Oil and water don't mix
Explore the patterns made by dropping food coloring into vegetable oil.
A clear plastic cup
A dropper bottle of blue food coloring
isopropanol, (rubbing alcohol)
an eye dropper
Fill the cup at least 2 cm, an inch, deep with oil
To Do and Notice
Drop one drop of food coloring into the oil.
Notice how it sinks and forms a beautiful spherical ball.
Fill the eye dropper with isopropanol.
Drop one drop of isopropanol onto the oil.
Notice how it stays on the surface .
Insert the tip of the eye dropper into the ball of food coloring. Slowly add it to the food coloring. Move the tip about to gently stir the isopropanol into the food coloring.
Notice that the colored sphere begins to rise. You can make it almost neutrally buoyant. But, in general, it will rise slowly to the surface.
Notice that when it hits the surface the sphere "explodes" across the surface. Continue to watch. The smooth distribution of color breaks up into a myriad of small dots of color.
What's Going On?
The food coloring is in a solution of water and glycerin.
Water is a polar molecule it has an electrical positive side and a negative side. Oil is non-polar. Water and oil do not mix.
Water is denser than oil.
As a result, the food coloring sinks to the bottom and forms a sphere.
The spherical shape minimizes the contact between water and oil.
Isopropanol also does not mix with oil, although it will form a solution with water. Isopropanol is less dense than oil and so floats on oil.
By mixing isopropanol and food colored water a ball can be made with nearly the same density as the oil. This ball is called neutrally buoyant and will neither rise not sink.
If the ball does rise to the surface it will spread across the surface.
The alcohol evaporates, leaving behind the food coloring, which is denser than the oil. The thinly spread food coloring pulls itself together as little balls forming a pattern on the surface.
You can also experiment by adding oil to water, the oil will float on the surface of the water. Then use an eye dropper to add isopropanol to the water just underneath the oil layer. The oil layer will begin to sink. After you have added enough isopropanol the oil will form a ball that will float in the center of the glass.
Planets are pulled together into spherical shapes by the force of gravity. Food color drops are pulled together to form spherical drops by cohesive forces, electrical forces, between the molecules in the liquid.
Scientific Explorations with Paul Doherty
28 July 2003