Jamming and flow of soft particle suspensions

This thesis focuses on the study of soft spherical particles near jamming. N-isopropylacrylamide (NIPA) microgel spheres were studied in both static and flowing states. As the particles are soft, they may be packed above the jamming volume fraction &phis; ∼ 0.64, corresponding to random close packing. To study them in static jammed configurations, we developed a novel centrifigation technique that allowed us to probe the elasticity of the packed bed of particles. Using effective medium theory, we were then able to obtain information about the elastic properties of the individual spheres, discovering they are Hertzian with a Poisson ratio ν = 0.5. Next, we studied the flowing state of the particles in a microfluidic channel. Through a careful choice of the channel geometry, we were able to relate the velocity profiles to stress and strain rate, thus obtaining rheological flow curves for the system. We found we could collapse these curves onto two master curves: one above the jamming density and one below the jamming density. Such collapse had been seen before in simulation, but not in experiment prior to this work. Interestingly, the critical exponents for this collapse seem to be dependent on the interparticle interaction, thus undermining previous notions of universality for the jamming transition. Finally, we looked for evidence of a growing time and length scale near jamming by looking at the dynamical heterogeneities in the system. We found evidence of these time and length scales, which seem to grow in the same way above and below jamming. We also found critical exponents for these parameters, and saw again that the interparticle potential governs their values. We conclude that jamming is similar to a phase transition, but with interaction-dependent critical exponents