Medium Electrical Conductivity Governs the Collective Behaviors of Quincke Rollers

Active matter systems can convert environmental energy into continuous motion and exhibit collective behaviors. As a typical electrically driven active particle system, Quincke rollers can display a variety of collective behaviors under a uniform DC electric field. However, a systematic understanding of the emergence and evolution of different collective behaviors is still lacking, and it remains unclear how external environmental conditions, such as temperature and humidity, influence the dynamics of collective behaviors. In this work, we find that increasing temperature and humidity drives a continuous transition of the collective behavior of Quincke rollers from vortices to polar clusters, rotating clusters, and finally active crystals. In situ electrical measurements show that the medium conductivity increases by nearly 10-fold as temperature increases from 20 ℃ to 60 ℃, and around 30-fold as relative humidity increases from 5% to 65%, whereas the permittivity changes by around 10–15%. Finite-element calculations further reveal that the increase in medium electrical conductivity strongly modifies the magnitudes of the electric dipolar and hydrodynamic interactions between rollers. These forces shift from pure repulsion to anisotropic attraction and then to isotropic attraction, resulting in the emergence of vortex, polar clusters, rotating clusters and active crystals in turn. These results provide a simple physical picture linking the conductivity of the medium to the collective states in Quincke roller systems, and establish a theoretical framework that incorporates dipolar interaction, hydrodynamic interactions and electro-hydrodynamic interactions for understanding the dynamics of collective behaviors. In addition, this work demonstrates that temperature and humidity can serve as simple, efficient, and reversible control parameters for the in situ regulation of the collective behaviors of Quincke rollers.