Structure Formation and Collective Behavior of Dipolar Active Particles

Abstract

Dipolar active particles describe a class of self-propelled, biological or artificial particles. These particles are equipped with an internal, typically magnetic, dipole moment. The combination of dipolar particle-particle interactions and activity leads to emerging complex collective behavior in systems of such particles.In this thesis, we use Brownian dynamics simulations to explore and characterize the plethora of structural dynamics and pattern formation in systems of dipolar active particles. This study can be divided into four parts. First, we focus on structure formation in small systems. Here, we mainly observe chain and ring formation and characterize how activity and spatial confinement affects these structures. Second, we move to large systems of dipolar active particles, classify the collective patterns we observed and summarize our results in diagrams of states. We show that activity plays a crucial role in the emergent collective behavior in systems of dipolar active particles. Third, we investigate the effect a constant homogeneous external magnetic field has on collective dynamics of the system. We observe that band formation is suppressed by strong external magnetic fields and columnar structures form. In addition, we notice a previously not characterized transient state of oscillating chains of active dipolar particles. We hypothesize that these oscillations are caused by buckling instabilities. Finally, we introduce a time dependent external magnetic field and study the dynamics of structure formation driven by that field.This thesis demonstrates how dipolar interactions, external magnetic fields, or confinement, can further add to the rich complex collective behavior in systems of active particles.