MadSci Network: Physics Query:

Re: why do drinking glasses slide across the counter when they're wet?

Date: Wed Mar 8 14:43:19 2000
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Physics
ID: 950840511.Ph
Message:
```
Suzanne;
When a wet glass is placed on a smooth, wet surface, a ring of water first
makes a seal around the bottom edge of the glass.  Then, as the glass
continues toward the countertop, air that is under the glass bottom is
compressed.  Under ideal conditions, this small volume of pressurized air
is able to support the weight of the glass.  The water around the bottom
edge of the glass functions as a seal to prevent the air from escaping.
For a short time, the glass is floating on the surface water, and moves
with almost no friction.  A different, but related behavior is possible
with a container that has a very smooth, flat bottom.  Here, the water can
not escape quickly, and temporarily supports the glass on a squeeze film
of water alone.

This is much like an ice skate, which melts the ice under the blade, so
the skater glides freely on a thin film of water.  The first case is like
a hovercraft, where a large fan pressurizes the air under the craft so it
can move freely over water or even smooth land.

Technically, the glass uses what is called a 'squeeze film' bearing to
temporarily support the glass.  Hovercraft are supported by 'externally
pressurized' bearings.  Ice skates employ 'self-lubricating' bearings.
These are all ways to reduce friction, using lubrication.  Another way to
reduce friction involves the use of magnets.  Then it is called magnetic
levitation.

You might want to experiment with different smooth-bottomed containers to
learn more about this behavior.  Note especially how large a recess is on
the bottom of the containers.  Very flat
Bottomed containers can support themselves for several seconds.  Some
glycerin or liquid detergent added to the water will increase the support
time.

Containers with a very large recess on the bottom will not glide well, as
their large air volume will not pressurize enough.  Remember to be
careful.  Glasses can easily slide off a table or countertop during these
experiments.

```

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