eCapillary: a disposable microfluidic extensional viscometer for weakly elastic polymeric fluids

We report a disposable microfluidic extensional viscometer based on an optimized hyperbolic contraction–expansion geometry. This “eCapillary” device works by measuring pressure drop as a function of flow rate while accounting for viscous contribution to the pressure drop. The viscometer operates by applying a constant pressure and using an image-based approach to measure the flow rate. The device is fabricated entirely out of polydimethylsiloxane with no embedded sensors, making it disposable. We tested our approach using weakly elastic polymer solutions whose relaxation times were characterized by dripping-on-substrate rheology. Flow visualization was used to determine the onset of inertioelastic instabilities in the eCapillary device, thereby establishing the operating limits for extensional rheological measurements. Holography-based velocimetry analysis showed that extensional strain rate is uniform in a narrow section of the contraction throat necessitating correction for the shear contribution to the measured pressure drop. We observed the onset of extensional thickening to occur at Deborah number ≈ 1 and found that the apparent extensional viscosities are 2–4 orders of magnitude higher than the shear viscosities. Finally, we compared our data with those from other microfluidic extensional viscometers reported in the literature and found good agreement.