A 3D View on Elastic Instabilities of Polymeric Solutions in Cross-Channel Flow

Over a decade ago Jerry Gollub along with two talented Haverford College undergraduate students and one of the authors (PEA) investigated the flow of viscoelastic fluids near hyperbolic points using a cross-slot channel. We found two novel flow instabilities of a planar extensional flow under steady forcing: (i) a symmetry breaking and steady instability and (ii) a time-dependent flow as strain-rate is increased. Here, new experiments using 3D particle tracking velocimetry are presented. We find a new symmetry-breaking instability \textit{normal} to the extension plane, marked by a bi-stable flow switching in that direction and ``buckling'' of the separatrix between the impinging inflow streams. A strong connection exists therefore between temporal fluctuations and 3D flow. We explore this connection in some detail comparing the distribution of 3D modes with fluctuations of local pressure in inlet channels