A rheological study of wormlike micelles flows in microchannel

Complex fluids show non linear properties under simple shear flows since they have various microstructures leading to flow induced phase transitions and instabilities. Such a coupling has widely been studied for wormlike micelles. Their flow curve exhibits a stress plateau separating high and low viscosity branches, corresponding to a shear-banding flow. Our aim is to understand the structure/concentration/flow coupling of wormlike micelles. A microfluidic chip is easy to couple with many analytical methods; it is hence well adapted to our study. We both perform particle image velocimetry and microscopy on a microfluidic chip consisting in a straight channel with dimensions: 250 $\mu $m large and 1 mm deep. Such a ``canyon'' geometry enables us to simply relate the measured velocity profiles to the local rheology. We evidence shear banding flow and slip at the walls. Strikingly there is no single rheological law that describes the velocity profiles at different pressure drops. Using microscopy, we point out turbid bands at the walls of the channels corresponding to the highly sheared bands. It seems that the shear induced phase has a lower concentration than the low sheared band. At low pressure drops, these bands are stable in time and their widths increase with increasing pressure until a limit where they fluctuate in space and time.