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Anomalous propulsion regime in axonemal-propelled cargoes

Bio-actuated micro-swimmers provide a platform to understand physical principles related to the motion of micro-organisms at low Reynolds numbers. Here, we used isolated and demembranated flagella from green algae Chlamydomonas reinhardtii as an ATP-fueled bio-actuator for propulsion of micron-sized beads. Chlamydomonas flagella have an asymmetric waveform, which can be accurately described as a superposition of a static component corresponding to an arc-shaped intrinsic curvature, a mode describing the global oscillations of the axonemal curvature, and a main base-to-tip traveling wave component. By decomposing experimental beat patterns in Fourier modes, and applying resistive force theory, we performed numerical simulations and obtained analytical approximations for the mean rotational and translational velocities of a flagellum-propelled bead. Our analysis reveals the existence of a counterintuitive anomalous propulsion regime where the speed of the flagellum-driven cargo increases with increasing the cargo size. Further, it demonstrates that in addition to the intrinsic curvature and even harmonics, asymmetric bead-flagellum attachment also contributes in the rotational velocity of the micro-swimmer. This turning mechanism induced by sideways cargo attachment has potential applications in fabrication of bioactuated medical micro-robots in the subject of targeted drug delivery and synthetic micro-swimmers.