Collective dynamics and control of collective polar states of electro-hydrodynamically bonded Quincke pairs

Active colloidal fluids composed of micro-rollers powered by the Quincke effect exhibit remarkable collective dynamics, including directed motion, spontaneous demixing, and formation of vortices. Here, we report the intricate role of electrostatic and hydrodynamic interactions in the self-assembly dynamics of a binary mixture of rollers. We find that the rollers form robust heterogeneous pairs depending on the ratio of particle size at certain range of the excitation parameters. Such Quincke pairs move on curved trajectories with the curvature controlled by the energizing electric field. At high enough number densities, the Quincke pairs form collective states ranging from a polar state (global vortex) to multi-vortical states comprised of free-standing long-lived vortices with quasi-antiferromagnetic arrangements of the vorticities. We further demonstrate that an activity temporal modulation (cessation-restoration cycle) results in a swift reversal of the global polar state upon reenergizing the system. The process of the polar state reversal does not proceed through a chaotic flocking regime and does not rely on structural dynamic memory of a roller ensemble reported for polar states comprised of spherical rollers [1]. Our findings provide insights into collective behavior of active heterogeneous ensembles with collective motion orchestrated by a fine interplay between electrostatic and hydrodynamic interactions.



The research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.



[1] B Zhang, H Yuan, A Sokolov, MO de la Cruz, A Snezhko, Nature Physics 18 (2), 154-159 (2022)