Nordstrom, Kerstin N.

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  • Publication
    Microfluidic Rheology of Soft Colloids above and below Jamming
    (2010-10-21) Nordstrom, Kerstin N.; Arratia, Paulo E; Basu, Anindita; Gollub, Jerry P.; Verneuil, E.; Durian, Douglas J.; Zhang, Zheng; Yodh, Arjun G.
    The rheology near jamming of a suspension of soft colloidal spheres is studied using a custom microfluidic rheometer that provides the stress versus strain rate over many decades. We find non-Newtonian behavior below the jamming concentration and yield-stress behavior above it. The data may be collapsed onto two branches with critical scaling exponents that agree with expectations based on Hertzian contacts and viscous drag. These results support the conclusion that jamming is similar to a critical phase transition, but with interaction-dependent exponents.
  • Publication
    Centrifugal Compression of Soft Particle Packings: Theory and Experiment
    (2010-01-01) Nordstrom, Kerstin N; Verneuil, Emilie; Lubensky, Tom C; Durian, Douglas J; Ellenbroek, Wouter G; Gollub, Jerry P
    An exact method is developed for computing the height of an elastic medium subjected to centrifugal compression, for arbitrary constitutive relation between stress and strain. Example solutions are obtained for power-law media and for cases where the stress diverges at a critical strain—for example as required by packings composed of deformable but incompressible particles. Experimental data are presented for the centrifugal compression of thermo-responsive N-isopropylacrylamide (NIPA) microgel beads in water. For small radial acceleration, the results are consistent with Hertzian elasticity, and are analyzed in terms of the Young elastic modulus of the bead material. For large radial acceleration, the sample compression asymptotes to a value corresponding to a space-filling particle volume fraction of unity. Therefore we conclude that the gel beads are incompressible, and deform without deswelling. In addition, we find that the Young elastic modulus of the particulate gel material scales with cross-link density raised to the power 3.3±0.8, somewhat larger than the Flory expectation.