Talk:Weissenberg effect

Explanations are wrong[edit]

The entanglement explanations are wrong, I believe. Very low polymer concentration is sufficient to obtain this effect, far from the concentrations at which entanglement can occur.

The simple explanation is that the elasticity of the polymers creates a tension in the streamlines : imagine polymers circling around the rod, this (imaginary) circle is called a streamline. When close enough to the rod, polymers are extended due to shear (they incur friction as they are between faster moving fluid to the inside and slower moving fluid to the outside). As they are like small springs, they exert a force to get back to their unextended shape, and thus the circle is "tensed" : the sum of all the contribution of these polymers is a force towards the center. This force drives the fluid towards the rod, by incompressibility it needs to find some space by bulging at the surface (climbing the rod) until an equilibrium with gravity is reached.

See John Hinch lecture, page 7. Josce (talk) 12:06, 18 October 2010 (UTC)[reply]

Very true, also happens in dilute solution. The polymer backbone gets elongated along the shear field (this is entropically unfavourable), and the polymer tries to evade by moving towards smaller shear fields, which are towards the center of a conical field. --132.230.157.60 (talk) 06:53, 17 August 2012 (UTC)[reply]

Here are the disputed explanations (which are unreferenced and may be "original research"):

[...] entanglements cause the polymer chains to be drawn towards the rod.

Technically, it is due to a non-zero first normal stress difference. The strain [[tensor]] of the motion of turning the rod produces a non-zero difference between the normal components of the resulting stress tensor, so there is a force up and down.

The polymer chains get wrapped around the rod and then as it keeps turning, the free ends in the bulk solution are trapped in the tangled bulk. As the rod is rotated, the end of the chain wrapped on the rod is under tension (a force on each end). To try to reduce the distance between the two ends it tries to move up or down the rod to a region where fewer chains are wrapped round and hence the effective diameter (the diameter of the rod plus wrapped-around chains) is less and hence the distance is shorter.

Please provide the correct explanation in the article. A reference won't harm. All the best, --Jorge Stolfi (talk) 02:52, 25 February 2013 (UTC)[reply]