Active Matter under static and dynamic confinement

Abstract

This thesis presents an experimental investigation of active matter under confinement. The experiments build on an established platform that allows individual control over the motion of suspended microscopic particles by using an optical feedback control setup. Here, we extend this setup to generate arbitrary-shaped light-induced effective potential landscapes that constrain the motion of these self-propelled microparticles. This provides a versatile model system for exploring how geometric boundaries and confinement affect the collective dynamics and emergent behavior in active matter. We detail the implementation of our technique and characterize the motion of the self-propelled microparticles and their interaction with the repulsive boundaries through benchmark measurements. Furthermore, we employ our experimental model to investigate active matter under both static and dynamic confinement in three distinct situations. First, we confine active particles in narrow, soft channels and examine how this influences motility induced cluster formation. We find a novel reentrant behavior regarding cluster formation as a function of particle velocity and strength of confinement. Second, we study the flow of active particles through bottleneck constrictions and examine the mechanisms leading to clogging. We find that clogging is due to arch-formation as known from granular matter up to a threshold value of interparticle attraction, above which clogging is governed by cohesive droplets. Third, we perform indirect measurements of the force on a beating, flagellum-shaped object moving through a suspension of active matter. We demonstrate that slow object motion promotes the accumulation of active particles at its rear, leading to a notable reduction in drag force. A similar phenomenon is known as active thinning. These studies are corroborated by coarse analytical models and computer simulations aligning overall well with the experimental results, indicating that the observed phenomena are rather generic features occurring in various realizations of active matter systems. All in all, our results sustain the rapidly evolving research field of active matter in complex environments and highlight the importance of considering boundary effects when designing microfluidic devices or micromachines that are intended to operate with or within active matter. BEINHALTET EINE DEUTSCHSPRACHIGE ZUSAMMENFASSUNG