High Deborah number flows through 3D contractions at the microscale

In this work, we report a rich sequence of elastic transitions captured experimentally using a solution of polyethylene oxide in a water/glycerol mixture that exhibits shear-thinning and viscoelastic behavior. The experiments were carried out in a planar microfluidic channel with a sudden contraction followed by a smooth expansion, taking advantage of the distinctive conditions provided by microfluidic flows, i.e the capability of achieving high Deborah numbers (De) while concomitantly keeping the Reynolds numbers low enough so that inertia does not have a significant impact. We studied the effect of De on the flow patterns experimentally using epifluorescence long exposure photography and video, and were able to achieve a flow regime, at sufficiently high De, in which the flow becomes unsteady with the main vortices formed upstream of the contraction varying in size substantially and very rapidly resembling what Afonso et al. [J. Fluid Mech., in press] coined as the back-shedding regime. This process is accompanied by the formation of secondary vortices upstream of the main ones, which are shed in the upstream flow direction. Furthermore, the experimental results were compared with the numerical simulations obtained using our numerical code with both the Oldroyd-B and PTT constitutive equations and the results show good qualitative agreement.