Liquid Crystal Microswimmers - from single entities to collective dynamics

Recently, artificial self-propelled systems at low Reynolds numbers have gained a lot of attention. The main interest lies in mimicking well-studied systems, e.g. the active Brownian motion of bacteria, or in gaining deeper insight into nature's schemes like search of food or light. Moreover, fundamental and hitherto unanswered questions like the Plankton paradox prevail. Due to the complexity of living (eco-)systems, computational simulations or experiments in nature can often only tackle parts of the underlying mechanisms. In all these aspects, reproducible artificial microswimmers in well controlled laboratory conditions offer a possibility to gain insights into these open questions. Hitherto, preceding studies on artificial systems mainly focused on Janus particles. These introduce an intrinsic symmetry breaking onto the system, and often propel due to external addition of energy. The presented thesis discusses a well controllable and adaptable artificial system, consisting of nematic liquid crystal (LC) droplets in ionic surfactant solutions above the critical micelle concentration. In a spontaneously induced process, the droplets undergo micellar solubilization. If the surfactant concentration surpasses a threshold, this is accompanied by self-sustained propulsion, with the incorporation of the LC molecules in the surfactant micelles as the energy source. The active emulsion system generates a symmetry breaking on its own due to an inhomogeneity in surfactant coverage along the droplet interface. This generates Marangoni flows in an otherwise fully symmetric system. The main advantages of our system are the well-studied microfluidic droplet preparation, active swimming periods up to hours and the tuneability of the buoyancy. The droplet system is therefore a prime candidate for studying the behaviour in various nature-like environments. This thesis provides systematic studies covering a substantial range of the parameter space of interest in terms of dimensionality, driving force and number density of entities. Through the investigation of the solubilization behaviour, the propulsion of the droplets is linked to the molecular pathway of solubilization. In this respect, the (auto)chemotactic behaviour of the droplets is discussed by investigating maze solving and avoidance dynamics. The analysis of one-dimensional propulsion in capillaries is followed by the investigation of the hydrodynamic flow profile and wall interactions in two dimensions. One main aspect is the influence of the nematic nature of the droplets on the characteristic motion. It is found that both, curling in 2D and helical swimming in 3D, result from a second symmetry breaking in the system. The latter arises from the interaction of the nematic matrix with internal flows. Yet, this behaviour vanishes for temperatures exceeding the nematic-to-isotropic transition, giving rise to ballistic motion. Eventually, dense suspensions of droplets are examined in three dimensions, showing convection-roll stabilized large scale clustering when buoyancy is present, while in freely floating suspensions with buoyancy set to zero, autochemotactic caging dynamics occur.