Dresden 2026 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 10: Droplets, Wetting, and Microfluidics (joint session DY/CPP)

CPP 10.9: Talk

Monday, March 9, 2026, 17:15–17:30, ZEU/0118

Effect of Flow Coupling on Defect Binding and Unbinding in Nematic Fluids — •Jayeeta Chattopadhyay, Simon Guldager Andersen, Kristian Thijssen, and Amin Doostmohammadi — Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, Copenhagen, Denmark.

Topological defects play a central role in the ordering and dynamics of nematic fluids. We investigate how coupling to fluid flow modifies defect-mediated phase transitions in two-dimensional nematics using fluctuating nematohydrodynamic simulations. The system is driven by tuning the fluctuation strength, with increasing and decreasing fluctuations defining forward and backward protocols. Without flow coupling, the system undergoes a Berezinskii–Kosterlitz–Thouless (BKT)–like transition via the reversible binding and unbinding of ± 1/2 defect pairs. When hydrodynamics is included, the transition depends on the flow–alignment parameter: non-aligning nematics (λ = 0) retain BKT-like behavior, whereas strain-rate–aligning nematics (λ ≠ 0) form bend–splay walls, lowering the defect-creation threshold and preventing recombination, leaving defects unbound across all fluctuation strengths. In active nematics, defects remain unbound for all λ, showing that self-generated flows also inhibit bound-pair formation. These results demonstrate that coupling to fluid flow fundamentally alters topological phase behavior, suppressing the equilibrium BKT binding mechanism.

Keywords: Topological defects; Nematic liquid crystals; Flow alignment; Phase transitions; Active matter