  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.
