Flow Induced Unstable Structure of Liquid Crystalline Polymer Solution in L-Shaped Slit Channels

An experimental study has been conducted on unstable structures induced in two dimensional slit flows of liquid crystalline polymer solution. 50wt% aqueous solution of hydroxyl-propylcellulose (HPC) was utilized as a test fluid and its flow behavior in L-shaped slit channels with cross section of 1mm height and 16mm width was measured optically. The inner corner of the L-shaped channel was rounded off in order to clarify the influence of the radius of curvature on the unstable behavior. A conversing curved channel was also tested. The flow patterns of HPC solution in the channels were visualized with two crossed polarizers and we observed that typical wavy textures generated in the upstream of the corner almost disappeared after the corner flow. However, an unstable texture was developed again only from the inner corner in downstream flow. The fluctuation of orientation angle and dichroism were also measured with a laser opto-rheometric system and it was found that the unstable behaviors of HPC solution have periodic oscillatory characteristics at a typical frequency. In the inner side flow after the corner, the periodic motion became larger toward the downstream and then higher harmonic oscillations were superimposed. Larger rounding off of the inner corner suppressed the redevelopment of unstable behavior, and it is considered that the rapid re-growth of unstable behavior was caused by rapid deceleration at the corner flow. Moreover, the unstable structure was stabilized with accelerated (elongated) region in the corner flow and the converging channel was helpful to obtain stable structure in the downstream region.