(390f) Transient Flows of Colloidal Dispersions

Transient development of the microstructure in colloidal
hard- and soft-sphere dispersions is studied using simulation by the
Accelerated Stokesian Dynamics algorithm. The cases of startup of a steady
shear flow, cessation of steady shear flow, and large-amplitude oscillatory
shear (LAOS) have been examined. A methodology for use of simulation to
extract time-resolved ensemble averages of structure and properties is
presented and the findings of the study are summarized. The flows have been
studied for monodisperse spherical particles over a range of solid volume
fractions varying from 30-55%, at Peclet numbers (characterizing the relative
strength of shear to Brownian motion) from Pe << 1 (near equilibrium) to
Pe = 1000. In each case, the time-resolved structure is related to the
rheological response during the transient flow. For soft spheres interacting
through long-range electrostatic repulsion, the solid fractions examined are
lower.

It is found that during startup from an isotropic
equilibrium configuration that both the shear stress and normal stress
differences in hard-sphere dispersions exhibit an overshoot before returning to
their steady values. This overshoot is particularly pronounced for the second
normal stress difference, N₂, and is found to be correlated to a
development of a strong pair correlation along the compressional axis, which is
relaxed through rotational flow and diffusion. Upon flow cessation, the
hydrodynamic stress relaxes immediately while the Brownian stress relaxes due
to Brownian motion (and through repulsive forces in the soft-sphere case). The
relaxation of the first normal stress difference, N₁, is found to be
comparable to the shear stress, with both relaxing significantly more slowly
than slower than N₂; this is explained in part by N₂ being
controlled by the very small length scale structural feature of the pair
correlation boundary layer scaling roughly as aPe^-1 where a . The primary features of the LAOS study are reviewed using the higher
harmonics of the Fourier transform description of the nonlinear rheology for a
wide range of Pe and maximum imposed strain of 0.1 to 5.