Hi,

Is no-slip condition valid at the wall all the time? If I have a very "slippery" wall such as teflon, is it still valid to apply no-slip at the wall. If not, what is appropriate to model it?

Thanks!

Pei-Ying

Pei-Ying,

based on my understanding of kinetic theory, I believe that no-slip will always apply at the boundary between a fluid and a solid.

Particles in the fluid will collide with particles in the solid at the interface. For a solid to be solid, the solid particles will remain in place. This creates a layer at the wall where the average fluid particle velocity will be zero.

Teflon has low friction when rubbed by another solid, but the above argument still applies to fluids. For no-slip to fail, the surface would need to be a non-solid (whatever that means).

David Hunt

Hi, Pei-Ying,

No-slip condition is believed to be valid as far as the characteristic scale of the flow is much greater than the mean length of the path of the fluid molecular between collisions.

The wall material does not matter as far as it is rigid.

Rgds, Sergei.

There is some doubt as to whether the no-slip condition is valid for Low Reynold's number flows. Simple physical examples are sufficient to convince oneself that this is true. A single drop of water may slide down a slippery surface. Intuitively one knows that capillary, viscous and gravitational forces are involved and that a simple newtonian law for viscosity will not work here.

I recall some research published a few years ago in which correlations of various "slip conditions" were made with experiment, for airlows. In summary, low Reynolds flows or Rare-fied gas flows present problems of special interest and difficulty where present day physical models are inadequate.

(1).Even the material like teflon (a commercial product), does not slip off the surface of a pan. (2). So, it is non-slip.

Hi,

Thank everyone for the info. I think it comes to an agreement that the "no-slip" condition fails under rare-fied gas condition. Mr. Venkat also mentioned "no-slip" fails under low Reynolds number condition. Could you please shed some more light on this? What range the Re # is?

The problem I am interested is oil flow in a small ID Teflon tube.

I did read about this in a paper some years ago. I forget exactly in which Journal, but it is either the AIAA Journal or the "Journal of Aircraft". The conditions are very specialized, and the Reynolds number range is narrow. Sorry I could not shed more light on this. I cannot of course vouch for the correctness of those published results, and so you should proceed with caution here.

Cheers,

C.S.Venkat

Oh, I am sorry, I forgot about this. Thanks Mr. Venkat for reminding. Indeed, there is a well-known problem of a moving contact line. With no-slip boundary condition a contact line cannot move which contradicts everyday observation. This is indeed low-Re staff. Look at

http://www.mat.bham.ac.uk/J.Billingham/mpf.htm

for further references. Search with Altavista on 'moving contact line' can also produce something interesting.

Sergei.