This week, I have been working on my new storm show. While working on some classroom demos to for teachers to use, I got side tracked again. It is amazing how often that happens. I was looking at some different ways to look at hurricane patterns, and wound up with a fun experiment, using food coloring. As I played with it, I accidentally found a second, and then a third fun experiment. This is the original.

Materials:

Plan:

I used a cereal bowl, filled with water. Use the spoon to give it a quick stir. Now wait about 30 seconds, so the water can slow down a bit. It may seem to have stopped, but don't worry. It is still moving. Next, add one drop of coloring near the center of the bowl. Watch carefully to see what happens.

You might have expected the color to spread out quickly and color all the water in the bowl. Instead, it tends to form thin streamers that slowly move outwards around the bowl, making some incredible patterns. Why does it do this?

A lot of it has to do with density. If you added a large drop, you will notice that the color tends to form a "pool" on the bottom of the bowl. The colored water is denser, so it tends to sink. It also tends to spread out slower. The water in the bowl is made up of tiny molecules that are vibrating and moving around. As these water molecules move, they bump into the dye molecules in the food coloring, causing them to move too. This is called Brownian Motion. Imagine a cardboard box sitting on the floor. If you bounce a large ball against it, the box will be moved. Next, imagine that you put some bricks into the box. Now the box is heavier, so it will not move nearly as far if you hit it with the ball again. The dye molecules are like the box of bricks. Eventually, the water molecules will bump them all around the bowl, but it takes a while.

Another part of the answer has to do with surface tension. In the past, we have done LOTS of experiments showing that water molecules tend to stick together, forming a skin-like surface that can hold up grains of pepper, carefully placed needles, and small insects. The surface tension can also form a barrier between two liquids. Adding dyes can change the surface tension in the water, so the pure water would have a different surface tension than the dye/water mix. That would form a skin-like barrier, helping to keep the dye separate. This skin is slowly broken down as Brownian Motion carries some of the color into the surrounding water.

Together, density and surface tension help to keep the dyed water and clear water apart. What causes the color streamers to move and grow? Remember when you stirred the water? Although the water seemed to have stopped moving, there was still a lot of turbulence, currents of moving water swirling through the bowl. This turbulence helps to move the dye streamers.

I am tempted to go on with all the other fun that I am having with food coloring, but I need to get back to work, so we will save them for next week.