UNSTABLE SLIT FLOW OF A LIQUID-CRYSTALLINE POLYMER-SOLUTION

A lyotopic solution of 27 wt% hydroxypropylcellulose [HPC] in m-cresol has been studied in pressure-driven slit flow. At high flow rates an instability leads to large wavelength disturbances in fluid structure. A combination of image analysis and time signal processing is used to determine the velocity at which the structural disturbances are conve cted downstream, which is shown to be equal to the independently measu red and predicted centerline velocity. This implies that the disturban ce structure is confined near the midplane of the slit flow. Upstream of the onset point of the wavy fluid structures, the fluid exhibits un usual optical properties when viewed between crossed polarizers that a re rotated relative to the flow direction. Specifically, the optical p roperties indicate that there must be some variation in the macroscopi c optical axis of the sample as light passes through the slit flow. A discrete optical model consisting of birefringent elements twisted awa y from and back to the now direction as a function of depth in the sam ple is able to predict the essential optical characteristics; however, independent x-ray scattering measurements show that the macroscopic m olecular alignment is along the flow direction. The wavy textures appa rently emerge as a result of an inhomogeneous transition of orientatio n back to the flow direction, trapping thin bands of fluid in the twis ted configuration.