Surface Tension

When you put water in a beaker, test tube, or in the case of the WatrLevel - tubing, you can see a small curved surface (figure 1).  This curved surface is called the meniscus and it occurs when the molecules of the liquid attract those of the container (to properly read the meniscus read the bottom of the curve). Surface tension acts on the concave menisci to pull the liquid up. Therefore, increased surface tension results in a larger meniscus, essentially "sticking" to the interior surface of the tubing.

The WatrLevel juice contains a small amount of soap which helps reduce this surface tension and therefore reduces the "sticking" action. To see an example of this, splash some water on a window and you'll see beads of water form as they "stick" to the glass, next, splash soapy water on the window and you'll see that fewer beads of water form.

The soap also helps prevent bubbles from forming in the tubing since it is more difficult for them to stick to the inside of the tubing.