Rheo-microscopy and flow properties of crystallizing agitated sucrose dispersions

Crystallization from highly supersaturated, agitated (sheared and stirred) solution is a highly non-equilibrium process. Despite being an often observed and intended process, basic scientific research on the rheology of such systems is lacking. This paper presents findings on changes in flow properties and particle formation during crystallization out of highly supersaturated sucrose solution under shear. For this, sucrose solution is subjected to steady shear in a plate-plate rheometer until a high crystal fraction is reached. Rheo-microscopy is performed in order to allow in situ visual observation of the crystal formation. These experiments show the complex interactions of crystal growth and rheological properties. While a high amount of hard particles is formed, the system changes from a homogeneous solution to a highly concentrated hard-particle suspension. At the same time, overall viscosity and loss modulus drop significantly, while the storage modulus rises during crystallization. In addition, image analysis is performed on microscopy images taken at different stages of the process. It confirms that significant decrease in viscosity coincides with major crystal growth, which is explained by the decrease in solute concentration in the continuous phase.