Title: Boojums, Hedgehogs, Rings and Walls – Defects in Confined Nematics
When: Tuesday, December 02, 2025, 12:00
Place: Department of Theoretical Condensed Matter Physics, Faculty of Sciences, Module 5, Seminar Room (5th Floor)
Speaker: Alberto Fernandez-Nieves / Department of Condensed Matter Physics. U Barcelona. Barcelona, Spain ICREA – Catalan Institution for Research and Advanced Studies (ICREA). Barcelona, Spain Institute for Complex Systems, U Barcelona, Barcelona, Spain
Nematics have orientational but not positional order, and often consist of elongated particles that align, on average, along a common axis or director. When confined within curved surfaces, the geometry and topology of the confining surface, together with the boundary conditions for the director field, all affect and often dictate how the material organizes. In this talk I will focus on the 3D confinement of nematics. In particular, I will discuss how we generate non-trivial confining spaces and novel methods to quantify the order of the nematic inside them, paying special attention to the topological defects that we often observe. Inside tori with tangential anchoring, structures with broken reflection symmetry, as well as with pairs of boojums, are both observed. Applying magnetic fields results in the formation of inversion (Helfrich) walls both of the splay-bend and twist types. When confined inside surfaces with constant mean curvature, and subjected to perpendicular anchoring, we find states with either hedgehogs or rings. In many cases, the structures observed correspond to long-lived metastable states that are stabilized by the presence of large barriers between elastic free energy minima.
