Anupam Sengupta

Liquid crystal microfluidics: surface, elastic and viscous interactions at microscales

The hydrodynamic properties of nematic liquid crystals are characterized by a complex mutual coupling between flow, viscosity, and nematic order. While the flow behaviour of nematic bulk samples is well known, corresponding studies in microfluidic settings are still at an early stage. The presence of the four confining channel walls – and in particular the nature of the surface anchoring of the nematic order on the walls – adds new phenomena to the already rich and multifaceted flow behaviour. We present an overview of recent studies focusing on the microfluidics of nematic liquid crystals. Particular topics are the functionalization of the channel walls for defined surface anchoring conditions and the resulting structures of the nematic director field, the controlling and tuning of the flow velocity profile and director field configuration and resulting opto-fluidic applications, and the behaviour of topological defects in the flowing nematic and their application for a guided colloidal transport.

Topological microfluidics for flexible micro-cargo concepts

State-of-the-art microfluidic techniques rely usually on an isotropic carrier fluid, the flow of which is modulated using morphological patterns on the microchannels, or application of external fields. In the present work, we demonstrate that replacing the isotropic fluid by an anisotropic liquid crystal introduces a flexible but versatile approach to guided transport of microscopic cargo in microfluidic devices. We show that topological line defects can be threaded at will through the microfluidic channels and used as a ‘soft rail’ whose position is controlled through easily accessible experimental parameters. Colloid particles and small water droplets, the ‘working horses’ of microfluidics, are trapped and consequently guided by the defect line through the microfluidic device. Furthermore, we demonstrate controlled threading of the defect line at a channel bifurcation. Topological microfluidics introduces a unique platform for targeted delivery of single particles, droplets, or clusters of such entities, paving the way to flexible micro-cargo concepts in microfluidic settings.

Nematic textures in microfluidic environment

We study the flow of a nematic liquid crystal through microchannels possessing degenerate planar anchoring conditions on the channel walls. Depending on the channel dimensions and the flow rate, the formation of different textures and topological defect structures is observed and studied using polarizing optical microscopy and fluorescence confocal polarizing microscopy. The observed structures comprise π-walls, disclination lines pinned to the channel walls, disclination lines with one pinned and one freely suspended end, and disclination loops freely flowing in a chaotic manner. We focus on the creation, evolution and morphology of different kinds of π-walls and pinned disclinations. Such defects and textures in the nematic fluid can potentially be applied for guiding the transport of individual particles and larger colloidal assemblies inside an appropriate microfluidic device.

Flow of a nematogen past a cylindrical micro-pillar

We study the flow of a nematic liquid crystal past a micron-sized cylindrical pillar within a microfluidic confinement of a rectangular cross-section. The liquid crystal molecules are anchored perpendicularly (homeotropic anchoring) to the surface of the pillar and the channel walls. Flow past the cylindrical obstacle generated topological defect structures whose nature, dimensions and morphology varied with the flow velocity and channel dimensions. On increasing the flow speed, we observed sequential evolution of a semi-integer loop, which transformed into an integer hedgehog defect, and finally equilibrated to an extended defect wall. On stopping the flow, the topological defect states reversed its sequence of appearance. Additionally, we introduce dual-focus fluorescence correlation spectroscopy as a general velocimetry technique for microfluidics of liquid crystal systems – with or without topological defect structures.

Opto-fluidic velocimetry using liquid crystal microfluidics

The coupling between flow and orientation of nematic liquid crystal molecules has been utilized to devise a non-intrusive opto-fluidic velocimetry technique on a microfluidic platform. The flow-induced reorientation of the liquid crystal molecules in a diverging channel possessing homeotropic surface anchoring produced distinct birefringent domains, directly observable through their interference colors, which are characteristic to the local flow velocity. The flow-induced effective birefringence was characterized using polarizing optical microscopy, confocal fluorescence polarizing microscopy, and particle tracking methods.

