College Park, Maryland      June 6 - 10 , 2004

MP28: Rheo-SANS microstructure investigations of concentrated anisotropic particle dispersions near the shear thickening transition

R. G. Egres, N. J. Wagner (Department of Chemical Engineering, University of Delaware, Newark, DE 19716), L. Porcar, B. S. Greenwald (NIST Center for Neutron Research; University of Maryland, College Park)

An experimental Rheo-SANS configuration with rheologically accurate quartz shear cell is described and validated using a rheometer in the NG7 SANS beamline at NIST. Rheo-SANS experiments were performed on concentrated dispersions of anisotropic particles exhibiting shear thickening behavior as a means of exploring microstructure evolution during shear flow. Concentrated dispersions of anisotropic particles exhibiting shear thickening behavior are ubiquitous in industry, yet little has been done to investigate the microstructure of these dispersions during flow in order to obtain a mechanistic understanding of this observed rheological behavior. Our research involves rheological and microstuctural investigations of stable polyethylene glycol based dispersions of acicular precipitated calcium carbonate (PCC) particles having three average particle aspect ratios. Rheological measurements performed on these dispersions demonstrate that increased particle aspect ratio results in extensive (discontinuous) shear thickening behavior at lower particle volume fractions. Small angle neutron scattering measurements have been used to determine particle geometry and microstructure as a function of concentration and aspect ratio. Rheo-SANS experiments show a significant degree of particle alignment with flow direction at low to moderate shear rates. Surprisingly, this alignment is maintained at the higher stresses associated with shear thickening. These observations invalidate earlier hypothesis suggesting that the shear thickening observed in anisotropic particle dispersions was the result of particle rotations out of flow alignment leading to particle jamming. Rather, the Rheo-SANS measurements support the hydrocluster mechanism of shear thickening known for spherical particle dispersions.

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