Artificial microswimmers formed by liquid crystal droplets

点击翻译按钮获取中文摘要

The aim of our experimental project is the study of self-propelled liquid crystal droplets in an aqueous surfactant solution and their biomimetic capabilities. Swimming microorganisms like plankton and single-cell organisms are of high interest with regard to their propulsion mechanisms, as in chemotaxis, autochemotactic signalling and helical swimming, as observed, e. g., for sperm, interactions with interfaces and confined geometries like soil, and collective processes like bioconvection, especially with respect to oceanic plankton dynamics. During the first funding period, we have shown that even in a very simple three component droplet system, many of these characteristics can be observed and tuned. We are able to mass produce monodisperse droplets with reproducible speed and long running times, which show switchable helical swimming due to their nematic anisotropy, follow chemotactic and autochemotactic gradients of surfactant micelles, interact with straight and curved surfaces, can be tuned from sedimented to free buoyant swimming by admixture of heavy water and which feature to confinement and gravity dependent convective flows in large collective ensembles. We observed these droplets in microfluidic cells under light microscopy to reproduce complex and confined geometries and developed a fluorescence light sheet microscope for the observation of buoyant droplets and large convective ensembles under real three dimensional conditions. We were able to image flow fields inside and around the droplets via particle image velocimetry and gained insight into the nematic structure via polarised microscopy. During the second funding period, in collaboration with theory collaborators from the priority programme, we aim to address open questions, further applications and variations of the basic system. We want to understand quantitatively how the swimming behaviour depends on the internal structure of the droplets, which can be affected by admixing other nematogens or chiral dopants, inclusion of water droplets to create nematic shells, and external magnetic fields. The morphology of the helical trajectories strongly depends on negative autochemotaxis, which inhibits self-crossing, an effect we aim to model and quantify. The wall interaction of the swimmers, depending on hydrodynamic and phoretic fields generated by the moving droplet, will be quantified at the transition from quasi two dimensional to three dimensional geometries and under variation of wall curvature. The most important aim is a thorough understanding of the collective behaviour resulting in the formation of convective patterns and clusters. Using our light sheet microscope, we will study the dynamics of large swimmer ensembles under varying effective gravity and swimmer velocity, imaging both individual droplet dynamics and the flow fields in the bulk phase.