Rheo-optical determination of flow birefringence and flow dichroism with the pulsed laser method

Rheo-optical measurements of the flow birefringence, flow dichroism and their respective orientations to the flow direction allow for a detailed direct investigation not only of the configuration of polymers in a flow field but also of the behavior of associated structures or multiphase systems under flow. However, the standard setup of an optical train, using phase modulation for the rheo-optical investigation of a chosen flow field, is a rather expensive, complex and delicate structure that needs careful adjustment and calibration. This article presents a new, simple and robust design of a measurement unit, consisting of several pulsed lasers, to determine the desired rheo-optical material functions. The setup uses three laser beams with different polarization planes that pass in the same path through a fluid sample. Modulation patterns that alternately operate only single lasers in the kHz regime and the subsequent determination of intensity changes caused by the anisotropy of the sample allow for the calculation of the flow birefringence or dichroism. Comparison of experimental results to conventional methods show a lowest resolution limit of $Δn′∼3.5×10−8$ for the birefringence, determined by the signal-to-noise ratio of the pulsed laser system.