Ordered patterns of liquid crystal toroidal defects by microchannel confinement

Abstract

In this article we present experimental results demonstrating an approach to controlling the size and spatial patterning of defect domains in a smectic liquid crystal (LC) by geometric confinement in surface-modified microchannels. By confining the LC 4-octyl-4-cyanobiphenyl in m-sized rectangular channels with controlled surface polarity, we were able to generate defect domains that are not only nearly uniform in size but also arranged in quasi-2D ordered patterns. Atomic force microscopy measurements revealed that the defects have a toroidal topology, which we argue is dictated by the boundary conditions imposed by the walls of the microchannel. We show that the defects can be considered to be colloidal objects, which interact with each other to form ordered patterns. This method opens the possibility for exploiting the unique optical and rheological properties associated with LC defects to making new materials. For example, the control of the shape, size, and spatial arrangement of the defects at the mesoscale suggests applications in patterning, templating, and when extended to lyotropic LCs, a process leading to uniform-sized spherical particles for chemical encapsulation and delivery. S mectic (Sm) liquid crystals (LCs) are composed of elongated molecules that are aligned and arranged in layers. Local disruptions in the orientational, positional (layering), or morphological (bending) order in a Sm LC result in defects, which, when observed under crossed polarizers, exhibit complex textural patterns. The texture, which results from the highly anisotropic polarization of the molecules, has been extensively characterized and categorized based on the type of the LC structure (1-5). Understanding and controlling defects in LCs are important to many technological applications (e.g., displays and optical switches). Because the defects are considered to be anomalies in the LC structure, general efforts have been directed at minimizing or annealing the defects, including by using microstructures (grooves and gratings) (6-8). However, defect domains possess unique rheological and optical properties, which could potentially be exploited to make novel materials. The prerequisite for doing so would be the control of the type, size, and spatial distribution of the defects, which had not been well addressed in previous studies.In this article we present experimental results demonstrating the control of the size and spatial distribution of toroidal defects in the Sm LC 4Ј-octyl-4-cyanobiphenyl (8CB) by using surfacemodified microchannels. By changing the depth and width of the m-scale channels with a controlled surface polarity, we were able to generate focal conic defects that are not only nearly uniform in size but also arranged in 2D ordered patterns. This process is analogous to colloidal crystallization, except here the ''colloids'' are the ''soft'' m-sized defects within the LC. In addition, whereas the ordering in colloids is caused by interparticle interactions, here the defect ordering results from the competitio...

Cooperative liquid-crystal alignment generated by overlaid topography

“…Uncovered LC-filled channels exhibit periodic organization of smectic defects [15]. There have also been demonstrations of alignment in cells in which both plates are topographically patterned, showing uniform planar alignment and stabilization of the twisted nematic geometry [16].…”

Organization of the polarization splay modulated smectic liquid crystal phase by topographic confinement

“…As for many smectics appearing upon cooling directly from the isotropic phase via a first order transition, i.e., having no intervening nematic phase, the B7s are not easily aligned into macroscopically oriented domains of any sort with the anisotropic surface treatment methods such as optical-or mechanical-rubbed polymer surfaces or obliquely deposited silicon monoxide (SiOx) (7)(8)(9)(10)(11) that are typically useful for aligning LCs. Topographic patterning has been demonstrated to align LCs (12)(13)(14) and has recently emerged as an effective alignment technique for both nematic and smectic LCs, and in particular for layered LC phases growing directly from the isotropic (15)(16)(17)(18).…”

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

“…In the more recent past, microfluidics was starting to be used as a tool to generate liquid crystal droplets [30][31][32], to study their wonderful properties [33][34][35][36], and to investigate confinement and motion of topological defects [37][38][39][40], and potential applications [41]. However, the possibility to use the available microfluidic techniques for studying the fundamental behaviour of liquid crystal flows within minute confinements was never explored.…”

“…Thus we can write the conservation equations involving the mass, linear momentum, and angular momentum terms: 38) where ρ is the density of the NLC, F is the external force (body force) per unit mass. The above equations assume homogeneous incompressibility of the nematic (∇ρ = 0) and negligible inertial effects.…”

“…Explorations beyond this Poiseuilledetermined regime could open new microfluidic phenomena and applications [218,219]. Although experimental investigations of complex fluids in microfluidic confinements have been initiated [31,38,39,220,221], understanding of their dynamics is mostly limited to numerical studies [222][223][224][225][226][227][228]. In particular, investigations on liquid crystals as a natural choice for an anisotropic replacement of the isotropic fluids have been directed towards effects mediated by topological defects [24,60,114,121,192,[229][230][231].…”