Topological defect transformation and structural transition of two-dimensional colloidal crystals across the nematic to smectic-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>A</mml:mi></mml:math>phase transition

Abstract

We observe that topological defects in nematic colloids are strongly influenced by the elasticity and onset of smectic layering across the nematic $(N)$ to smectic-$A (\mathrm{Sm}A)$ phase transition. When approaching the $\mathrm{Sm}A$ phase from above, the nematic hyperbolic hedgehog defect that accompanies a spherical colloidal inclusion is transformed into a focal conic line in the $\mathrm{Sm}A$ phase. This phase transformation has a strong influence on the pairwise colloidal interaction and is responsible for a structural transition of two-dimensional colloidal crystals. The pretransitional behavior of the point defect is supported by Landau--de Gennes $Q$-tensor modeling accounting for the increasing elastic anisotropy.