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 an AC electric field. Both the speed and the direction of the self-propulsion and the strength of attractive interaction between the particles can be controlled by tuning the frequency of the applied electric field and the ion concentration of the solution. The strong attractive force at high ion concentration give rise to chain formation of the Janus particles, which can be explained by the quadrupolar charge distribution on the particles. The chain formation is observed irrespective of the direction of the self-propulsion of the particles. When both the positions and the orientations of the heads of chains are fixed, they exhibit beating behavior reminiscent of eukaryotic flagella. The beating frequency of the chains of the Janus particles depends on the applied voltage and thus on the self-propulsive force. The scaling relation between the beating frequency and the self-propulsive force deviates from theoretical predictions made previously on active filaments. However, this discrepancy is resolved by assuming that the attractive interaction between the particles is mediated 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 interaction mechanism, which had been dismissed previously, can modify dispersion relations of beating behaviors. In addition, hydrodynamic interaction within the chain and its effect on propulsion speed are discussed. These provide new insights on active filaments such as optimal flagellar design for biological functions.