Direct measurement of the intermolecular forces confining a single molecule in an entangled polymer solution

We use optical tweezers to directly measure the intermolecular forces acting on a single polymer imposed by surrounding entangled polymers (115 kbp DNA, 1 mg/ml). A tubelike confining field was measured in accord with the key assumption of reptation models. A time-dependent harmonic potential opposed transverse displacement, in accord with recent simulation findings. A tube radius of 0.8 mu m was determined, close to the predicted value (0.5 mu m). Three relaxation modes (similar to 0.4, 5, and 34 s) were measured following transverse displacement, consistent with predicted relaxation mechanisms.