Velocity statistics of dynamic spinners in out-of-equilibrium magnetic suspensions

Abstract

We report on the velocity statistics of an out-of-equilibrium magnetic suspension in a spinner phase confined at a liquid interface. The suspension is energized by a uniaxial alternating magnetic field applied parallel to the interface. In a certain range of the magnetic field parameters the system spontaneously undergoes a transition into a dynamic spinner phase (ensemble of hydrodynamically coupled magnetic micro-rotors) comprised of two subsystems: self-assembled spinning chains and a gas of rotating single particles. Both subsystems coexist in a dynamic equilibrium via continuous exchange of the particles. Spinners excite surface flows that significantly increase particle velocity correlations in the system. For both subsystems the velocity distributions are strongly non-Maxwellian with nearly exponential high-energy tails, P(v) ∼ exp(-|v/v0|). The kurtosis, the measure of the deviation from the Gaussian statistics, is influenced by the frequency of the external magnetic field. We show that in the single-particle gas the dissipation is mostly collisional, whereas the viscous damping dominates over collisional dissipation for the self-assembled spinners. The dissipation increases with the frequency of the applied magnetic field. Our results provide insights into non-trivial dissipation mechanisms determining self-assembly processes in out-of-equilibrium magnetic suspensions.

Active turbulence in a gas of self-assembled spinners

“…While the spinners are not self-propelling entities (activity comes from rotation only), they get advected by the flows generated by the neighboring spinners. The motion of the spinners induces a large-scale vortical flow field ( 40 ). The spinners are the dominant active component in our system that induces a diffusive motion of the inert particles.…”

Complex collective dynamics of active torque-driven colloids at interfaces

“…Since for the case of uniaxial magnetic field the clock/counterclockwise senses of rotation are equally probable, the initial direction is selected by a variety of factors, like interactions with neighboring particles and flows. The rotating chains exert viscous torques on the liquid which trigger strong long-range [64] hydrodynamic vortical flows at the interface. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65…”

Dynamics of inert spheres in active suspensions of micro-rotors

“…Hence, hydrodynamic interactions may have a significant impact on the collective motion of other types of active particles. Active systems consisting of particles rotating due to a magnetic field display macroscopic generated fluid flows [26]. Previous theoretical studies [35] highlighted the critical role played by the hydrodynamic coupling of rotors in the phase behavior of the system.…”

“…Unlike self-propelled particles such as bacteria and colloids, spinning particles ("rotors") and their collective dynamics are far less explored [23] mostly because there are fewer experimental realizations, e.g., magneticallydriven colloids [24][25][26] and electrically-driven "Quincke" colloids [27,28]. Self-organization in rotor suspensions have been analyzed mostly computationally [29][30][31][32][33], though not all studies consider the effects of the immersing liquid on the rotor motions.…”