, A Single Myosin Head Moves along an Actin Filament with Regular Steps of 5.3 Nanometers, Nature, Vol 397, pages 129-134, (Jan 14, 1999). \newline \noindent[M99] G. Musser, Taming Maxwell's Demon: Random molecular motions can be put to good use, Scientific American, (Feb 1999). http://www.sciam.com/article.cfm?articleID=000510B0-4B92-1C6D-84A9809EC588EF21