  Hard spheres in Newtonian fluids serve as paradigms for Non-Newtonian
materials phenomena exhibited by colloidal suspensions. A recent experimental
study (Cheng et al. 2011 Science, 333, 1276) showed that upon application of
shear to such a system, the particles form string-like structures aligned in
the vorticity direction. We explore the mechanism underlying this
out-of-equilibrium organization with Steered Transition Path Sampling, which
allows us to bias the Brownian contribution to rotations of close pairs of
particles and alter the dynamics of the suspension in a controlled fashion. Our
results show a strong correlation between the string structures and the
rotation dynamics. Specifically, the simulations show that accelerating the
rotations of close pairs of particles, not increasing their frequency, favors
formation of the strings. This insight delineates the roles of hydrodynamics,
Brownian motion, and particle packing, and, in turn, informs design strategies
for controlling the assembly of large-scale particle structures.
