Control of viscoelastic instabilities by symmetry in microfluidic channels

We leverage the symmetry of the pillars and their organization in microfluidic arrays to control the existence and characteristics of viscoelastic fluctuations. We study these fluctuations in the form of waves that we observed recently in DNA at concentrations exceeding the overlap concentration flowing through square arrays with circular pillars [1] and that appear in two mirrored variants (LEFT and RIGHT). Breaking the lateral symmetry using triangular pillars, the waves form only in one of the mirrored orientations (LEFT or RIGHT).

Enhanced mixing is demonstrated at low Reynolds numbers using these waves by introducing DNA stained with different colors for each of two inlets in the microfluidic device. We observe the waves using epi-fluorescence microscopy and quantify the mixing by the overlap of the two DNA samples as they flow along the channel. We also quantify the mixing of the aqueous solvent by monitoring the change in fluorescence of an ion sensitive dye that is added to one of the DNA samples while a monovalent ion is added to the other sample.

Efficient suppression of the viscoelastic fluctuations is realized by alternating every second row in the pillar array, thereby creating a situation where a LEFT wave is initiated immediately followed by a RIGHT wave that neutralizes it.