Response of Moderately Concentrated Xanthan Gum Solutions to Time‐Dependent Flows Using Two‐Color Flow Birefringence

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Abstract
A new optical arrangement for the flow birefringence experiment, based on a two‐color laser, is described. Using this system, flow birefringence measurements can be performed in transient flow conditions with the birefringence Δn and orientation angle χ of the sample being obtained simultaneously in time. Unlike previous designs which have been devised to accomplish this capability, only a single experimental run is necessary. Measurements using a quarter‐wave retarder indicate that the experimental accuracy is 2.2% for the birefringence and 0.15 ° for the orientation angle. The minimum birefringence detectable by this system is estimated to be 2×10 −7 . Using this technique. Δn and χ of moderately concentrated xanthan solutions were measured following the inception and cessation of simple shear. The effects of increased ionic strength by adding NaCl to the xanthan solution were also examined. A qualitative comparison of measurements between the no salt and 0.1M NaCl solutions indicates that the macromolecules might be somewhat stiffened in the presence of salt, which agrees with results reported previously in the literature.

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