Simultaneous stress, pressure, and 3D velocity measurements of viscoelastic instabilities in a cross-slot.

Turbulent flow and the hydrodynamic instabilities which may appear, are generally observed when inertia dominates the fluid motion. However, polymeric fluids with viscoelastic properties do not require an inertial flow regime to exhibit such instabilities or even turbulent-like behavior. Instead, these types of fluids need only the correct flow gradients and curvature to develop what has been classified as purely elastic turbulence. Using holographic particle tracking, we elucidate the 3-D flow structures that underly the transition and early stages of purely elastic turbulence in a model mixed flow type system. Furthermore, via measurements of the pressure fluctuations upstream of the cross-channel middle section, and using birefringence polarization to visualize the fluid stresses, we correlate the changes in retardance with both pressure and velocity measurements to show how stress disturbances propagate upstream even in the presence of viscous dissipation. Finally, we further study the spatiotemporal dynamics of the pressure and velocity measurements to fully describe the different regimes in the elastic turbulent regime.