Where the water is steeper, such as at the bottom of the vortex, the centripetal force on the water is greater. Water moving with higher speeds and in smaller radius curves requires larger forces. The water at the bottom of the vortex is doing just this, and so the wall of the vortex is steepest at the bottom. The hole in the vortex allows air from the lower bottle to flow easily into the upper bottle. This enables the upper bottle to drain smoothly and completely.
Spiral waves form in the water surface of the vortex.
These waves appear to move in slow motion as they travel upward through the downward flowing water. The Exploratorium's Vortex exhibit was created by artist Douglas Hollis. Related Snacks are Momentum Machine and Spinning Blackboard .