content_image/360839.jpg
## III. INSTABILITIES IN EXTENSIONAL MICROFLOWS OF POLYMERIC SO- LUTIONS 

Creating a visually observable fluid interface by flowing dyed fluid from reservoir 1 and undyed fluid from reservoir 2 to reservoir 4, Fig. 1.a, allows us to investigate fluid flow properties. The channel geometry is designed with 2:1 constrictions in order to induce extensional flow and the fluid interface is monitored in the center of this region (at the white spot in Fig. 1.b). The Reynolds numbers here are < 0 . 01, below the nominal creeping flow limit of Re < 1. Without polymer creeping laminar flow persists to the highest applied voltages (Fig. 2.b), with the PMT traces appearing similar to the no-flow case (see Fig. 2.a). 

<img src='content_image/360839.jpg'>

In contrast, very large fluctuations arise, at sufficiently large driving electric fields, when polymer is added to the solution (Fig. 2.c). This arises from an instability of the flow causing the local dye concentration to vary under the green excitation spot. The presence of polymer results in large movement of the dye/undyed interface relative to the illumination spot, generating order-unity fluctuations in Fig. 2.c. We note that extensional flow also exists in the cross-channel region, due to acceleration, which excites instabilities in polymeric fluids 

