Flow pattern and molecular visualization of DNA solutions through a 4:1 planar micro-contraction

The present work explores unusual flow behavior of entangled fluids in an abrupt contraction flow device. Fluorescent imaging was carried out on four different entangled DNA solutions with concentrations ranging from 0.1 to 1.0% (with a wide range of entanglements per chain Z = 7-55). For weakly entangled solutions (Z < 30), vortex flow was dominant at high flow rates. However, for well-entangled DNA solutions (Z ≥ 30), unusual time dependant shear banding was observed at the contraction entrance. Upon reducing the slip length by adding sucrose to the well-entangled DNA solution, vortex flow became dominant again. In vortex flow, most DNA chains remained coiled at the corner in regular recirculation. However, when jerky-shear-banding flow developed, significant stable stretching of DNA chains occurred at the center-line, with quasi-periodic switching between stretching and recoil at the corner.