Short Communication, 82.70.Dd, 82.70.-y, 64.60.-i.
Y. Mao
Cavendish Laboratory, Madingley Road,
Cambridge, CB3 OHE, UK.
The entropic depletion force, in colloids,
arises when large particles are placed in a solution of smaller
ones, and sterically constrained to avoid them. We calculate the
interaction between two flat plates or large hard spheres (of radius R) in a
solution of small and polydisperse hard spheres (of average
diameter 
) to second order in small sphere
concentration. We find that the effect of polydispersity
is to generally smoothen out the depletion force while maintaining
the recently predicted repulsive barrier;
but for high polydispersities, the repulsion could
be eventually wiped out.