Entropic Colloidal Interactions in Concentrated DNA Solutions

We explore the entropic interactions between a pair of micron-sized colloidal spheres in DNA solutions. By confining the particles in a line-scanned optical tweezer, we directly measured the functional form of the interaction potential with sub- $k_{B}T$ resolution in samples where the spheres and the polymer coils were of comparable size. The potential is well described by the Asakura-Oosawa depletion model even in the semidilute regime where DNA coils overlap strongly. Its range and depth increase with increasing concentration in a manner consistent with a crossover from a dilute solution of Gaussian coils to the weakly fluctuating semidilute regime dominated by two-point collisions which is unique to semiflexible polymers.