= Depletion force
{title2=$U_{\mathrm{dep}}=-\Pi_p V_{\mathrm{ov}}$}
A <depletion force> is an effective interaction caused by exclusion of smaller particles or macromolecules from regions around larger objects. Overlap of the excluded regions increases the accessible <volume> for the smaller species. At fixed reservoir <osmotic pressure> $\Pi_p$, the associated pair <free energy> is $-\Pi_p$ times the overlap <volume> in the ideal dilute-depletant approximation. It is an entropic attraction for two large hard particles. Surface adsorption can introduce different interactions, so adding an arbitrary polymer does not by itself guarantee depletion attraction.
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