  Suspensions of anisotropic Brownian particles are commonly encountered in a
wide array of applications such as drug delivery and manufacturing of
fiber-reinforced composites. Technological applications and fundamental studies
of small anisotropic particles critically require precise control of particle
orientation over defined trajectories and paths. In this work, we demonstrate
robust control over the two-dimensional (2D) center-of-mass position and
orientation of anisotropic Brownian particles using only fluid flow. We
implement a path-following model predictive control scheme to manipulate
colloidal particles over defined trajectories in position space, where the
speed of movement along the path is a degree of freedom in the controller
design. We further explore how the external flow field affects the orientation
dynamics of anisotropic particles in steady and transient extensional flow
using a combination of experiments and analytical modeling. Overall, this
technique offers new avenues for fundamental studies of anisotropic colloidal
particles using only fluid flow, without the need for external electric or
optical fields.
