Open Access

Alexander Geiseler

• Living microorganisms are capable of a directed response to external stimuli, such as light or certain chemicals, by swimming toward or away from the stimulus source. They do so by means of complex signal transduction pathways, which allow them to elaborate a physiological response to the stimulating environment. Usually, tactic stimuli are not static, but rather modulated in the form of spatio-temporal signals, like traveling wave pulses. Interestingly, many microorganisms are capable of locating the pulse source and heading toward it, irrespective of their response to a static gradient of the respective stimulus. Their biomimetic counterpart, artificial microswimmers, also propel themselves by harvesting kinetic energy from an activating medium, but in contrast lack any adaptive capacity. In the present work, we investigate the transport of such swimmers subject to traveling activation pulses and show, by means of analytical and numerical methods, that they can actually drift inLiving microorganisms are capable of a directed response to external stimuli, such as light or certain chemicals, by swimming toward or away from the stimulus source. They do so by means of complex signal transduction pathways, which allow them to elaborate a physiological response to the stimulating environment. Usually, tactic stimuli are not static, but rather modulated in the form of spatio-temporal signals, like traveling wave pulses. Interestingly, many microorganisms are capable of locating the pulse source and heading toward it, irrespective of their response to a static gradient of the respective stimulus. Their biomimetic counterpart, artificial microswimmers, also propel themselves by harvesting kinetic energy from an activating medium, but in contrast lack any adaptive capacity. In the present work, we investigate the transport of such swimmers subject to traveling activation pulses and show, by means of analytical and numerical methods, that they can actually drift in either direction with respect to the propagation of the pulses, depending on the pulse speed and waveform. Moreover, chiral swimmers, which move along spiraling trajectories, may drift preferably in a direction perpendicular to the pulse propagation. Such a variety of tactic responses is explained with a combination of two mechanisms: angular fluctuations, which help the swimmer explore its surroundings and thus diffuse faster toward more active regions, and self-polarization, a mechanism inherent to (phoretic) self-propulsion, which tends to orient the swimmer's velocity parallel or anti-parallel to the local activation gradients. By determining the relative magnitude of both effects, we characterize the selective transport of artificial microswimmers in inhomogeneous activating media.…