Mean inter-particle distance

Mean inter-particle distance (or mean inter-particle separation) is the mean distance between microscopic particles (usually atoms or molecules) in a macroscopic body.

Ambiguity

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From the very general considerations, the mean inter-particle distance is proportional to the size of the per-particle volume  , i.e.,

 

where   is the particle density. However, barring a few simple cases such as the ideal gas model, precise calculations of the proportionality factor are impossible analytically. Therefore, approximate expressions are often used. One such estimation is the Wigner–Seitz radius

 

which corresponds to the radius of a sphere having per-particle volume  . Another popular definition is

 ,

corresponding to the length of the edge of the cube with the per-particle volume  . The two definitions differ by a factor of approximately  , so one has to exercise care if an article fails to define the parameter exactly. On the other hand, it is often used in qualitative statements where such a numeric factor is either irrelevant or plays an insignificant role, e.g.,

Ideal gas

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Nearest neighbor distribution

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PDF of the NN distances in an ideal gas.

We want to calculate probability distribution function of distance to the nearest neighbor (NN) particle. (The problem was first considered by Paul Hertz;[1] for a modern derivation see, e.g.,.[2]) Let us assume   particles inside a sphere having volume  , so that  . Note that since the particles in the ideal gas are non-interacting, the probability of finding a particle at a certain distance from another particle is the same as the probability of finding a particle at the same distance from any other point; we shall use the center of the sphere.

An NN particle at a distance   means exactly one of the   particles resides at that distance while the rest   particles are at larger distances, i.e., they are somewhere outside the sphere with radius  .

The probability to find a particle at the distance from the origin between   and   is  , plus we have   kinds of way to choose which particle, while the probability to find a particle outside that sphere is  . The sought-for expression is then

 

where we substituted

 

Note that   is the Wigner-Seitz radius. Finally, taking the   limit and using  , we obtain

 

One can immediately check that

 

The distribution peaks at

 

Mean distance and higher moments

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or, using the   substitution,

 

where   is the gamma function. Thus,

 

In particular,

 

References

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  1. ^ Hertz, Paul (1909). "Über den gegenseitigen durchschnittlichen Abstand von Punkten, die mit bekannter mittlerer Dichte im Raume angeordnet sind". Mathematische Annalen. 67 (3): 387–398. doi:10.1007/BF01450410. ISSN 0025-5831. S2CID 120573104.
  2. ^ Chandrasekhar, S. (1943-01-01). "Stochastic Problems in Physics and Astronomy". Reviews of Modern Physics. 15 (1): 1–89. Bibcode:1943RvMP...15....1C. doi:10.1103/RevModPhys.15.1.

See also

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