Hard spheres

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Hard spheres are widely used as model particles in the statistical mechanical theory of fluids and solids. They are defined simply as impenetrable spheres that cannot overlap in space. They mimic the extremely strong repulsion that atoms and spherical molecules experience at very close distances. Hard spheres systems are studied by analytical means, by molecular dynamics simulations, and by the experimental study of certain colloidal model systems.

Formal definition

Hard spheres of diameter \sigma are particles with the following pairwise interaction potential:

V(\mathbf{r}_1,\mathbf{r}_2)=\left\{ \begin{matrix}0 & \mbox{if}\quad |\mathbf{r}_1-\mathbf{r}_2| \geq \sigma \\ \infty & \mbox{if}\quad|\mathbf{r}_1-\mathbf{r}_2| < \sigma \end{matrix} \right.

where \mathbf{r}_1 and \mathbf{r}_2 are the positions of the two particles.

Hard-spheres gas

The first three virial coefficients for hard spheres can be determined analytically

\frac{B_2}{v_0} = 4{\frac{}{}}
\frac{B_3}{{v_0}^2} = 10{\frac{}{}}
\frac{B_4}{{v_0}^3} = -\frac{712}{35}+\frac{219 \sqrt{2}}{35 \pi}+\frac{4131}{35 \pi} \arccos{\frac{1}{\sqrt{3}}}\approx 18.365

Higher-order ones can be determined numerically using Monte Carlo integration. We list

\frac{B_5}{{v_0}^4} = 28.24 \pm 0.08
\frac{B_6}{{v_0}^5} = 39.5 \pm 0.4
\frac{B_7}{{v_0}^6} = 56.5 \pm 1.6

A table of virial coefficients for up to eight dimensions can be found on the page Hard sphere: virial coefficients.

File:Hard-sphere phase diagram pressure packing fraction.svg
Phase diagram of hard sphere system (Solid line - stable branch, dashed line - metastable branch): Pressure P as a function of the volume fraction (or packing fraction) \eta

The hard sphere system exhibits a fluid-solid phase transition between the volume fractions of freezing \eta_\mathrm{f}\approx 0.494 and melting \eta_\mathrm{m}\approx 0.545. The pressure diverges at random close packing \eta_\mathrm{rcp}\approx 0.644 for the metastable liquid branch and at close packing \eta_\mathrm{cp}=\sqrt{2}\pi/6 \approx 0.74048 for the stable solid branch.

Hard-spheres liquid

The static structure factor of the hard-spheres liquid can be calculated using the Percus–Yevick approximation.

Literature

  • J. P. Hansen and I. R. McDonald Theory of Simple Liquids Academic Press, London (1986)
  • Hard sphere model page on SklogWiki.