BUTTERFLY LIGHT-SCATTERING PATTERN IN SHEAR-ENHANCED CONCENTRATION FLUCTUATIONS IN POLYMER-SOLUTIONS AND ANOMALY AT HIGH SHEAR RATES

The effects of simple shear flow on concentration fluctuations in semidilute solutions of high molecular weight polystyrene are investigated by the flow light scattering method. The shear-rate (gamma) dependence of scattering was found to fall into four regimes: Regime I at gamma<gamma(c) where the scattering is that of homogeneous semidilute solutions, and Regimes II to IV at gamma>gamma(c), gamma(c) being the critical shear rate for the shear-enhanced concentration fluctuations. In Regime II, a unique butterfly-type scattering pattern with a dark streak perpendicular to the flow direction appears. The intensity parallel to the flow increases by about 100 times with increasing gamma. In Regime III at higher shear rates, the intensity increase with gamma tends to saturate, and finally in Regime IV at even higher shear rates, the intensity further increases and the butterfly pattern changes into a two-point pattern elongated normal to the flow, with a broad intensity maximum along the flow. This pattern change is closely related to anomalies in rheological behavior.