Measuring the Thickness of an Oil Slick
"Oil and water don't mix." This common expression can be easily seen in salad dressing: the oil floats on top of the vinegar. Even when you shake the bottle, the vinegar remains together in cohesive
Cohesion is the attraction between molecules of the same substance.
Droplets spread throughout the oil. "Oil quiets troubled waters" is another expression that comes from a very interesting fact: if you pour a small amount of oil in a large body of water, the slick will spread evenly across the surface, reducing wave activity. About 40 cc of oil will calm more than 1 ha (hectare) of water. Oil, unlike water, does not cling to itself well, so the molecules spread out in a thin film instead of forming droplets. For many oils, the slick that forms is actually only one molecule thick. You can easily calculate this thickness.
Benjamin Franklin observed the calming effects of oil on water in 1765. He wrote: "Where there is on the common a large pond which I observed one day to be very rough with the wind, I fetched out a cruet of oil and dropped a little of it on the water. I saw it spread itself with surprising swiftness upon the surface … The oil, though not more than a teaspoonful, produced an instant calm over a space several yards square, which spread amazingly and extended itself gradually … Making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass."
- safety goggles
- oil samples (Regular olive oil works well. You can also use household oils, such as WD-40, lubricating oil, and furniture polish, to compare their different behaviors.)
- dropper pipette or paper clip
- paper towels
- metric ruler or meterstick
- child's wading pool or a large bathtub
- garden hose
- talcum powder (The unperfumed, unmedicated talcum powder used in hospitals works best.)
- notebook and pencil
- 8-cm stick of balsa wood
- utility scissors
- petroleum jelly
- gum camphor
- Put on your safety goggles and begin by practicing handling and measuring the oil. Use a dropper pipette or paper clip to make tiny droplets of oil. Practice dropping them on a paper towel. Measure them with a ruler. You want to make as small a measurable droplet as possible—l to 2 mm is good.
- Clean the wading pool or bathtub thoroughly with a few drops of detergent and a strong stream of water from a garden hose. The cleaner the better—you want to remove all residual oils from human skin and lotions.
- Fill the pool or tub with clean water. When the water is calm, sprinkle a small amount of talcum powder over the center of the water. Use just enough of a dusting to see the powder form a small "cloud" at the surface of the water.
- Measure the diameter of an oil droplet at the mouth of the pipette or the end of the paper clip. Gently drop it or touch it into the center of the powder. The oil will rapidly spread over the surface, pushing the powder outward and forming a circle of oil.
- As soon as the circle stops growing, measure the diameter.
- Remove your safety goggles and calculate the thickness of the oil slick as follows. For oil monolayers (a monolayer is a layer one molecule thick), this is the thickness of individual molecules.
Thorough cleaning is essential. A flyspeck of contaminant can ruin your results.
To make an oil slick-powered boat, fashion a toy boat from a flat stick of balsa wood about 8 cm long. Cut the bow end (the front) to a point, and the stern end (the back) to a rectangular notch. Smear everything but the stern with white Vaseline or generic petroleum jelly. Press a small lump of gum camphor into the notch, and launch the boat in the pool or tub. The camphor molecules rushing from the stern will push the boat. If you put your finger to one side of the boat, the natural oils on your skin can exert enough pressure to steer the boat in the other direction.
Use the same units for all your measurements (e.g., millimeters).
- Divide the diameter of the oil droplet by 2 to get r, the radius of the droplet.
- Divide the diameter of the oil circle by 2 to get R, the radius of the oil slick.
- Calculate the volume of the oil droplet. Use V = 4/3(πr3).
- Calculate the area of the oil slick. Use A = πR2.
- Divide the volume by the area (V/A) to get t the thickness.
- It's a very small value.
π = approximately 3.14
Sample thickness calculation: if r = 1mm and R = 1,200 mm, then t = 9.25 815; 10–7 mm, or approximately one millionth of a millimeter! The thickness of your oil slick depends on the type of oil you use–whether it forms mono layers, and how large the molecules are.
Molecules are so small they are often measured in angstrom units. One angstrom is one ten-billionth of a meter.
After the Spill: The Exxon Valdez Disaster, Then and Now by Sandra Markle (New York: Walker, 1999).
Experiments That Explore Oil Spills by Martin Gutnik (Fresno, Calif.: Millbrook Press, 1991).
Warning is hereby given that not all Project Ideas are appropriate for all individuals or in all circumstances. Implementation of any Science Project Idea should be undertaken only in appropriate settings and with appropriate parental or other supervision. Reading and following the safety precautions of all materials used in a project is the sole responsibility of each individual. For further information, consult your state’s handbook of Science Safety.