Packing fraction

For the hard sphere fluid

$\eta ={\frac {\pi }{6}}{\frac {N\sigma ^{3}}{V}}$

where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma } is the hard sphere diameter.