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The migration of helical particles in viscous shear flows plays a crucialrole in chiral particle sorting. Attaching a non-chiral head to a helicalparticle leads to a rheotactic torque inducing particle reorientation. Thisphenomenon is responsible for bacterial rheotaxis observed for flagellatedbacteria as Escherichia coli in shear flows. Here we use a high-resolutionmicroprinting technique to fabricate micro-particles with controlled andtunable chiral shape consisting of a spherical head and helical tails ofvarious pitch and handedness. By observing the fully time-resolved dynamics ofthese micro-particles in microfluidic channel flow, we gain valuable insightsinto chirality-induced orientation dynamics. Our experimental model systemallows us to examine the effects of particle elongation, chirality, andhead-heaviness for different flow rates on the orientation dynamics, whileminimizing the influence of Brownian noise. Through our model experiments wedemonstrate the existence of asymmetric bistability of the particle orientationperpendicular to the flow direction. We quantitatively explain the particleequilibrium orientations as a function of particle properties, initialconditions and flow rates, as well as the time-dependence of the reorientationdynamics through a theoretical model. The model parameters are determined usingboundary element simulations and excellent agreement with experiments isobtained without any adjustable parameters. Our findings lead to a betterunderstanding of chiral particle transport, bacterial rheotaxis and might allowthe development of targeted delivery applications.

Asymmetric bistability of chiral particle orientation in viscous shear flows