Abstract
Spherical confinement of nematic liquid crystals leads to the formation of equilibrium director field configurations that include point and line defects. Driving these materials with flows or dynamic fields often results in the formation of alternative metastable states. In this article, we study the effect of magnetic field alignment, both under static and dynamic conditions, of nematic gems (nematic droplets in coexistence with the isotropic phase) and emulsified nematic droplets of a lyotropic chromonic liquid crystal. We use a custom polarizing optical microscopy assembly that incorporates a permanent magnet whose strength and orientation can be dynamically changed. By comparing simulated optical patterns with microscopy images, we measure an equilibrium twisted bipolar pattern within nematic gems that is only marginally different from the one reported for emulsified droplets. Both systems evolve to concentric configurations upon application of a static magnetic field, but behave very differently when the field is rotated. While the concentric texture within the emulsified droplets is preserved and only displays asynchronous oscillations for high rotating speeds, the nematic gems transform into a metastable untwisted bipolar configuration that is memorized by the system when the field is removed. Our results demonstrate the importance of boundary conditions in determining the dynamic behavior of confined liquid crystals even for configurations that share similar equilibrium bulk structures.
Highlights
Liquid crystals are anisotropic liquids that feature long-range orientational order, locally characterized by a director field, whose distortion from uniform alignment incurs in an elastic free-energy cost [1,2]
We have reported experiments in which the dynamic behavior of either nematic gems or emulsified nematic droplets of Sunset Yellow (SSY) is studied under rotating magnetic fields
We have found that the equilibrium director field is only marginally different when comparing the two scenarios in the field-free twisted-bipolar texture, and apparently identical in the concentric director configuration when aligned by the external magnetic field
Summary
Liquid crystals are anisotropic liquids that feature long-range orientational order, locally characterized by a director field, whose distortion from uniform alignment incurs in an elastic free-energy cost [1,2] Spatial confinement of these materials often triggers the formation of patterns and complex defect configurations of the director, as a result of the interplay between the drive towards free-energy minimization and the topology imposed by boundary conditions on the enclosing container walls [3]. Within droplets, is the simplest topology where fixed boundary conditions on the droplet surface are incompatible with a uniform director field, leading to the formation of point and line defects [7] This confinement can be realized at the coexistence between the isotropic and nematic phases of a mesogen, where nematic droplets with internal orientational order are suspended within the isotropic phase. Materials based on dispersed liquid crystal droplets have found applications as functional materials [11,12] or biosensors [13,14]
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