Abstract
We study the polar collective dynamics of Janus colloidal particles fueled by an AC electric field. When the density is high enough, the polar interactions between the particles induce a polar orientationally ordered state which exhibits features reminiscent of the Vicsek model such as true long-range order and giant number fluctuations. Independent measurements of the polarity and velocity at the single particle level allowed us to investigate the single particle dynamics within the ordered state. We discovered theoretically-unaddressed statistical properties of the ordered state such as the asymmetric relation of polarity and velocity, enhanced rotational diffusion stronger than in the disordered state, and an algebraic auto-correlation of the polarity. Our experimental findings, at the crossroad of the Vicsek physics and the Active Brownian Particles physics, shed light on the so-far-unexplored physics arising from the interplay between the polarity and the velocity.
Highlights
Active matter systems consume and dissipate energy at the level of their local units to generate systematic motion based on their internal degrees of freedom [1,2,3,4]
The results show that particles within 6 μm range had a narrower distribution with a peak near θ = 0, which suggests the existence of polar interaction within the Janus particles
The interaction mechanism of Janus particles under an AC electric field is still under discussion [9,38], our previous study experimentally validated the existence of polar interaction induced by the effective electrostatic dipole-dipole interaction between two particles both in the induced charge electrophoresis (ICEP) and the sDEP regimes [9]
Summary
Active matter systems consume and dissipate energy at the level of their local units to generate systematic motion based on their internal degrees of freedom (e.g., polarities) [1,2,3,4]. The phenomenological hydrodynamic equations by Toner, Tu, Ramawsamy, and their coworkers have predicted the existence of true long-range order and giant number fluctuations (GNF) for the orientationally ordered state [16,17,18,23,25] In these theoretical and numerical studies, each particle’s polarity is assumed to be parallel to its velocity. Through single particle measurements, we show that the particles counterintuitively exhibit larger diffusion in the ordered state than that in the disordered state for short time scales We suggest that this anomalous enhanced diffusion stems from the coupling between particle polarity and velocity through computations of the cross-correlation function, which is a dismissed feature in Vicsek-style models. Through the observation of both velocities and polarities, unravels hitherto unexplored statistical properties within orientationally ordered active colloidal systems
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