The "Collisions Cube" Molecular Dynamics Simulator John J.
ash' and Paul E. Smith
Purdue University, West Lafayette, IN 47907 Over the past 50 years, several simple devices have been designed to illustrate molecular motion and molecular dynamics (143).The more recent "molecular motion simulators" are constructed easilv. can be used with or without a n overhead projector, and a r e available commercially in some cases. Even though these simulators can be quite useful in the classroom, their design does not permit a true, three-dimensional representation of a n atomic or molecular system. I n this we describe the "Collisions molecular dynamics simulator. The simulator employs ping-pong balis a s the "atoms" or "molecules" and is suitable for either large lecture halls or small classrooms (it was originally designed for the former). We have successfully used the simulator to illustrate many of the fundamental concepts related to molecular motion and dynamics. Moreover, t h e simulator provides a three-dimensional perspective of molecular motion. Simulator Design
A schematic diagram of the simulator is shown in the figure. The "collision chamber" (24 x 24 x 36 in.) is constructed from clear, 114-in. Plexiglas. Approximately 1 in. from the top of the collision chamber are four Plexiglas "rails" that support a metal-framed, wire-mesh lid (the wire mesh is very coarse to prevent the restriction of air flow). A hinged, Plexiglas "trap-door" (12 x 8 in.) exists a t the back of the collision chamber so that the ping-pong balls may be easily and quickly removed. The collision chamber i s mounted on a metal cart (46 x 24 x 28 in.) that has a wooden top. Two forced-air, "squirrel-cage" furnace blowers are mounted, side-by-side, underneath a hole (9 x 14 in.) cut in the center of the top of the cart. The hole is covered with a piece of wire refrigerator shelving. The cart is enclosed on the front and the two sides with 118-in. ~ l v w o o d(oainted black) to make the simulator more vis&lly appealing. The back of the cart has been left uncovered to ~ e r m isufficient t air intake and easy accessibility to the operational controls. Several small panels of 114-in. Plexiglas are mounted on a slight angle a t the bottom of the collision chamber to improve the efficiency with which the ping-pong balls return to the center of the air flow. Slots (118 in.) were cut (at the front and a t the back) in the bottom Plexiglas panels and in the rails a t the top of the chamber to allow various dividers to be inserted. The dividers are constructed from 118-in. Plexiglas, and they bisect the collision chamber. This design allows individual control of the air flow on either side of the divider because the two blowers are each connected to a separate variable transformer (it should be noted that direct-drive blowers, a s opposed to belt-drive blowers, are controlled more easily with a variable transformer). Finally, the two variable transformers are connected to a single 120-V power strip. Bulk quantities of white and orange ping-pong halls were purchased from Martin-Kilpatrick Table Tennis (Wilson, NC). White ping-pong balls can be dyed easily 'Author to whom correspondenceshould be addressed. 'The name for this apparatus was proposed by Justine Schuller, a student in one of our general chemistry courses.
Schematic diagram of the "Coilsions Cube". The labels in the figure are as follows: (A) collision chamber. (6)wire-mesh lid. (C)hinged, trapdoor, (D) forced-air blower motors, (E) angled Plexiglas panels, (F) Plexiglas divider, (G) variable transformers, (H) power strip. to produce balls of various colors. The following procedure is representative. I n a 5-L metal beaker, approximately one-half of a package of Rit dye (CPC International, Inc., Indianapolis, IN) was dissolved in about 2.5 L of water that had been heated to 70-80 "C (it is important to keep the temperature below 80 "C because higher temperatures lead to significant deformations of the ping-pong balls). About 72 ping-pong balls were then placed in the dye solution, and the temperature was maintained for about 2 h (for a n intense color).Aweighted, perforated Plexiglas disk was placed on top of the balls to keep them submerged. Finally, stimng the balls every 30 min was necessary to ensure complete coverage of the surfaces of the balls. Simulations
Atornic/Molecular Motion of Gases, Liquids, and Solids Although gaseous motion can be illustrated easilv in the simulator, we believe that it is more instructive toilace a solid divider (hereafter, this divider will be referred to as divider A) in the simulator and contrast the motion of a gas versus a liquid, or a liquid versus a solid. For example, a few gross of ping-pong balls can be placed on each side of the divider. Each hlower can'then he adjusted, using the Volume 72 Number 9 September 1995
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attached variable transformers, to provide gaseous motion (i.e., blower a t maximum power) on one side of the divider and liquid motion (i.e., blbwer a t low power) on the other side. The simulation of a liquid works surprisingly well. The equilibrium tbat exists between the "vapor" (above the liquid) and the bulk "liquid" is clearly visible. However, this should be pointed out to the students because it mav not be immediately obvious to them. Because verv fine control of the blowers is not oossible. we turn the blower off completely to illustrate a soiid. his; of course, can lead to a n additional discussion of absolute zero temperature as well as zero-point vibrational enerm. -. Moreover, the ping-pong balls assume a well-ordered arraneement a t the bottom of the simulator. This could oerhaps provide a useful prelude for a discussion of the crystalline structure of solids. Finally, whether or not a divider is employed for illustrating gaseous motion, a distribution of kinetic energies of the ping-pong balls is clearly visible in the simulator. This very easily provides the opportunity to discuss the kinetic theory of gases.
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Pressure Pressure effects on the dynamics of a gas can be illustrated in the following manner. First,. gaseous motion a t "low pressure" is illustrated by placing a certain number of pine-pone -. balls in the chamber with both blowers a t maximum power. Next, the same number of ping-pong balls is placed on only one side of divider A with the blower a t maximum power. The increased frequency of collisions between individual ping-pong balls as well a s between the ping-pong balls and the walls of the container a t "high pressure" are easily noticeable.
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Chemical Equilibrium We have illustrated chemical equilibrium by placing another type of divider witb a 5-in. hole (cut halfway between the too and the bottom of the divider. hereafter referred to as divider B) into the simulator. ~ n u k b eof r red ping-pong balls are placed on one side of the divider and a n identical number of blue ping-pong balls are placed on the opposite side. Both blowers are then run a t maximum power until equilibrium is established (-2-3 m i d . Althouah the time required to reach equilibrium depends on the diameter of the hole in the divider as well as on the power of the blowers, equilibrium can he achieved in a reasonable amount of time Ibr a lecture prcwntation. If the two blowers arc well-matched 1i.e.. . , similar air output), a n approximately equal number and distribution of red and blue ping-pong balls on each side of the divider will result. However, because the Dower of the blowers can be adjusted easily, &pilibrim states with unequal numbers of pine-oone balls on each side of the divider are also nossible. We have found that many students believe that: regardless of t h e magnitude of a n equilibrium constant, there will be equal proportions of reactants and oroducts when chemica
