Y. Mao 
, M. E. Cates 
and H. N. W. Lekkerkerker 
Cavendish Laboratory, Madingley Road,
Cambridge, CB3 OHE, UK.
Department of Physics and Astronomy,
University of Edinburgh, King's Buildings,
Mayfield Road, Edinburgh EH9 3JZ, UK.
Van't Hoff Laboratory, University of Utrecht,
Padualaan 8, 3584 Utrecht, The Netherlands.
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 hard spheres (of radius R) in a
semidilute solution of hard rods of length L and diameter D
( 
) to second order in rod concentration.
In addition to the well-known attractive force for separations
less than L,
we find a repulsive force between the spheres at larger separations.
For semidilute rods, the
resulting barrier can be large compared to 
, permitting
kinetic stabilization.