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Debye shear regime with polymers [19]. Our pumping scheme therefore excludes the shear component that is ubiquitous to mechanically pumped flows. In addition to deformations caused by shearing there are possible Lagrangian chaos effects [20], where flow fields result in chaotic trajectories which can lead to rapid mixing. We stack identical constriction units equally spaced down a microchannel, imposing symmetry as to reduce possible Lagrangian chaos effects [21]. 

## II. EXPERIMENTAL SETUP 

See Ref. [22] were we demonstrate viscoelastic instabilitites under steady electro-osmotic pumping and discuss our device in more detail. Briefly, the experimental details are as follows: a microfluidic toolkit ( µ TK) from MicraLyne Inc. [44] is used to impose EOF flow 

