Effect of external fields on soft matter systems: liquid crystals in elliptic confinement

Abstract

Soft matter has large response functions to external fields such as shear forces, electric/magnetic fields, and confinements. The studies of these response functions have contributed to the fundamental understanding of soft matter and the development of various applications such as electro-optic displays and sensors. In this point of view, we discuss the following problems.

1) We report the first study on the confinement-induced transition of the topological defects of liquid crystals (LCs) using smectic-A LCs confined in prolate spheroids with homeotropic anchoring. Upon increasing the aspect ratio of a LC droplet, dispersed in a stretched elastomer film, the topological defect undergoes a transition from a point to a line of which the length is a function of the aspect ratio. Additionally, when the size of a droplet is larger than a certain value, the defect has a pearl-necklace-like texture. We propose a simple model to understand the formation of these defects in terms of the misorientation and undulation instability of the smectic layers.

2) We present the first observation of liquid crystals confined in elliptic cylinders. To fabricate the elliptic cylinder, poly(dimethlysiloxane) (PDMS) micro-channels of circular cross sections are stretched uniaxially along the direction of the diameter. Upon increasing the aspect ratio of the elliptic cross section, confined nematic liquid crystals maintain the escaped-radial configuration with homeotropic anchoring. In smectic-A liquid crystals, defect regions of focal conic domains appear as a function of the aspect ratio. We propose a model to understand the formation of these defects in terms of the confinement-induced misorientation, and thereby the tilt grain boundaries.

3) We introduce a new one-step method to prepare magnetic Janus particles and observe their supra-structures under external magnetic field. We utilize the phase separation of polymers inside microfluidics-generated emulsion droplets dur...