Topological constraints in a microfluidic platform

Microfluidics has evolved as a major technological platform for biotechnology, material science and related fields. In virtually all of the areas of application, the flowing matrix is an isotropic fluid. However, replacing the typically isotropic fluid with an anisotropic liquid crystal opens up avenues beyond the viscous-dominated isotropic microfluidics. Especially, the material anisotropy of the flowing LC matrix and the consequent incorporation of topological constraints within the microfluidic device offer smart capabilities ranging from tunable flow-shaping to flexible micro-cargo concepts. The key to such capabilities lies in exploiting the possible topological constraints offered by the microfluidic confinement. As an example, we shall demonstrate how long-range ordering and consequent anisotropy in liquid crystals (LCs) could be utilised to devise a novel route to guided transport of microscopic cargo on ‘soft rails’, i.e. topological defect lines (disclinations). We create, position and navigate disclination lines within the LC matrix by tuning the coupling between flow and LC orientation. As model cargo elements, we have used isolated or self-assembled chains of colloidal particles, and demonstrated the broader capability of this method by transporting aqueous droplets on the defect lines. Topological constraints in combination with flow-director coupling thus endow LC microfluidics with features distinct from its isotropic counterparts.

Cross-talk between topological defects in different fields revealed by nematic microfluidics

Significance Topological defects play a defining role in systems as extensive as the universe and as minuscule as a microbial colony. Despite significant advances in our understanding of topological defects and their mutual interactions, little is known about the formation and dynamics of defects across different material fields embedded within the same system. Here, using nematic microfluidics as a test bed, we report how topological defects in the flow and the orientational fields emerge and cross-talk with each other. Although discussed in a nematofluidic context, such multifield topological interactions have potential ramifications in a range of systems spanning vastly different length and time scales: from material designing, to exploration of open questions in cosmology and living matter. Topological defects are singularities in material fields that play a vital role across a range of systems: from cosmic microwave background polarization to superconductors and biological materials. Although topological defects and their mutual interactions have been extensively studied, little is known about the interplay between defects in different fields—especially when they coevolve—within the same physical system. Here, using nematic microfluidics, we study the cross-talk of topological defects in two different material fields—the velocity field and the molecular orientational field. Specifically, we generate hydrodynamic stagnation points of different topological charges at the center of star-shaped microfluidic junctions, which then interact with emergent topological defects in the orientational field of the nematic director. We combine experiments and analytical and numerical calculations to show that a hydrodynamic singularity of a given topological charge can nucleate a nematic defect of equal topological charge and corroborate this by creating −1, −2, and −3 topological defects in four-, six-, and eight-arm junctions. Our work is an attempt toward understanding materials that are governed by distinctly multifield topology, where disparate topology-carrying fields are coupled and concertedly determine the material properties and response.

Tuning Fluidic Resistance via Liquid Crystal Microfluidics

Flow of molecularly ordered fluids, like liquid crystals, is inherently coupled with the average local orientation of the molecules, or the director. The anisotropic coupling—typically absent in isotropic fluids—bestows unique functionalities to the flowing matrix. In this work, we harness this anisotropy to pattern different pathways to tunable fluidic resistance within microfluidic devices. We use a nematic liquid crystalline material flowing in microchannels to demonstrate passive and active modulation of the flow resistance. While appropriate surface anchoring conditions—which imprint distinct fluidic resistances within microchannels under similar hydrodynamic parameters—act as passive cues, an external field, e.g., temperature, is used to actively modulate the flow resistance in the microfluidic device. We apply this simple concept to fabricate basic fluidic circuits, which can be hierarchically extended to create complex resistance networks, without any additional design or morphological patterning of the microchannels.

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

We study nematic LCs and nematic colloids flowing through microchannels using polarizing transmission microscopy and fluorescence confocal polarizing microscopy. Depending on the channel dimensions, the anchoring conditions on the channel walls, and the flow rate, the formation of different textures and defect structures is observed: π-walls, disclination lines (DL) pinned to the channel walls, DL with one pinned and one freely floating end, and DL and loops freely floating in a chaotic-like manner. Preliminary observations of nematic LCs containing colloidal particles indicate that textures and defects of the nematic matrix can be used to guide the transport of the particles through the microchannels.

Materials and Experimental Methods

This chapter focuses on the materials and the experimental methods employed for this work. In the context of materials, the properties relevant to the current work are emphasized. The list of materials broadly includes nematic liquid crystals, colloidal particles, fluorescent dyes, and reagents for generating specific surface attributes on the colloidal particles and water droplets. Additionally, experimental tools used for characterizations will be presented here.