Electrically Programmable Nematofluidics with a High Level of Selectivity in a Hierarchically Branched Architecture

Abstract

The development of simple and generic principles for microfluidics should have an impact on a wide range of distinct areas, [1] including chemical synthesis, [2] medical diagnostics, [3] and designer emulsions. [4] For advanced applications such as the purification of nucleic acids from a large number of cells, [5] multiple connection of liquid-core/liquid-cladding waveguides, [6] and sorting colloidal emulsions or vesicles in complex solutions, [7][8][9] a high level of selectivity in a highly branched architecture is an essential element of designing a robust platform. However, existing technologies including an electro-kinetic [10] or a hydrodynamic [11] method, successfully employed for a simple or parallel Y-channel structure with a single junction, are not easily applicable for serial branches, due to the requirement of either a high voltage (or pressure) between the junctions or a complicated structure.Herein, we present a concept of nematofluidics that provides a simple, programmable, and hierarchically branched architecture of microfluidics where a nematic liquid crystal (LC) is used as an anisotropic fluid. This anisotropic-fluid-based approach allows precise and concurrent control of the flow resistance and streamlines in multi-junction networks, in analogy to the resistance and current in electrical circuits, by the application of a low electric field to each channel. The anisotropic flow resistance, associated with the nematodynamic viscosities, [12] is electrically controlled through molecular reorientation to steer nano-or microparticles immersed in the LC into a specific channel at each junction. Moreover, due to the channel-independent controllability of the LC, a hierarchically branched architecture of microfluidics with high-level of selectivity is designed.For the flow selection capability at a channel junction, two different microfluidic environments that can be switched are required, namely the effective dynamic viscosity along the flow direction (h 1 ) and that perpendicular to the flow direction (h 3 ). The perpendicular viscosity h 3 is much larger than the parallel shear viscosity h 1 because the rotational motion of the LC molecules is mostly involved in the former. The backflow effect incurred during the switching of one environment to the other should not play a significant role in the steady state.[12] Note that dynamic viscosities will depend on the surface interactions of the LC with a wall as well as the geometric shape of a channel, particularly in a microfluidic system with a high surface-tovolume ratio.In our case, the anisotropic nature of the nematodynamic viscosities is produced through the change in the direction of the LC alignment by an electric field applied perpendicular to the flow. A LC material, 4-cyano-4'-pentylbiphenyl (5CB), was filled into a rectangular microfluidic channel with a cross section of 100 mm in width (w) and 15 mm in height (h), by capillary action. The contact angles of the LC on the glass substrate and on polydimethylsiloxane (PDMS) were measured a...

Tuning Fluidic Resistance via Liquid Crystal Microfluidics

“…In addition, electric field effects offer a promising approach to tune nematic flows [28,29]. The anisotropic coupling between the viscous and elastic interactions in a flowing NLC material is fundamental in modulating the flow properties.…”

Microfluidic control over topological states in channel-confined nematic flows

“…Nematic liquid crystals (NLCs) 22 are fluids with orientational order of their anisotropic building blocks, which affects their rheological behaviour when confined to thin planar cells or a network of channels. Confined nematic microflows are of use in optofluidic applications 23–25 and for guided material transport through microfluidic networks 26,27 . Coupling between nematic orientational order and the flow drives structural organisation and hydraulic conductivity in porous networks 28–30 , and leads to shaping of the molecular field by Poiseuille flows 31–35 .…”

Field generated nematic microflows via backflow mechanism

“…The functionality of nematic materials is strongly related to their structure [20][21][22] . The dynamics of non-equilibrium nematic fluids is dependent on the temperature, pressure, and -in the context of this work -on the application of external fields, which cause effective elastic deformations in the molecular ordering 46 .…”

“…Microfluidic functionality can be further expanded by using fluids with internal structure, such as for example nematic liquid crystals, where transport of colloidal cargo 20 , electric field switching of channel resistivity 21 , fabrication of microresonators 22 , manipulation of colloidal particles by groovy interfaces 23,24 , and generation of intertwined field structures 25 have been demonstrated utilizing nematic orientational order and high responsiveness to external fields. Liquid crystals can form complex orientational structures 26 , which are then strongly coupled to the material flow 27 and can lead to flow-induced structural transitions 28 and also activity-driven microfluidics 29 .…”