Fluids: Simple and complex, are ubiquitous in our day-to-day life. While water, for all practical reasons, is asimplefluid exhibiting Newtonian behaviour, most of the food, personal care products and polymer solutions arecomplexnon-Newtonian fluids. Complex fluids can be broadly considered as ’deformable solids’ with physical attributes intermediate between solids and liquids. Typically, at short time scales the solid properties manifest more prominently over the liquid ones. However, at long times – from fractions of seconds to hours or days – the overall flow properties become visible. These different time scales define the characteristic non-linear mechanical response (deformation) of complex fluids to shear stresses and render themviscoelastic[1]. The non-Newtonian behaviour of fluids has been exploited in a number of engineering applications. Particularly, the use of tiny amounts polymers (few ppm) in water astonishingly reduce the turbulent drag [2], thereby significantly lowering the pumping power required for the flow. Furthermore, such non-Newtonian polymer solutions can lead to elasticity-mediated turbulent flows [3] even at low Reynolds number microfluidic flows [4].
Viscoelastic fluids like the polymer solutions exhibit these special properties due to breaking down of the internal fluid structures by flow, finally resulting in the change in the entropic elasticity [5].
Liquid crystals are mesophases of rod-like or disk-like molecules, intermediate between ordered solid phase (crystalline) and disordered liquid phase (isotropic). While classical vis-coelastic liquids are isotropic, liquid crystals constitute a special class of complex fluid pos-sessing anisotropic properties due to the spontaneously broken internal symmetry [6]. On one
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hand they possess hydrodynamic properties of complex fluids, on the other hand, they ex-hibit anisotropy in physical properties due to the inherent ordering existing in the mesophase.
Hence, the anisotropy in elasticity, viscosity, and other physical properties of liquid crystals arises due to their orientational order, and is crucial for devising different kinds of applications, e. g. liquid crystal displays. Consequently, the flow of such anisotropic fluids is not only more complex due to the coupling between the liquid-crystal-ordering and the flow field, but also provides a physically rich system to investigate different competing effects characteristic to these systems.
1.2 Microfluidics
Microfluidics is a miniaturized and versatile platform to study the flow through micron-sized confinements, typically capillaries, channels, or a network of such conduits. While the miniaturization owes to the micron or sub-micron dimension of the individual conduits, trans-porting fluids in volumes of microliter (µL), nanoliter (nL), or picoliter (pL), the versatility of this platform is due to the wide range and highly diverse set of investigations that can be carried out in them. Since its emergence in the early 1990s [7], microfluidics has seen an ever-expanding reach in practically all fields of modern science and technology. Thereby, it has significantly contributed to the development of interdisciplinary research – converging physics, chemistry, biology, and technology – all on one platform.
Microfluidics derives its forte from the ability to control and manipulate flows precisely.
Using a variety of techniques available [7], the conduits can be fabricated with high di-mensional precision with or without morphological structures for flow manipulation: micro-pumps, micro-valves, and flow-guiding paths etc [8]. The microfluidic devices can be seam-lessly integrated to an external field for enhanced functionality [9], or to existing flow char-acterization techniques [10]. The individual components can be subsequently put together to construct large-scale-integrated networks of varied levels of complexity, resulting in highly efficient lab-on-a-chip devices [11].
1.3 Motivation
Considering the umpteen possibilities microfluidic techniques can offer, my decision to investigate liquid crystals flowing through micron-sized confinements was rather
straightfor-ward. The use of an anisotropic fluid as a continuous phase could offer possibilities beyond microfluidics based on isotropic fluids. However, quite astonishingly, a consequent review of the existing literature (till 2009, when I started my doctoral research) on liquid crystal flows revealed that this domain was practically unexplored. Nevertheless, the fundamentals of liquid crystal flows were already laid down by the numerical works of Ericksen [12, 13], Leslie [14, 15], and Parodi [16]. Almost parallely, a combination of numerical and experimen-tal investigations were undertaken by theOrsay Group in Paris. While Dubois-Violette and Manneville [17–19] provided the numerical support during the 1970s, it was the experimental works of Pieranski and Guyon [20–23] that provided significant insights to the practical as-pects of liquid crystal flows. A period of lull followed till the novel colloidal interactions in liquid crystals were discovered by the seminal work of Poulinet al. in the late 1990s [24].
Several numerical investigations were undertaken thereafter, especially on flow past inclu-sions [25] and the effects of flow on topological defects [26] around the inclusions [27–29].
In the more recent past, microfluidics was starting to be used as a tool to generate liquid crystal droplets [30–32], to study their wonderful properties [33–36], and to investigate con-finement and motion of topological defects [37–40], and potential applications [41]. However, the possibility to use the available microfluidic techniques for studying the fundamental be-haviour of liquid crystal flows within minute confinements was never explored. Especially, the effects of multiple surfaces in close proximity, and the influence of different surface properties on them, could be studied in a great detail by exploiting the precision microfluidics offered.
Due to lack of adequate fabrication techniques previously, the fundamental experimental stud-ies by Pieranskiet al. were conducted within confinements with only one characteristic length, i. e. the channel depth, typically few hundred micrometers. A set of promising experiments investigating the effect of confinement and surface properties were initiated by Sambles and co-workers inExeter, UK [42–44] using wide channels (widthdepth ≈30µm).
Thus, in this doctoral thesis, I have tried to conduct the first systematic experiments in strict microfluidic confinements, and to explore the possibilities of novel applications based on liquid crystal microfluidics. Investigations were carried out for different surface function-alities within channels having a range of depths, from 5 µm – 100 µm, and aspect ratios.
The promising outcomes provided subsequent impetus to investigate flows within channels of different geometries and flow past microscopic obstacles. The ability to control the genera-tion and navigagenera-tion of flow-induced topological defects led to the idea of controllably guiding colloids and droplets using defect lines as soft rails. However, the number of prospective pos-sibilities which emerged during the course of this work outweighs that of the realized ones.
Due to the distinct yet generic capabilities offered by theanisotropicliquid crystal microflu-idics vis-`a-visisotropicmicrofluidics, I consider this work still at its infancy.