Microfluidic confinement and flow set-up

The nematic liquid crystal and the nematic colloids were flowed through microchannels of different geometry and dimensions. The microchannels were fabricated using the tech-niques of soft lithography. Additionally, the walls of the channels were functionalized to induce different kinds of nematic anchoring, providing a wider platform to investigate ne-matic micro-flows. While most of the investigations were done at a constant volume flow rate, Q, some experiments also employed a constant pressure gradient along the channel to create the nematic flow. The director dynamics were studied using a combination of polarization and fluorescence confocal microscopy techniques. The measurements of flow speeds were carried out either by tracking tracer particles over time, or using the technique of dual-focus fluorescence correlation spectroscopy.

3.3.1 Fabrication of microfluidic devices

Microfluidic devices were fabricated using the techniques ofsoft photolithography [145].

Photolithography is a standard method employed to pattern silicon substrates. Figure 3.3 shows the steps involved in the fabrication process schematically. The microfluidic devices were fabricated within an in-house clean room facility. A photo-sensitive material called pho-toresistis first spin-coated on a silicon substrate and baked at moderate temperatures to drive away traces of solvent, see Fig. 3.3a(i). The speed and duration of the spin-coating pro-cess dictates the thickness of deposited photoresist, which is effectively also the depth of the microchannels. A patterned photomask is then placed on the photoresist, followed by brief exposure to UV light, as depicted in Fig. 3.3a(ii). In this present work, I have used a neg-ative photoresist: Negneg-ative photoresists cross link post exposure. UV light thus cures the photroresist layer only at regions exposed to it. After this process, the coated silicon substrate is immersed and stirred in a suitable solvent, known as the developer. Portions of the

pho-Figure 3.3: Fabrication of microfluidic devices. (a) Photolithography technique. (i) Deposition of photoresist on a silicon substrate. (ii) Exposure to UV light through the photomask. (iii) Post development, the UV-cured photoresist layer remains, whereas rest of the layer is washed away. (b) Preparation of PDMS cast. (i) PDMS and cross-linker mixture is poured over the silicon substrate with photoresist. (ii) The PDMS layer is thermally cured and pulled out from the silicon substrate, leaving behind the PDMS relief. (iii) Glass and PDMS are exposed to plasma and surface bonded, producing the PDMS-glass microchannel. (c) Microfluidic geometry. Optical micrographs of microchannel sections, from left to right: Linear, linear channel with micro-pillar, diverging, Y, and U shaped microchannels. Scale bar: 20µm

toresist film which did not undergo cross-linking are washed away by the developer, leaving behind patterned features, Fig. 3.3a(iii).

Subsequently, PDMS reliefs were prepared following the standard soft lithography tech-niques [146]. Polydimethylsiloxane, PDMS (Sylgard 184, Dow Corning) was first mixed with a cross-linker in 10:1 proportion (by weight), and poured on the photoresist (Fig. 3.3b(i)). The moulded reliefs were obtained by pulling offthe PDMS after thermal curing, see Fig. 3.3b(ii).

Subsequently, the reliefs were bonded to glass substrates after being exposed to air/oxygen plasma, as shown in Fig. 3.3b(iii). Different geometries – linear, U, T, L, and Y shaped, me-andering, diverging, and channels with micro-pillars – were prepared using above techniques

(Fig. 3.3c). The channels had rectangular cross-section, with the channel depthdvarying be-tween 5 and 100µm, while the channel widthwwas typically maintained between 50 µm to 500µm. The distance between the inlet and the outlet port was generally set to 20 mm, defin-ing the lengthlof the channel. To ensure that the measurements were carried out in the fully developed flow regime, the lengthlwas maintained much longer than the entrance length – the minimum channel length required for the flow to be fully developed. No perceptible swelling of PDMS by 5CB was observed during the course of experiments. This is in agreement with the observed dependence of the material swelling ratio on the substance polarity (≈ 4.8 D for 5CB) [147].

3.3.2 Flow setup

At both ends of the microchannel, cylindrical holes with 750 µm diameter were punched through the PDMS to provide housing for the flow tubings. Two teflon tubes with 300 µm inner diameter and 760µm outer diameter were inserted into each of the housings and served as connectors for inlet-to-source and outlet-to-sink, respectively. Figure 3.4 shows such a microchannel. The housing diameter was kept smaller than that of the tubes to ensure good mechanical fitting and to avoid leakage.

Figure 3.4: PDMS-glass microchannel and flow equipments. (a) Image shows a typical PDMS-glass microchannel. The teflon tubings inserted into the PDMS cast serve as the inlet and outlet connectors. (b) Pumps used for micro/nanoliter flows. (c) Equipment for constant pressure flows. Image courtesy for (b) and (c): J. C. Baret and group.

The inlet tube was connected to a gas-tight microlitre syringe (1001LT, Hamilton Bonaduz), which was driven by a gear pump (neMESYS, Cetoni) with a flow rate precision of nano-liter per hour, which is around±0.02% of the corresponding flow rates used. Alternatively, Poiseuille flow was created within the microchannels by applying a pressure difference

be-tween the inlet and outlet ports using commercially available Fluigent Maesflow Flow Con-troller (France). The outlet pressure in the experiments was set to atmospheric pressure. In some cases, pressure-driven flow was created by using a gravitational head as well.

3.3.3 Functionalization of microfluidic devices

Microfluidic devices were functionalized for investigating nematic flows within microchan-nels possessing a variety of surface anchoring conditions. Specific anchoring states within the microfluidic devices were achieved using a combination of different physical and chemical methods. Due to a rather diverse set of steps involved, Chapter 4 is separately devoted to-wards studying the functionalization techniques (see (3) on the publication list).