How To Rip A Fluid

Gizmo

1st June 2007 - 07:33 PM

Who's seen the article in 'PhysOrg News' about two scientists 'ripping a fluid' with an ordinary knife?

What would be the cause of this?

I'm quoting the article in question here: -

QUOTE

The knife moved through the solution at slow speeds, but when the knife was moved faster, it cut the fluid like a rubbery compound.

Is this due to the fact that the knife, at a slow speed, moves through the mixture normally, but at quicker speeds, the force of the knife changes the molecular structure of the fluid, causing it to become (more) solid?

This would certainly be an easy explanation.

(I love that little blinking smiley

Zephir

1st June 2007 - 10:56 PM

QUOTE (Gizmo+Jun 1 2007, 10:33 PM)

..the force of the knife changes the molecular structure of the fluid, causing it to become (more) solid?.

Every fluid has a cellular large scale structure, which gives it a behavior of gelatin. This is a consequence of molecules cohesion, which results in polymerization of molecules into larger groups. Furthermore, the turbulent fluid behaves as a mixture of vortex pairs, which are behaving like individual particles (vortex rings). This leads to the non-Newton behavior of real fluids, too.

MDT

2nd June 2007 - 09:15 PM

A do it at home example of this affect, uses silly putty, which is a viscous fluid. If you pull it slowly it will stretch. If you pull it with a quick jerk, it will shear in half ,with smooth surfaces appearing at the place of shear. The slow pull allows the stress to become distributed, so many molecules slide over each other sharing the burden. The fast jerk causes the stress to happen so fast that it can not redistribute, resulting in local shear.

The forces that bind silly putty are weak versions of the EM force. The slow pull will distribute the EM force potential, since the pull is itself a force. The quick jerk doesn't allow enough time for the force potential to redistribute focusing it along a plane of molecules.

rpenner

2nd June 2007 - 09:30 PM

Or in the kitchen, a thick paste of cornstarch and water will seize up if you stress it.

Euler

2nd June 2007 - 11:49 PM

There is a fair amount of theory on this kind of thing: an elementary course on non-Newtonian fluid dynamics should cover this stuff and a fair amount more. I'm afraid I'm not a fluid dynamicist so I can't go into this area in detail, but it is accessible to anyone comfortable with the basics of viscous flow and all that it entails.

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