When a container of liquid is rotated on its base in an upright position, the liquid inside doesn’t turn with it and remains static. Why is this?
Chris Daniel, Glan Conwy, Conwy, UK
The statement that precedes your question only contains an element of truth. Take a bucket of water by its handle and rotate it quickly by, say, half a turn and back again, and most of the water won’t move.
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But look carefully and you will see ripples close to the wall of the bucket. This is because a layer of water molecules is in adhesive contact with the inside of the container and moves with it as you turn the bucket. The motion of this layer of water creates a force known as shear stress that affects the rest of the liquid at a rate dependent on its viscosity.
If you turn the container for long enough, this effect continues until all .
To see this in action, you could try the following experiment: put a bucket of water on a rotating surface (I used an old office chair) and sprinkle a few dry leaves on the surface of the liquid to help see it moving. Rotating the bucket at about one revolution per second causes the water to turn, starting with the outer edge and spreading towards the centre. The whole body of water rotates with the bucket after about 90 seconds.
Of course, the reverse also occurs. If you stop spinning the bucket, the water will continue to turn for some time.
If you take a thicker, more viscous liquid, such as household paint, you will see this effect much more quickly. At one revolution per second, it is almost instantaneous. With a rapid half-turn of the tin you will see all of the paint try to catch up with the rotation, along with some pleasing spiral ripples in its surface.
Mike Follows, Sutton Coldfield, West Midlands, UK
The liquid does spin, though it can take a little while to get going. Once spinning, the speed of the liquid varies from zero at the centre of the container to a maximum at the walls.
At this point, the surface of the liquid is no longer flat but becomes an inverted parabolic dome: the surface becomes lower at the centre and raised at the walls. The water molecules on the surface behave as if they are on a banked racing track: the faster the liquid is spinning, the steeper the slope becomes. This wouldn’t be observed with a superfluid, which has zero viscosity, but then such a liquid would probably escape by spontaneously climbing the sides of the container.
Richard Swifte, Darmstadt, Germany
The answer lies with friction, or lack of it. Two surfaces in contact and trying to move against each other experience friction, due to the roughness of the surfaces along with the molecular structure and the bonding characteristics of the materials.
Generally, liquids result in less friction than solids, which is why oil is used as a lubricant between moving metal parts in machinery.
A further factor is the internal friction, or viscosity, of the liquid. An extreme case of a viscous liquid is pitch. This is technically a liquid because, given enough time (years for some types), it will gradually flow, but for practical purposes it behaves like a solid.
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