  We report an alternative method for electrophoretic separation of large DNAs
using steric confinement between solid walls, without gel or obstacles. The
change of electrophoretic mobility vs confinement thickness is investigated
using fluorescence video microscopy. We observe separation at small confinement
thicknesses followed by a transition to the bulk behaviour (no separation) at a
thickness of about 4 &#956;m (a few radii of gyration for the studied DNA
chains). We present tentative explanations of our original observations.
