In physics, the mean free path is the average distance traveled by a moving particle (such as an atom, a molecule, a photon) between successive impacts (collisions),[1] which modify its direction or energy or other particle properties.
The value of the mean free path is available for the gas phase of many substances.
Is there any estimate ever reported for the mean free path of a liquid such as water?
Many thanks for your answers.
it's defined for any phase, including, in particular, the liquid phase of matter.
Thanks for your answer.
How can I find the mean free path of water in the liquid phase?
Yes you are right. The mean free path is in the order of inter-molecular distance. For liquid water it should be in the order of some Angstroms (2-3 Angstroms).
Now consider a nano-particle with diameter about 1-10 nano-meters. For such a particle, the Knudsen number which is the ratio of the mean free path, Landa, to the particle diameter, D, (Kn=Landa/D) can be in the order of 1e-2. This Knudsen number is in the slip flow regime (0.001
Essentially it's determined by the so-called ``structure factor'', deduced from the scattering cross section. Cf. any textbook or lecture notes on simple liquids.
Hi Dr. Morteza Anbarsooz ,
I know this is an old post, but I find your question and discussion very interesting!
The MFP of liquid water is indeed between 2-3 Angstroms. But I believe that doesn't much affect the continuum description of the base-fluid in a nanofluid since slip flow effects start becoming important for Kn > 0.015.
The Kn of most nanofluids will be less than that limit (or within it), and any slight slip flow effects can usually be handled by means of a slip correction factor to Stokesian drag.
But I would say that for very small nanoparticles (e.g., 1-3 nm) in liquid water, slip flow effects can only be accurately considered by means of discrete particle models where the base-fluid is not treated from a continuum viewpoint.
Dear Dr. Morteza Anbarsooz ,
I am wondering if you have any references that show how the MFP of liquid water molecules is calculated or measured?
I am also wondering if you have any references that show the range of Kn (for the case of a liquid base fluid) under which slip effects with nanoparticles can be safely neglected?
Dear Omar Z. Sharaf
For the first question, maybe these books can help:
1. Sinha, S.K. and Dey, T.K.: Molecular Physics, Alpha Science, 2006.
2. Frenkel, D. & Smit, B. (2002), Understanding Molecular Simulation (Second ed.), San Diego: Academic Press, ISBN 0-12-267351
3. D. C. Rapaport (2004). The Art of Molecular Dynamics Simulation. Cambridge University Press. ISBN 978-0-521-82568-9.
The mean free path of the liquid molecules are in the order of the mean molecular spacing.
For the second question, unfortunately, I don't have any suitable reference.
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