Mixing of Yield-Stress Fluids Using a Multilamination Mixer

Fluids that posess a yield-stress or are highly shear-thinning  are difficult to mix in conventional agitated vessels due to the presence of unmixed low-speed zones far from the high-shear surfaces.  Static mixers can be used to achieve mixing, though conventional ones have complex flow patterns that may complicate the experimental and  theoretical assesment of mixing quality. A split-and-recombine (SAR) mixer on the other hand can generate a uniform pattern of parallel  layers that are more easily characterized through optical or tomographic methods and are amenable to simplified theoretical treatments. A SAR  mixer was constructed by stacking a series of plane plates with the desired flow pattern. The mixer was used to mix two carbopol gels of the  same concentration and rheological properties, except that one contained a magnetic resonance contrast agent. Magnetic resonance imaging was used  to determine the spatial distribution of the doped and undoped material at different stages of the static mixer. The images obtained show the feasibility of achieving a duplication in the number of layers upon exiting each stage. The three dimensional nature of the images obtained  revealed detailed features of the fluid layers in planes normal or parallel to the flow direction. The effect of image resolution, imaging time and solute diffusion on image quality is discussed.