# Why does squashed clay 'hug' the press rather than spreading out?

In this demonstration a piece of clay (or clay-like material) is squashed under a press. The lump of clay initially spreads out under the ram of the press. But once it reaches the edge of the ram instead of continuing to expand outwards it instead rises UP and seems to 'hug' the sides of the ram.

Why does it behave this way?

It reminds me somewhat of surface tension in liquids but since its a solid material (though 'flowable' to some extent) I'm not sure that is really the explanation.

The sample just about to be pressed:

The result:

(The video was obviously meant just to be entertainment, not any sort of real experiment, but I think its interesting nonetheless to see the effects of common materials subjected to uncommon situations. Sometimes these examples don't match one's intuition or expectations, a good opportunity to learn something.)

• This is a viscoelastic flow. It would be interesting to calculate the speed and strain. – Alex Trounev Nov 11 '19 at 12:01

Adhesion between the components of the clay is sufficient here.

As the clay is compressed, the material is reduced vertically, but extended in both horizontal dimensions. We can imagine that a small ring in the original material becomes a thinner, but larger ring during the compression.

Once this ring of material grows beyond the size of the piston, the compression stops. The extruded material is no longer forced to become larger, but other material behind it is forcing it away from the edge. You can imagine this ring of material has three obvious possible responses to this pressure:

• Continue to expand away from the interface as a thinning ring
• Tear into segments and move away from the interface without expanding
• Move along the piston without expanding or tearing

With strong adhesion between the portions of the material, the first and second require a lot of energy (to either stretch or tear). The third requires energy to bend the material and then lift it.

It appears that for this material, the third takes the least energy. Other materials (and other configurations of this material) might be different.

• This is good, but I am not sure it explains everything. Clearly the clay turns a sharp 90 degree corner as it leaves the piston and rises without expanding or tearing. But at the end, it turn a 180 degree corner and does expand. This leads me to look for a reason why the top flows slower than the bottom. E.G. something like a temperature difference between the piston and the table. – mmesser314 Nov 11 '19 at 11:07