Functionalization of microfluidic devices for investigation of liquid crystal flows

Abstract

Systematic studies of thermotropic liquid crystals in confinement, such as liquid crystals in microfluidic channels, require control of the anchoring conditions on the surfaces. Especially for the case of uniform planar anchoring, the standard method involves a mechanical treatment (rubbing) of the surface that is not applicable to microfluidic devices. In the present study, we report methods for the achievement of well-defined anchoring conditions for liquid crystals in microfluidic channels consisting of polydimethylsiloxane and glass. Various physico-chemical techniques enable to establish homeotropic, degenerate planar, uniform planar, and hybrid anchoring conditions on the surface of the channel walls. We characterize the treated surfaces in terms of wettability and liquid crystal anchoring and determine the director field in the microchannels for the different anchoring configurations using polarizing optical microscopy and fluorescence confocal polarization microscopy. The relevance of the surface anchoring for the flow behavior of the liquid crystal in the microchannel is demonstrated by studying the onset of defect-mediated chaoticlike flow at high Ericksen numbers for the different anchoring cases.

Transport of particles in liquid crystals

“…Simplicity of the phenomenon that does not require pumps nor even electrodes to produce dramatic optical and mechanical changes suggests that it might find applications in sensors, photonics, lab-on-a-chip, microand optofluidics. All these fields started to explore benefits offered by LC as a functional microfluidic and optofluidic medium [168][169][170][171][172][173] . …”

Topological microfluidics: present and prospects

“…In this work, we primarily refer to four different anchoring conditions on the walls of the microchannels: [20] degenerate planar anchoring (Figure 2d, i), uniform planar anchoring (Figure 2d, ii), homeotropic anchoring (Figure 2d, iii) and hybrid anchoring (Figure 2d, iv). In degenerate planar conditions, the NLC molecules anchor parallel to the microchannel surfaces, however, without any uniform direction of alignment.…”

“…(a) Topological defect line generated at the air-NLC meniscus during the filling up of a microchannel by 5CB; [8] (b) flow-induced locomotion of an integer defect leads to imprinted director pattern within a microchannel; [17] snapshots of a defect structure due to flow past at a micropillar undergoing (c) topological transformation and (d) director field reorientation, respectively; [18] (e) topological defect line used as a soft rail to transport colloidal microcargo; [19] and (f) defect-mediated chaotic flow of nematic 5CB at very low Reynolds numbers. [20] with the channel reliefs were prepared and were surface-bonded to the glass substrates via plasma exposure (Figure 2b, i-iii). Most of the channels that will be discussed here possessed rectangular cross section, with typical length l ≈ 20 mm, depth d ≈ 25 µm and width w ranging from 50 µm to 500 µm.…”

“…Further details of the channel functionalisation can be found in ref. [20] LC material and its dispersions Single component pentylcyanobiphenyl (5CB; Synthon Chemicals, Bitterfeld-Wolfen, Germany), a room temperature NLC that goes to isotropic phase at~33°C was flowed through the microchannels. We have used two different dispersions -of silica microspheres with~5 µm mean diameter (Microparticles GmbH, Berlin, Germany) and aqueous droplets (polydispersed) in 5CB.…”

Electro-osmosis of nematic liquid crystals under weak anchoring and second-order surface effects

“…The existence of topological defects in NLC flows greatly depends on the channel dimensions and flow rates. Both experimental [11,70] as well as theoretical studies [72][73][74] exist in literature which show that the topological defects become significant when the Er is very high. On the other hand, we find that the actual maximum value of Er for the present problem falls within the order of ~10 ;…”