Effect of particle anisotropy on suspension rheology and phase behavior with tunable attractive strength

Kim, Yun Soo
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University of Delaware
Particle shape plays a very important role in the thermodynamic and rheological properties of colloidal and nanoparticle suspensions and new phenomena arise as compared to dispersions of isotropic particles. Previously, a state diagram has been constructed for the simplest, fundamental system that includes both excluded volume and attractive interparticle interactions, i.e., adhesive hard spheres. This was performed by combining state-of-the-art simulations, rheology and small angle neutron scattering (SANS) experiments on a model dispersion. In this study, we extend the state diagram to include shape effects by characterizing the dynamical arrest and strength of attraction of model adhesive hard spheres (AHS) comprised of octadecyl-grafted silica nanoparticles in n-tetradecane. We synthesize model, anisotropic colloidal particles of aspect ratio 2.5 ± 0.4 by first by growing ellipsoidal titanium dioxide particles as the core and then coating these templates with a silica shell. The silica coating layer was added to achieve the same surface chemistry as the previous isotropic systems and to reduce the strong van der Waals attractions induced by the titanium cores. Octadecyl chains are grafted onto the silica coating as done previously for the spherical analogs to develop a system exhibiting thermoreversible gelation. Gel temperatures were determined by dynamic oscillatory rheological measurements whereas SANS experiments performed in the vicinity of the rheological gel temperature provided microstructural information. The rheological results shows the adhesive hard ellipsoids suspension formed a gel-like structure as determined by applying the Winter and Chambon criterion SANS results show a more compact structure is formed in comparison with the structure of the aggregates formed by spherical particles of comparable size. The results show that the phase behavior strongly depends on particle shape at a given particle volume fraction and provide guidance for the formulation of gels of anisotropic colloidal particles.