Flagellar dynamics of chains of active Janus particles fueled by an AC electric field

We study the active dynamics of self-propelled asymmetrical colloidal particles (Janus particles) fueled by anAC electricfield. Both the speed and direction of the self-propulsion, and the strength of the attractive interaction between particles can be controlled by tuning the frequency of the applied electricfield and the ion concentration of the solution. The strong attractive force at high ion concentration gives rise to chain formation of the Janus particles, which can be explained by the quadrupolar charge distribution on the particles. Chain formation is observed irrespective of the direction of the self-propulsion of the particles.When both the position and the orientation of the heads of the chains arefixed, they exhibit beating behavior reminiscent of eukaryotic flagella. The beating frequency of the chains of Janus particles depends on the applied voltage and thus on the selfpropulsive force. The scaling relation between the beating frequency and the self-propulsive force deviates from theoretical predictionsmade previously on active filaments. However, this discrepancy is resolved by assuming that the attractive interaction between the particles ismediated by the quadrupolar distribution of the induced charges, which gives indirect but convincing evidence on the mechanisms of the Janus particles. This signifies that the dependence between the propulsion mechanism and the interactionmechanism, which had been dismissed previously, canmodify the dispersion relations of beating behaviors. In addition, hydrodynamic interactionwithin the chain, and its effect on propulsion speed, are discussed. These provide new insights into active filaments, such as optimal flagellar design for biological functions.