Spiral arms of water from a spinning sponge ball
Jearl Walker www.flyingcircusofphysics.com
November 2015 Here is a novel video in which a water-soaked sponge ball is released with a significant rotation. The water seems to be thrown off the ball in spirals around the equator, in a pattern that resembles the arms of stars, gas, and dust around spiral galaxies.
Can water really be shot out into spirals?
When the ball is stationary, the water stays within the ball because of the mutual attraction between water molecules and also between the water molecules and the sponge molecules. If the ball is rotated slowly, those forces can still hold the bits of water inside the ball. The forces are said to be centripetal forces because they are radially inward and provide the centripetal acceleration for the bits of water to move in circles around the rotation axis.
The greatest inward force is needed along the ball’s equator because the radial distance of the surface from the rotation axis is greatest there and so the water bits travel in a large circle. Little or no force is needed to hold on to the water bits at the north and south poles because they travel in small circles.
When the ball is rapidly rotated, the forces along the equator are insufficient to hold on to the water bits, and so the bits begin to leave the surface, each traveling in an approximately straight line from the release point. Their composite gives the illusion of a spiraling path but that is only because the bits leave the equator one after another while the ball rotates between the releases. The first bits out are the ones on or near the surface, but then water bits from deeper in the ball come to the surface and leave.
I am surprised that we see somewhat discrete spiral arms instead of a continuous, horizontal spray of water. I think the reason is that the water bits are more easily released at several points on the equator than at other points. Perhaps those points are more directly connected to the maze of openings and connections in the ball’s interior.