Diffusion Model

Plastic Model to Demonstrate Diffusion and Thermal Conductivity

Harvey F. Blanck, Clarksville, Tennessee

Model overview:

The model I designed to demonstrate numerous diffusion and thermal conductivity situations uses water flowing through a small tube in the bottom of each partition. For example, when a water flow steady state condition is achieved, the water heights in the columns decrease in uniform steps as shown below which emulates diffusion through a porous barrier between two reservoirs of constant concentration. For other situations see my article in the Journal of Chemical Education, October 2005, Volume 82, Number 10, p 1523-1529. (See my Professional Information webpage for links to Diffusion, Spreadsheet,  and Gravity Current articles that use this or similar plastic models.)

 Construction of the model:

Following is detailed description of how I constructed the diffusion model. I am not an expert at working with acrylic sheets so it may not be the best possible way.

Materials:

1) Sheet of acrylic plastic (Plexiglas or equivalent) 32" x 44" x 0.080" (This is enough material for 2 models with plenty to spare.) ($20 at home supplies type store).

2) 6 foot length of 1/8" ID, 1/4" OD acrylic tubing. (See a window glass company or order online.) ($1 plus possible shipping.)

3) Strip of wood about 1" x 1" x 48". Two pieces of wood nominal 2 x 2 (or 2 x 4) x 20". Wood glue.

4) Methyl ethyl ketone (MEK) was used.  Commercial glues are probably satisfactory but I have not tried them.

Note: MEK is flammable so adequate ventilation must be used and vapors must be kept away from possible sources of ignition. As with all such organic solvents contact with the liquid and inhalation of vapors was avoided.

Tools:

1) Electric drill

2) Plastic 'cutting' knife (Used to score the plastic.) ( $3)

3) 17/64" Bradpoint bit ($5) or regular electric drill bit ($2) but Bradpoint bit is much better. (A 1/4" bit will probably be just a little too small for the acylic tubing. If the fit is too tight, the MEK will fracture the plastic sheet.)

4) 10" general purpose mill file (single cut bastard) ($6)

5) Six 4 inch 'C' clamps (or equivalent)

6) Metal straight edge > 32" in length (For use with scoring tool)

7) Two metal straight edges ~10-12" in length (For use with scoring tool)

9) Bench vise helpful.

10) Yardstick

11) Thin 6" flat steel ruler with 1/32" and 1/64" subdivisions

12) Hacksaw

How the plastic was cut:

1) Using C clamps, the straight edge was clamped in the desired location to the plastic by placing the threaded end of the clamp on the metal and the fixed end of the clamp against the plastic. The clamp was not allowed to rotate while tightening it because the fixed end might scratch the plastic. I found almost no scratching doing this even though the metal clamp was on the plastic.

2) The plastic sheet was supported with boards with the plastic flat against a board so that the cutting tool could be drawn along the metal straight edge.

3) The cutting tool is very sharp. I was careful not to let it slip while scoring the plastic. The directions that come with the cutting tool describe how to use the tool. (The plastic sheet can not be broken easily using just one score.)

4) With the plastic securely in place and my fingers out of the way in case the tool slipped, the tool was drawn firmly along the straight edge. (I usually started about 1/4" inch from the end and drew the cutter all the way to the opposite end.) The scoring was repeated several more times. I then pulled the knife in the opposite direction on the other end to finish the scoring. Once the plastic was scored several times, the metal guide was no longer needed. Each draw produced a plastic 'thread'.

Note:  The plastic 'thread' that formed during the scoring was saved and put into a small, capped glass bottle containing 10 mL of MEK. The acrylic plastic and MEK slowly formed a high viscosity solution that was later used to easily seal joint imperfections. 

Pieces cut:

1) Along a 32" side of the plastic sheet, a metal straight edge was clamped 8" down. A cut was made to form an 8" x 32" piece. This piece was then cut in half to form two 8" x 16" pieces for the sides of the model.

2) The main plastic sheet was cut to form a piece 3" x 32" and then cut to form a 3" x 18" piece for the bottom and a 3" x 14" piece used to make the drainage channel.

3) Another cut on the main plastic sheet was made to form a piece 8" x 32". This piece was used to form 1" x 8" partitions.

Partition holes and tubes:

1) Holes: Drilling holes in plastic is different than drilling in wood or metal. The bit must not become hot enough to melt the plastic. The plastic chips easily when the bit exits. If the bit is dipped in water and the drilling done slowly, the bit will not melt the plastic. If light pressure is used, the bit will not chip the plastic upon exiting. I placed the plastic firmly on a piece of wood, dipped the bit in water often and drilled slowly. My drill was not variable speed so I just triggered it briefly and did most of the drilling as it coasted to a stop and kept the end of the bit wet while drilling. (I tried drilling a smaller hole first but it was very difficult to then drill slowly enough with the final bit to prevent chipping upon exiting.) The center of the 17/64" holes were placed about 1/4" from both the bottom and one side of the partition. Since hole location might have a slight effect on the water flow characteristics, it seemed best to locate all the holes the same distance from the bottom and the edges. Holes were drilled in 16 of the 18 partitions.

2) Tubes: Using a hacksaw with fine teeth, slightly longer than 1.25 inch lengths of the 1/8" ID acrylic tubing were cut. Burrs were removed and the interior was checked to be certain that has no chips were present. The tube ends must be flat and at 90 degrees so one end was filed at 90 degrees. Each tube was then inserted into a hole that was 1.25" deep and the remaining end filed down to produce tubes of uniform length.

