Transient flow birefringence measurement system for microgravity sciences

Abstract

Current state-of-the-art techniques measuring transient flow birefringence require too much space, power, mass, and astronaut intervention to be practical for microgravity science applications. The modulation scheme, electronic packaging, optical alignment and calibration are the most prohibitive limitations of present-day systems. We have overcome these limitations by developing a novel instrument capable of transient birefringence and molecular chain orientation angle measurements in complex fluids. We perform a mathematical derivation of the time-variant optical intensity using the Mueller-Stokes matrix algebra for polarized light. Using the equivalent system matrix, we then derive the mathematical equations necessary to perform optical retardance and molecular orientation angle measurements from quantities obtained by two heterodyning lock-in amplifiers. We derive an equation for the intensity at a photodetector that depends on the in-phase and in-quadrature signal components. By measuring the optical path length through the sample, we calculate the transient birefringence. To reduce electrical power consumption, we modulate in the transverse mode with a resonant circuit. This optical diagnostic instrument was developed for the Extensional Rheology Experiment in order to analyze a non-Newtonian polymer fluid undergoing extensional deformation in microgravity. This instrument will be flown for the first time on an unmanned sounding rocket in July of 1999.%