Velocity measurement of velocity profile of complex fluids through a microchannel and the effect of the channel size on the flow pattern

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Flows of polymer solutions through a 3.71 abrupt contraction were observed in a micro- and a regular-size channels. The test fluids are 0.2 wt% aqueous solutions of polyacrylamide whose molecular weights are 6×10^6 (fluid A), 1.5×10^7 (fluid B), and 1.9×10^7 (fluid C). The microchannel was made of PDMS and it is mounted on the glass plate. The growth of the salient corner vortex is discussed as a function of the shear rate, the Weissenberg number We and the viscoelastic Mach number M. Consequently, for the flows of fluid B and C, the vortex size in the microchannel is smaller than that in the regular-size one. The vortex grows with increasing the shear rate, We and M, but more quickly through the microchannel compared with the results in the regular-size one. However, the difference of tendency of the vortex enhancement between flows in the micro-size and the regular-size channels was not observed for the fluid A. Velocity profiles of polymer solutions were directly measured. The velocity profile in the depth direction is discussed as a function of the flow rate and shear stress on the wall. Consequently, for the flows of fluid A, a symmetric profile is observed in the region between the PDMS wall and the glass wall in a low flow rate, but the velocity profile slightly becomes asymmetric with increasing the flow rate : the velocity near the PDMS wall is larger than that near the glass wall. For the flows of fluid B, a slightly asymmetric profile is observed even in the low flow rate and a largely asymmetric one in the large flow rate. This large apparent slip on the PDMS wall significantly affects the velocity profile in the flow of polymer solution through a PDMS microchip.