Dynamic tuning of the director field in liquid crystal shells using block copolymers

JungHyun Noh,Jan P. F. Lagerwall,Venkata Subba Rao Jampani,Hsin-Ling Liang,Apala Majumdar,Yiwei Wang

doi:10.1103/physrevresearch.2.033160

Abstract

When an orientationally ordered system, like a nematic liquid crystal (LC), is confined on a self-closing spherical shell, topological constraints arise with intriguing consequences that depend critically on how the LC is aligned in the shell. We demonstrate reversible dynamic tuning of the alignment, and thereby the topology, of nematic LC shells stabilized by the nonionic amphiphilic block copolymer Pluronic F127. Deep in the nematic phase, the director (the average molecule orientation) is tangential to the interface, but upon approaching the temperature TNI of the nematic– isotropic transition, the director realigns to normal. We link this to a delicate interplay between an interfacial tension that is nearly independent of director orientation, and the configurationdependent elastic deformation energy of an LC confined in a shell. The process is primarily triggered by the heating-induced reduction of the nematic order parameter, hence realignment temperatures differ by several tens of degrees between LCs with high and low TNI , respectively. The temperature of realignment is always lower on the positive-curved shell outside than at the negative-curved inside, yielding a complex topological reconfiguration on heating. Complementing experimental investigations with mathematical modeling and computer simulations, we identify and investigate three different trajectories, distinguished by their configurations of topological defects in the initial tangential-aligned shell. Our results uncover a new aspect of the complex response of LCs to curved confinement, demonstrating that the order of the LC itself can influence the alignment and thereby the topology of the system. They also reveal the potential of amphiphilic block copolymer stabilizers for enabling continuous tunability of LC shell configuration, opening doors for in-depth studies of topological dynamics as well as novel applications in, e.g., sensing and programmed soft actuators.

Highlights

Double emulsions of water in liquid crystal (LC) in water, known as LC shells, have over the last decade acquired a status as a prolific experimental platform for studying confinement effects in soft matter physics [1,2,3], in particular concerning topological defects and their interactions on curved spaces
We demonstrate reversible dynamic tuning of the alignment, and thereby the topology, of nematic LC shells stabilized by the nonionic amphiphilic block copolymer Pluronic F127
We have undertaken a comprehensive study of nematic shells stabilized by the amphiphilic block copolymer F127, using two different LC materials: 5CB and E7

Summary

Introduction

Double emulsions of water in liquid crystal (LC) in water, known as LC shells, have over the last decade acquired a status as a prolific experimental platform for studying confinement effects in soft matter physics [1,2,3], in particular concerning topological defects and their interactions on curved spaces. Experimental and theoretical, was restricted to nematic shells (orientational order only; molecules aligning along the director n) [4,5,6,7,8,9,10,11], later efforts have focused on smectic shells of SmA- [12,13,14,15,16,17,18,19] and SmC-type [17] as well as cholesteric [20,21,22,23,24,25,26] shells. For research as well as applications, it is imperative to control the director field configuration within the shell, a requirement typically fulfilled by choosing interface stabilizers that promote the desired orientation of n at each boundary [41,42]. Our understanding of how stabilizers affect the director field configuration in LC shells is far from complete

Results

Discussion

Conclusion