Three-dimensional structures and symmetry breaking in viscoelastic cross-channel flow.

Using holographic particle tracking, we report the three-dimensional flow structure organizing the viscoelastic instability in cross-channel flow. Beyond a critical Wi, the advective core flow undergoes an out-of-plane instability marked by the emergence of tertiary flow, resembling that of the toroidal vortices in Taylor-Couette geometry. The out-of-plane flow component distorts the separatrix between the impinging inflow streams, triggering symmetry breaking normal to the extension plane. As extensional rate increases, progressively higher order modes of the separatrix are observed, akin to Euler buckling of a rigid column. The disturbances propagate upstream via stress fluctuations despite viscous dissipation. These complex flow structures may be generic to elastic turbulence in mixed flows.