Colloidal Interactions in Suspensions of Rods

We report direct measurements of entropic interactions of colloidal spheres in suspensions of rodlike fd bacteriophage. We investigate the influence of sphere size, rod concentration, and ionic strength on these interactions. Although the results compare favorably with a recent calculation, small discrepancies reveal entropic effects due to rod flexibility. At high salt concentrations, the potential turns repulsive as a result of viral adsorption on the spheres and viral bridging between the spheres.