Micro/Macro mass flow near a surface

[amorrison]

1/ Consider - a ball bearing hitting a flat surface - it will bounce off at the angle of incidence similar to a light bean on a mirror.

2/ On a molcular scale there is no flat solid surface (because of surface roughness) so an individual atom will hit a many angled rough surface and can bounce off at any direction depending on the "angle" of the surface where the contact occurs.

3/ Assuming 1& 2 above are correct - what happens when a large mass of molecules (say 10^20)are travelling in the same direction (say out of a compressed air nozzle)and they contact the rough solid surface. Do the individual molecules bounce off at every direction - BUT - after they bounce off they are swept up by the greater mass flow whice means that all athe molecules "as a mass" bounce off at the same angle(angle of incidence = angle of reflection).

4/ Then there is the "Coanda effect" - maybe ? ?

Can anyone suggest search words or sites/papers on this topic.

Thanks

 

[btrueblood]

Agree with 1 and 2.

As for #3, no: molecule-to-molecule collisions become important, and the equations for fluid mechanics (Navier-Stokes) would seem to prevail (you don't state the ambient pressure/density conditions, so it is not clear if this is free molecular flow or ....)

#4, um, unless the impacted surface is moving/rotating, the Coanda effect is not applicable. But there will certainly be wall effects, and a boundary layer. CFD might help.

 

[IRstuff]

Perhaps you should start with what is driving these questions? There are lots of differences between ball bearings and molecules; one is not just a larger version of the other.

TTFN
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