We analyze self-propelling organisms, or active particles, in a periodic asymmetric potential. Unlike standard ratchet effect for Brownian particles requiring external forcing, in the case of active particles asymmetric potential alone produces a net drift speed (active ratchet effect). By using theoretical models and numerical simulations we demonstrate the emergence of the rectification process in the presence of an asymmetric piecewise periodic potential. The broken spatial symmetry (external potential) and time symmetry (active particles) are sufficient ingredients to sustain unidirectional transport. Our findings open the way to new mechanisms to move in directional manner motile organisms by using external periodic static fields.

Understanding the collective motion of self-propelling organisms in confined geometries, such as that of narrow channels, is of great theoretical and practical importance. By means of numerical simulations we study the motion of model bacteria in 2D channels under different flow conditions: fluid at rest, steady and unsteady flow. We find aggregation of bacteria near channel walls and, in the presence of external flow, also upstream swimming, which turns out to be a very robust result. Detailed analysis of bacterial velocity and orientation fields allows us to quantify the phenomenon by varying cell density, channel width and fluid velocity. The tumbling mechanism turns out to have strong influence on velocity profiles and particle flow, resulting in a net upstream flow in the case of non-tumbling organisms. Finally we demonstrate that upstream flow can be enhanced by a suitable choice of an unsteady flow pattern.

Spontaneous segregation of run-and-tumble particles with different velocities in microchannels is investigated by numerical simulations. Self-propelled particles are known to accumulate in the proximity of walls. Here we show how fast particles expel slower ones from the wall leading to a segregated state. The mechanism is understood as a function of particle velocities, particle density, or channel width. In the presence of an external fluid flow, particles with two different velocities segregate due to their different particle fluxes. Promising applications can be found in the development of microfluidic lab-on-a-chip devices for sorting of particles with different motilities.

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When a predator attacks a flock of starlings (Sturnus vulgaris), involving thousands of individuals, a typical collective escape response is the so-called agitation wave, consisting of one or more dark bands (pulses) propagating through the flock and moving away from the predator (usually a Peregrine falcon, Falco peregrinus). The mechanism underlying this collective behavior remains debated. A theoretical study has suggested that the individual motion underlying a pulse could be a skitter (in the form of a zigzag), that is copied by nearby neighbors, and causes us to temporarily observe a larger surface of the wing because the bird is banking during turning while zigzagging. It is not known, however, whether pulses during a wave event weaken over time. This is of interest, because whereas during the usual turning by an undisturbed flock the motion is copied completely without weakening, we may expect that pulses dampen during a wave event because individuals that are further away from a predator react less because of reduced fear. In the present paper, we show in empirical data that pulses during a wave event weaken over time. Our computational model, StarDisplay, reveals that this is most likely a consequence of a reduction of the maximum banking angle during the zigzag escape maneuver rather than by a reduced tendency to copy this maneuver with time. The response seems adaptive because of lowered danger at a larger distance to the location of attack. Significance statement Huge flocks of starlings display amazing patterns of collective escape when attacked by an avian predator, such as a Peregrine falcon. One of them is the "agitation wave" in which dark bands move away from the predator. Dark bands arise probably from the temporarily larger wing area, which is observed when birds perform a skitter escape motion (zigzag) while temporarily banking sideward. Whereas during regular flock turns birds copy each other's motion completely, it is unknown whether this happens during agitation waves, because individuals further away from the attack may be less frightened. Studying this both empirically at the group level only and in a computational model at both the level of the individual and the group, we show that pulses of waves fade out with time and that this is probably due to a reduced maximum banking angle during the zigzag maneuver rather than a lower tendency of copying. This seems an adaptive response.