(623e) Structure and Rheology of Depletion Mixtures of Polymers and Bidispersed Colloids

We used confocal microscopy, particle-tracking, and bulk rheology to study the effects of varying particle size dispersity on the viscoelastic and structural properties of bidispersed colloid-polymer mixtures, which serve as simple models of attractive bidispersed suspensions. We synthesized slightly charged bidispersed poly(methylmethacrylate) spheres with particle size ratio a_S/a_L ≈ 0.43 that were labeled with distinct fluorescent dyes and suspended them in an organic solvent mixture that nearly matched their index of refraction and density. To induce a controlled short-range interparticle depletion attraction, we added non-absorbing linear polystyrene (molecular weight Mw = 281,700 kDa) at a concentration of c_p= 25 mg/ml. We measured the bulk rheology, dynamics and structural properties of suspensions as a function of the ratio of volume fraction of large particles to total volume fraction (r) while holding constant the total volume fraction of particles (φ_T ≈ 0.15). A suspension of only the small particles (r = 0) exhibited strong shear thinning; appreciable changes in the viscosity as a function of shear rate appeared only when r was increased above 0.87. This rheological change coincided with a structural change in the suspensions: samples with r < 0.87 contained a connected network whereas those with r ≥ 0.87 contained disconnected clusters of large and small particles. Our results suggest that the viscoelastic properties of attractive bidispersed suspensions can be controllably tuned by varying the particle size dispersity. This result will aid the development of polydispersed suspensions with controlled rheology for a wide range of technological applications, including paints and coatings, lubricants, and drilling fluids.