24 days of kitchen oceanography

Experiment 8:

We place a paper towel with a cut-out in the middle over the edge of a container with water so that it hangs with one side in the water and with the cut-out side outside of the container. Using a water-soluble pen, we apply dots in regular intervals. The paper towel sucks water upwards and over the edge of the container like a sponge. When the water then flows down again, it dissolves the color of the dots and carries it along for a short distance before it is deposited again. Thus, at each of these dots, the lines of color indicate the direction of flow of water that flows around an "obstacle", the cut-out in the paper towel. If there were sufficient dots such that the stripe from one dot would run into the next dot, we would have streamlines. Streamlines describe exactly how water runs from the top edge of the paper towel all the way down.