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The oil-water-alcohol density timer!

Each year in May the American Visionary Art Museum in Baltimore, Md., hosts the Kinetic Sculpture Race - a race of human-powered sculptures that winds around (and in) the Baltimore harbor. For the second year in a row, the APS Physics Central team built a physics-themed timer for two of the race-day events, the water and mud obstacles. This year, the team was inspired by the mesmerizing display of water interacting with oil.

[Some of the sculptures in action at the 2011 Baltimore Kinetic Sculpture Race.]
To understand why this is so cool, you might first need a little background on density. Here is a Physics Central Physics@Home explanation of the interaction between water and mineral (baby) oil:
Density is the amount of stuff you have in a given space. Something that has a high density means that there is a lot of stuff in the space, where as something with a low density has less stuff in the same amount of space. Objects with a higher density sink when compared to objects of a lower density, which float. Whatever object has the lowest density will float. This explains why when you put a cork in a glass of water, it floats. The cork is less dense than the water. In the case of the baby oil and the water, the baby oil is less dense than the water; therefore, the baby oil floats on top of the water.

[Scale tests of the interaction of liquids with different densities. (Ignore the APS hot sauce collection behind the test tubes. The sauces are for lunch time testing purposes only.)]

After many tests of liquids with different densities, we found a couple of mixtures that separated well: Water and corn oil, and mineral oil and 91 percent rubbing alcohol. In each combination, we needed to use at least one liquid that would float to the top that was also dye-able. Since it is a timer, we went with a green-yellow-red stoplight theme. We dyed the water green, mixing it with yellow corn oil, and then dyed the alcohol red, mixing it with clear mineral oil.

[Equipment used in making the timer. It's surprising the people at Home Depot and CVS don't know the Physics Central team on a first-name basis yet.]

We created two tubes so that one could house the water (green) and corn oil (yellow) mixture and the other could house the alcohol (red) and mineral oil (clear) mixture. Two tubes also gave us more artistic flexibility, a very important consideration for the Race.

[The timer is halfway complete and tests of the corn oil (yellow) and water (green) tube show it takes a little over a minute for the corn oil to rise to the top (right end of the photo) while the water sinks.]

We wrapped the two tubes around a central PVC pipe. When the pipe was flipped end-over-end, the green water sank while the corn oil rose in the skinny tube. To keep the red alcohol from rising right away, we added a valve in the fatter alcohol-mineral oil tube. After the green water sank beyond a certain point, the operator opened the valve and the red alcohol rose to the top. In this way, we were able to make the timer run for two minutes.

[Close-up of the alcohol (red) rising to the top of the tube as the mineral oil (clear) sinks to the bottom.]

Sculptures taking part in the race had to make it through several obstacles on an approximately 15-mile long course. Obstacles included a sand pit, a mud pit, and a short water route through the harbor. If a sculpture got stuck in the mud pit or when exiting the water obstacle, it had two minutes to exit before being dragged out of the obstacle. The APS timer was at the ready for this year's two sticky events, but luckily for the artists, very few sculptures got stuck.

[The APS Wizard was the official timer at the 2011 Baltimore Kinetic Sculpture Race water and mud obstacles. Here, the Wizard posed for a photo-op while the green-yellow tube half of the timer was in action at the water obstacle.]

For more about this year's race, check out the Baltimore Kinetic Sculpture Race website.


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