Bonding the pieces with MEK:

1) For uniform step 'treads' and flow characteristics, the partitions should be uniformly spaced and at right angles to the sides. Although not shown in the diagram, a 17th 'cell' 1/2" wide was added at one end as an overflow channel. To form the overflow, 1/4" was removed from one of the partitions that did not have a hole. The partitions are 0.080" thick so 18 of them will consume 1.44" of the 16" of side-wall space leaving 14.56" for the 17 cells. The overflow cell is 1/2" in width leaving 14.06" for the remaining 16 cells, which is 0.879" or about 7/8" per cell. I found it useful to make a form to hold the partitions in place during bonding. I used a miter box to cut 32 uniform pieces of wood 7/8" in width from a 1" x 1" strip of wood. I also cut 4 pieces 1/2" in width. Using the plastic partitions as vertical spacers, 16 of the 7/8" wood blocks were glued along and about 1/4" above an edge of each of the two 20" 2x2 or 2x4 pieces of wood. Two of the 1/2" blocks were glued on one end of each row of 16.

2) Bonding the tubes into the partitions: In a well ventilated area using the jigs created in step 1 to hold the partitions in a vertical position, a tube was placed in each partition hole with the partition in the center of the tube. It was sometimes necessary to support the ends of the tubes so that they would remain square with the partition during bonding. A drop of MEK was placed on each joint. Although the MEK bonded the tube in place, an open region in the joint often remained. If a joint was not completely closed, a drop or two or the nearly saturated acrylic/MEK solution on the joint closed the opening. (If the solution had a viscosity similar to honey, a small amount of MEK was added until the solution flowed more readily. Better bonding occurs if the solution is slightly unsaturated so that some of the plastic being bonded will be involved in the bonding process.) Each joint was inspected carefully to be certain there were no openings in the joint. Once in place in the model, it is difficult to seal a leaking tube joint.

3) In a well-ventilated area, one of the 8" x 16" piece sides was placed on a flat surface. One of the forms with the slots was positioned at the top edge of the 8" x 16" piece. The partitions were slipped into position with the tube toward the bottom of the 8" x 16" piece but alternating the tube location top edge and bottom edge. (The tubes with respect to the finished model will all be on the bottom of the partitions but alternating left and right sides.) The partitions with no hole were placed on one end with the shorter being the second one to form the somewhat narrower overflow cell. The second slotted form was then placed on top of the partitions. The partitions were moved so that the ends were even with the bottom edge of the 8" x 16" piece. This step is critical for easy bonding of the bottom later.

4) Using a dropper a little MEK (not the acrylic/MEK solution) was placed on each joint. Surface tension drew the MEK into the joints. The forms were removed after the MEK had evaporated and the bonding was firm. After the partitions were firmly bonded to one side, it was turned over and placed on the other 8" x 16" side piece being certain that the bottom edge was lined up with this side piece. Numerous weights such as bottles of water were placed on the top 8" x 16" piece. A little MEK was then placed on each joint to be bonded. When this bonded, the cells were placed on the 3" x 18" bottom piece. The cells were positioned so that the overflow was on the left at the left edge of the bottom piece and about 1/8" from the edge of the bottom piece. That left about one inch of the bottom piece extending beyond the cell where water will exit the sequence of cells and it also left about a one inch width of the bottom piece extending along the back length of the cells. MEK was then put on the exterior joints between the sides and the bottom. It was not necessary at this time to put MEK inside each cell. When the MEK evaporated, the model was very sturdy but it typically had some joints that remained partially unbonded.

5) The partially unbonded joints can be sealed with the MEK/acrylic solution. If the solution was so viscous that it will not flow fairly easily, it was diluted by adding a small amount of MEK. By holding the model on its side, MEK solution was flowed down the joints toward the bottom being sure that some solution reached the bottom. The model was turned onto its other side and this process repeated. With the model in an upright position a bead of the MEK solution was run around the outside of the joints at the bottom. The model was allowed to dry in this upright position. After it is dry, all the joints were checked visually for imperfect seals. 

6) Two holes were drilled in the overflow cell channel near the bottom on the end for an overflow exit.

7) 3/4" wide strips were cut from the 3" x 14" piece and used to form an open channel starting after the last partition on the end and along the back of the model.

Wire plug construction and leak testing.

Two wire plugs were made by filing a notch near the end of a 10 gauge copper wire. A sharp 90 degree angle was formed by bending the wire toward the notch and soldering the closed notch. A shallow notch was then filed into each side of the end close to the bend to help hold a piece of heat shrink tubing in place. A piece of heat shrink tubing was placed on the end of the wire and cut so that the end extended just a little beyond the end of the wire. It was then heated to shrink the tubing. The wire was then attached to a dowel such that it could be easily inserted into the end of a connecting tube between partitions.  The wire plugs permit a set of cells to be filled with water to permit various starting configurations describe in the diffusion article. 

The wire plugs are also useful in locating leaks.  The first cell exit tube can be plugged and the cell filled with water.  If no leaks are detected, the second wire may be used to plug the second cell and the first plug removed.  The two cells can then be tested for a leaks into the third cell.  The process can be repeated until all the cells have been tested.  Once the water has been removed, leaks can be sealed with the MEK/acrylic solution.