demanding microfabrication) of a staggered herringbone shape allows Lagrangian chaos and effective mixing for Re < 1. Polymer coils in solution can be stretched in flows. Thermal agitation causes a relaxation back towards the globule coil state and polymers act as entropic springs [1]. Polymeric solutions therefore have a relaxation time, λ , and if flow times, t$_{f}$ = L$_{Char}$/v , where L$_{Char}$ is a characteristic channel dimension and v is the flow velocity, are driven to be shorter than the relaxation time λ significant elastic stress can develop and lead to secondary flows and instabilities (see Chapter 2 of Ref. [1] for a general overview). The Deborah number, De = λ/t$_{f}$ , is a dimensionless parameter describing viscoelastic solutions; when this number reaches a geometry-dependent threshold, roughly of order unity, instabilities often occur [1]. For example, it has been shown that chaotic instabilities can arise in Dean flow of polymer solutions through curvilinear serpentine microchannels [8], where earlier characterization of macroscopic-equivalent flows [9] demonstrated the presence of elastic turbulence [10]. In addition to these flows were elastic turbulence and chaotic flow are established it is known that rapid constriction/expansions can also lead to instabilities [1], for example modifica- tion of serpentine microchannels [8] to add asymmetric corrugation (triangular constrictions followed by sudden expansions along the channel) demonstrates the formation of unstable vortices [11] when polymeric fluid is pumped through the channel at sufficiently high rates. These unstable vortices are excited in linear channels as well and display apparent random growth and decay dynamics [12] which, due to the corrugation asymmetry, display differing dynamics depending on flow direction. The unstable vortices arise at Re < 1 for highly elastic polymeric liquids; in the same device Newtonian flow (water) also displays vortices at intermediate Reynolds numbers (Re ≈ 5) due to inertial effects. Microchannels were formed in glass with 100 µ m and 200 µ m width and 20 µ m depth to the bottom of the “D”-shaped chemically etched cross sections. Forming 2:1 constric- tions (see Fig. 1.a and b) causes flow acceleration and deceleration as liquid transverses through the constricted region, inducing an extensional (stretching) flow field. Polymer so- lutions were made by adding 18 × 10 6 Da high molecular weight (HMW) or 5 × 10 6 Da low molecular weight (LMW) polyacrylamide (PAAm) from Polysciences Inc. to a 20:80 vol.% II. EXPERIMENTAL SETUP 3