Constriction-length dependent instabilities in the microfluidic entry flow of polymer solutions

Real-world porous media are often mimicked in laboratories by microfluidic models with an abrupt contraction-expansion or a channel-centered cylinder. Various parameters have been investigated for the flow of polymer solutions through contraction and/or expansion microchannels. However, the majority of the previous studies has been focused upon the contraction flow. Little is understood about how the upstream contraction flow may be influenced by the high shearing flow inside the finite constriction as well as the transverse stretching and subsequent relaxation in the downstream expansion flow. We experimentally study the rheological responses of such flow instabilities in planar contraction-expansion microchannels differing only in the constriction length with three different polymer solutions.