Cross-talk between topological defects in different fields revealed by nematic microfluidics

Topological defects are singularities in material fields that play a vital role across a range of systems: from cosmic microwave background polarization to superconductors, and biological materials. Although topological defects and their mutual interactions have been extensively studied, little is known about the interplay between defects in different fields -- especially when they co-evolve -- within the same physical system. Here, using nematic microfluidics, we study the cross-talk of topological defects in two different material fields -- the velocity field and the molecular orientational field. Specifically, we generate hydrodynamic stagnation points of different topological charges at the center of star-shaped microfluidic junctions, which then interact with emergent topological defects in the orientational field of the nematic director. We combine experiments, and analytical and numerical calculations to demonstrate that a hydrodynamic singularity of given topological charge can nucleate a nematic defect of equal topological charge, and corroborate this by creating $-1$, $-2$ and $-3$ topological defects in $4-$, $6-$, and $8-$arm junctions. Our work is an attempt toward understanding materials that are governed by distinctly multi-field topology, where disparate topology-carrying fields are coupled, and concertedly determine the material properties and response.