A Model Collective System Made of Spinning Micro-Disks—From Fundamentals to Microrobot Swarms

Self-organization in colloidal systems has led to various fundamental studies on non-equilibrium systems and development of microrobot swarms. However, typical colloidal systems are limited in their behaviors and each system can only facilitate a few specialized studies. Therefore, developing a single model system that can controllably transition among diverse behaviors, would be very useful for various fundamental studies and for the development of microrobot swarms. We develop a collective system that utilizes the mutual interactions among its constituents, and its behaviors can be controlled via global stimuli. Our system consists magnetic micro-disks that interact via various physical interactions (magnetic dipole-dipole, hydrodynamic and capillary interactions) and the relative dominance of these interactions can be tuned by an external oscillating magnetic field. In this talk, we present various studies enabled by our system. First, we demonstrate that our system can form rotating collectives with varying degree of orientation order that can be controlled by a single control parameter: the rotation speed of the external magnetic field. These behaviors enable a fundamental study on development of a general-purpose order-parameter based on the information content of the self-assembled patterns. Next, we show that the mutual interactions among the micro-disks can be tuned to form various collective modes ranging from isotropic (rotating and static collectives) to anisotropic (chains) behaviors, and a gas-like behavior consisting self-propelling pairs. The mutual interactions are tuned by changing the profile of a 2D oscillating magnetic field, demonstrating that the external driving field and the mutual interactions can conjunctively generate transitions among behaviors and enable various controllable robotic functions. Last, we study the effect of heterogeneity in our system and demonstrate that our system can transition on-demand from a driven (following an external field) to an active (self-propelled) behavior or a mixture of both. Through these studies, we hope to convey that our system can act as a versatile model system facilitating fundamental studies that link driven and active behaviors, and also provide a step towards developing versatile and controllable microrobot collectives.