Simultaneous light scattering-rheology measurements for studying stress induced phase transitions

Abstract

There are a growing number of simultaneous rheological techniques that are of great interest for many applications. Simultaneous rheometry and small angle light scattering (SALS) is of particular interest for obtaining information on material structure, orientation, particle size and shape. A compact SALS setup for simultaneous measurements has been developed and integrated into a stress‐controlled rheometer. The laser source is positioned below a lower Peltier platen, which incorporates a small quartz window. The light beam is perpendicular to the surface of the Peltier platen. The beam passes through the quartz window, the sample and an upper quartz plate. As the beam is scattered by interactions with the electrons of objects within the sample, the pattern formed is collected by an optical assembly with collimator, redirected with a mirror away from the upper test fixture, and projected onto a 2D camera array. Quantitative measurements are possible by calibrating the setup with monodisperse micron size polystyrene beads. Two case studies are presented. The first study involves the use of SALS simultaneous with rheology to measure the onset of shear banding in a worm‐like micellar solution. A butterfly light scattering pattern that is a signature of shear banding is obtained, which indicates that the mechanism of shear banding in the fluid is due to shear‐induced phase separation. The second study tracks the kinetics of shear‐induced colloidal crystallization by large amplitude oscillatory shear using simultaneous rheology and SALS measurements of the crystal order parameter. This allows construction of a phase map for shear ordering and its dependence on oscillation frequency and amplitude.