Attractions between hard colloidal spheres in semiflexible polymer solutions

We explore the depletion attractions that arise between hard colloidal sphe res immersed in a nonadsorbing polymer solution of DNA. We spatially confin e two 1.25 mu m silica spheres in a scanning optical tweezer and quantitati vely examine their interaction potential in double-stranded DNA solutions o f different concentrations. The potentials obtained display variations in d epth and range that are consistent with scaling behavior expected for semif lexible polymers near the theta-point. In particular, we clearly observe th e crossover from a dilute solution of Gaussian coils to the weakly fluctuat ing semidilute regime dominated by two-point collisions (Schaefer, D. W.; J oanny, J. F.; Pincus, P. Macromolecules 1980, 13, 1280-1289). We also quant itatively test the Asakura-Oosawa model for these systems and show that it may be used in both the semidilute as well as the dilute regime. At fixed D NA concentration, we find that the range and depth of the interparticle pot entials do not change significantly for ionic concentrations between 1 and 50 mM.