Topological microfluidics for flexible micro-cargo concepts

State-of-the-art microfluidic techniques rely usually on an isotropic carrier fluid, the flow of which is modulated using morphological patterns on the microchannels, or application of external fields. In the present work, we demonstrate that replacing the isotropic fluid by an anisotropic liquid crystal introduces a flexible but versatile approach to guided transport of microscopic cargo in microfluidic devices. We show that topological line defects can be threaded at will through the microfluidic channels and used as a 'soft rail' whose position is controlled through easily accessible experimental parameters. Colloid particles and small water droplets, the 'working horses' of microfluidics, are trapped and consequently guided by the defect line through the microfluidic device. Furthermore, we demonstrate controlled threading of the defect line at a channel bifurcation. Topological microfluidics introduces a unique platform for targeted delivery of single particles, droplets, or clusters of such entities, paving the way to flexible micro-cargo concepts in microfluidic settings.