  The migration of helical particles in viscous shear flows plays a crucial
role in chiral particle sorting. Attaching a non-chiral head to a helical
particle leads to a rheotactic torque inducing particle reorientation. This
phenomenon is responsible for bacterial rheotaxis observed for flagellated
bacteria as Escherichia coli in shear flows. Here we use a high-resolution
microprinting technique to fabricate micro-particles with controlled and
tunable chiral shape consisting of a spherical head and helical tails of
various pitch and handedness. By observing the fully time-resolved dynamics of
these micro-particles in microfluidic channel flow, we gain valuable insights
into chirality-induced orientation dynamics. Our experimental model system
allows us to examine the effects of particle elongation, chirality, and
head-heaviness for different flow rates on the orientation dynamics, while
minimizing the influence of Brownian noise. Through our model experiments we
demonstrate the existence of asymmetric bistability of the particle orientation
perpendicular to the flow direction. We quantitatively explain the particle
equilibrium orientations as a function of particle properties, initial
conditions and flow rates, as well as the time-dependence of the reorientation
dynamics through a theoretical model. The model parameters are determined using
boundary element simulations and excellent agreement with experiments is
obtained without any adjustable parameters. Our findings lead to a better
understanding of chiral particle transport, bacterial rheotaxis and might allow
the development of targeted delivery applications.
