Elastic instabilities in a microfluidic cross-slot flow of wormlike micellar solutions

We present a series of experiments involving an elastic instability that occurs with the flow of wormlike micellar solutions in a microfluidic cross-slot device. We use four different concentrations of an aqueous solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal); two highly viscoelastic and two weakly viscoelastic. Flow in the microfluidic cross-slot device is examined using birefringence and PIV measurements. With all of the solutions we observe the formation of a birefringent band along the outflow axis, the intensity of which increases with the flow rate. In the two highly viscoelastic solutions, as the flow rate increases, the instability results in flow transitioning from a stable symmetric flow to a stable asymmetric flow, to an unsteady asymmetric flow. With the weakly viscoelastic solutions the instability results in the flow transitioning directly from a stable symmetric flow to an unsteady flow. The critical Weissenberg numbers at which the transitions occur, increase with increasing elasticity number (defined as the ratio of the Weissenberg number to the Reynolds number). With the two highly viscoelastic solutions we also observe the formation of lip vortices along the walls of the inlet channels. For one of the highly viscoelastic solutions the symmetry-breaking occurs before the formation of the secondary flow, whereas in another solution the secondary flow develops prior to the symmetry-breaking. This indicates that these instabilities are complex phenomena, whose behavior is likely influenced by a combination of factors such as Weissenberg number, inertial effects, and rheological behavior of the micellar solutions.