It's near Halloween and all the specialty food items are out.. What's this? Pumpkin seed oil?

Pumpkin seed oil has some pretty unique characteristics, and has the rare property of dichromatism. This means that under various conditions, it can be either green, red, or something in between! (Spooky?)

I'll show you how to set up some pumpkin oil to view this effect in Steps 1 through 9.

In Step 10, I'll explain why this occurs (with science, pretty graphs, and a smidgeon of math.) I highly recommend sticking around for this section, it's pretty awesome.

Step 1: Materials

Naturally, we're going to need some pumpkin seed oil and some other things. Here's a comprehensive list

(1x) Pumpkin Seed Oil (from your local Trader Joe's, or Amazon)
(1x) 60cc syringe (Amazon) *
(1x) small white or clear (sacrificial) plastic container**
(1x) Hot glue gun
(1x) Drill with 13/64" drill bit***
~6 inches of 3/16" Outer Diameter plastic tubing***
(1x) White light source (flashlight with white cloth over front)****
(1x) Caliper / Ruler / Tape Measure****

*The exact size of the syringe is not crucial, Larger sizes may decrease your sensitivity in the control of the fluid level while smaller sizes may not hold enough fluid to prime the tubing.

**I pick plastic because it's easy to put a hole through.

***The size of the plastic tubing and drill bit needed will vary depending on the syringe.

****(Optional)

Step 2: Getting Started / Forewarning

A quick word on the pictures in this instructable - the dichromatism effect that this is based on is partially due to pecularities in the human visual system. So the pictures I take to document do not adequately reflect what you would actually see trying this yourself. So you really do have to try this one to get the full experience!

For instance, when I opened the lid to the pumpkin seed oil, I see a combination of dark red, dark green, and light green colors on the cap. The picture predominately only shows the light and dark green, with just a hint of red.

In fact, I will show you two ways to set up the experiment, where one is much better for taking pictures. (And I'll explain why!)

Step 3: Syringe Basics

If you're confident in your syringe abilities, skip this step.

Take a syringe and draw water into it - the plunger may be difficult to move at first, but needs to be filled starting with the plunger all the way at the end. (Otherwise you'll end up with air in the syringe! Not so bad for us, but inaccurate if you're doing precise chemistry, and deadly if working in a medical setting.)

You read a syringe from the closest seal to the end. In the first picture, the syringe contains 20 ml, while in the second image the syringe contains 15 ml.

However, we will be adding a tube to the front of the syringe, which has non-negligible volume. So when we increase the fluid, we will be looking at the difference between the two markers to see the change in volume.

Step 4: Attaching the Tube

Now we're ready to start building our test system.

The plastic tube should fit firmly over the syringe tip, the friction and elasticity should keep it in place on its own. To secure this connection, use hot glue to create a better seal and hold the tube in place.

Step 5: Prime the Syringe and Tubing

Before we attach the tubing to our container, we need to fill it with pumpkin oil.

Put the tube in deep enough into the pumpkin oil so it will remain immersed in fluid as its being filled. Draw oil until there is about 30 mL of pumpkin oil in your syringe, depending on the size of your container.* My container had a cross sectional area of about 240 cm^2.

Once you've filled the syringe and tube, remove the tube from the pumpkin oil can and pull back on the plunger a little more. This fills the tube with a little more air and stops the oil from leaking from the end (paper towels recommended).

*Note - you will have air in your syringe from priming in this manner (the exact volume depends on how much was in your tube). If you want to avoid this, prime the syringe before attaching the tube - however this can get kind of messy.

Step 6: Sacrifice Your Container

Enjoy your last moments with your container... And then offer it up to science!

Using your drill, drill a small hole in the center of your container. I was unsure of what diameter to use at first, so I started with a drill size right at my outer diameter tubing size (3/16") and moved up until I was able to get a nice firm fit between the tubing and the drilled hole. (This was at 13/64" for me)

Step 7: Attach Your Container and Syringe

Similarly to how we attached the tube to the syringe, attach the other end of the tube to the bottom of your container. you want the top of the tube to be flush with the bottom of the container.

Apply hot glue around the base of the tube to seal the junction and hold the tube in place.

Step 8: Experiment! White Container

We're going to investigate on our own how the color changes with fluid thickness.

Note that you should have a good white light source - a white LED lamp works well, while a more yellow incandescent will make the effect less pronounced.

Measure the cross sectional area of the base of the small container you're using. My container has a diameter of 8.7 mm as measured with my caliper, giving a cross sectional area of 237.8 cm^2. Knowing this, we can determine how much the fluid level in the container changes for each ml the syringe puts in. A ml is 1 cm^3*, so the fluid rises 1/237.8 = 0.0042 cm (0.04 mm) for each ml. This gives us pretty sensitive control over the fluid height in the container!

For 0.5 mm fluid thickness, I would put in ~12 cc of pumpkin seed oil.
For 1 mm fluid thickness, I would put in ~24 cc of oil.
For 2 mm fluid thickness, I would put in ~48 cc of oil.

Slowly depress the syringe to add pumpkin oil to the base of the container. At first the color should be green, up until about 0.5 mm of depth. At this point, as more oil is added, the color will change to a murky yellow and then finally to a deep, dark red. (Like blood!)

*1 ml = 1 cm^3 = 1 cc (cubic centimeters)

Step 9: Experiment! Clear Container

It can be a little difficult taking pictures of an effect that relies on aspects of human vision. So in order to improve, I switched to a clear container, while lighting the bottom side of the container. This allows me to more easily put more light through the sample and better illuminate the oil.

The images once again show a progression from green color to a deep red color with increasing thickness of pumpkin oil. This looks even cooler and more pronounced in person!

Step 10: What's Going On? (Science)

There's a few studies on why dichromatism occurs in pumpkin oil and other substances, as shown in the first image. However, we can simplify even further!

Consider simplifying the set up to consider simple green, blue, and red light - together this is your typical white light. Pumpkin oil strongly absorbs the blue light, letting very little through. The green light is absorbed more strongly than the red light. Thus for the same number of photons of green and red light, more red light will always make it through the oil. Why then does the oil sometimes appear green?!

To answer this, I created some simulations in Octave* (Matlab compatible) which use Beer - Lambert Law. At each thickness of the fluid, some fraction of light is absorbed. This leads to an exponential decrease in the intensity of light that makes it through with the depth of the sample. Taking the red and green, I find the hue of the light at each depth. However, this model predicts that pumpkin oil should only change from yellow to red. Again, why green?!

The answer lies in the human visual system. Not all colors are created equally to the human eye - in fact, the human eye is more than 3 times as sensitive to green light as it is to red! So while less green photons may make it through the sample, these are weighted more heavily to human vision. In the second set of graphs, I take this into account by scaling up the green light. Now we see that at some thin sample depth, the perceived green light outweighs the red! Graphing the hue, we properly see the transition from green to red with increasing depth of the pumpkin oil.

The hue dependence on depth also explains why a clear container was easier to get better pictures. In the white container, the sample depth was effectively doubled as some light had to pass through the liquid twice.

*You can download Octave (free) from here. I've attached my source code to this step.