Diffusion of latex and DNA chains in 2D confined media.

We report a study on the dynamics of latex polystyrene beads and of DNA molecules confined in two dimensions, using fluorescence video-microscopy. We particularly focus on the character of the confined objects (hard or soft) and on the nature of the confinement: liquid (in a soap film) or solid (between two glass plates). For weak confinements, whatever the nature of confinement, we observe that DNA molecules and latex beads behave very similarly: the tighter the confinement, the slower the diffusion with a good agreement with theory. For strong confinements between solid walls (thickness of confinement smaller than the bulk radius of gyration), DNA coils are not immobilized and still diffuse. We show in this case that the conformation of DNA chains is in good agreement with the predictions of De Gennes and Brochard (radius approximately e (-1/4), with e the confinement gap); on the other hand, we cannot really check the theoretical predictions for the diffusion coefficient. Interestingly, strong confinement of latex beads in a soap film leads to a anomalous slow diffusion, certainly associated with an additional viscous drag generated by the interfaces.