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

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Flagellar dynamics of chains of active Janus particles fueled by an AC electric field
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We study the active dynamics of self-propelled asymmetrical colloidal particles (Janus particles) fueled by an AC electric field. 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 electric field 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 are fixed, 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 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 the dispersion relations of beating behaviors. In addition, hydrodynamic interaction within 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.
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Transcript: English(auto-generated)
We experimentally study the active dynamics of flexible chains of self-propelled colloidal particles. How can we understand the fascinating dynamics of biological flagella, as seen in Sperma's analogy? To answer this question, we created artificial flagella composed of self-propelled active colloidal particles
and compared their beating behaviors. First, we fabricated generous particles which are colloidal particles with distinct hemispheres. We suspended generous particles in NACL solutions and sandwiched them between two transparent two-dimensional electrodes. Due to gravity, the particles were segmented very close to the bottom electrodes.
Then we applied an AC electric field in the vertical direction, and the particles start to swim in the horizontal plane, interacting with each other. By tuning the frequency and the amplitude of the applied electric field and the ion concentration of the solution, the particles exhibit dynamical chain structures.
How do the particles interact and form such chains? Under the AC electric field with frequency higher than a certain threshold frequency, induced electric charges on the surface of the particles have opposite signs on the two hemispheres due to the difference in the response time of the dielectric and metal hemispheres.
This quadrupolar charge distribution results in attractive interaction between the particles, and this attractive interaction is responsible for the chain formation. Interestingly, we can also control the direction of motion of the generous particles by changing the frequency of the electric field.
In the low frequency regime, the particles move towards their dielectric sides. On the other hand, in the high frequency regime, they move towards their metal sides. In previous works, chain formation was only observed in the high frequency regime, but in this work, we have discovered that the chain formation at the low frequency regime
is possible by thoroughly exploring the parameter space. Depending on the constraints imposed on their heads, the chains exhibit oscillatory or rotary motions. In particular, when the formless particles of the chain are both positionally and orientationally fixed,
the chains exhibit stable fragile-like beating behavior. We studied this in detail. Such beating behavior can be observed when the formless particles are tethered on the bottom electrode or when they hit obstacles or aggregates of immobile particles. The beating chains exhibit characteristic trajectories in the real space.
However, as a matter of fact, the principal component analysis has shown that they can be interpreted as quite simple oscillations between the first and the second principal components. In our system, the self-proportional speed or the self-proportional force can be controlled by the applied voltage.
Taking this advantage, we studied how the beating frequency scales with the self-proportional force of the composing particles. Our experiment shows that the beating frequency was proportional to the self-proportional force. However, this scaling relation deviates from theoretical predictions made previously.
We have resolved this discrepancy by taking into account the quadrupole-quadrupole interactions of the generous particles. However, our measurement gives the indirect but first reliable experimental evidence on the quadrupolar charge distribution of the particles. Finally, we theoretically calculated the flow field around the beating generous chains
to examine the role of hydrodynamics in the beating behavior. For more details, please look at the paper. Thank you for watching.