| 研究概要 |
During this first year, I carried out experiments on asymmetric colloids called "Janus" particles that are able to self-propel by self-induced electrophoresis in a solution of water under an AC electric field. The self-propelling behavior of these particles strongly depends on the properties of the electric field and a transition from random-walks to regular rotations was observed when the amplitude of the electric field is increased. A simple system of coupled Langevin equations involving a self-propelling force and a constant torque can describe very well both the random-walks and rotations, even if the origin of this torque still requires some experimental evidences to be fully understood. One could imagine taking advantage of this rotational behavior to induce mixing in microfluidic systems. From a collective point of view, we were hoping to be able to use Janus particles to study a dynamical phase transition to collective motion that had been observed in various numerical simulations and in a few experimental devices. However, this phase transition does not appear in this particular system, even at high volume fraction. Understanding why some systems of self‐propelling objects never experience this phase transition is still an open question I am now trying to answer by analyzing their hydrodynamic interactions. For this purpose, numericalsimulations based on the smooth profile method were started. The first results suggest the hydrodynamics of Janus particles can be compared to "Pushers", a particular kind of swimmers unable to self-organize, even at high densitv, which could explain the absence of collective motion. Finally, Janus particles were predicted to induce strong rectification currents in ratchet geometries. Microfluidic ratchets channels have been made using photolithography methods but some technical difficulties stlll prevent us from checking this theoretical prediction.
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| 今後の研究の推進方策 |
Experiments on Janus particles can be very useful to investigate some unique properties of active matter. Continued efforts still need to be made to understand the mechanisms leading to the collective phase transition. Moreover, even if this transition was not reported for Janus particles, other interesting collective effects that deserve to be studied were observed, such as active turbulence or active jamming. Also, the experiments on ratchets still need to be completed.
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