# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 2 Materials and Methods
### LC:PDMS Structure Manufacturing  
The PDMS structures were obtained by the cast-molding method (with the main steps presented in Figure 1a-d), in which the convex micropattern from the master mold was replicated in the polymer material. Different molds were used as was extensively described in the previous subsection. Each mold was cleaned with distilled water, dried with compressed air, and then placed in a plastic Petri dish (Figure 1a). The PDMS was prepared by mixing the prepolymer (SYLGARD(tm) 184 Silicone Elastomer Base) with the crosslinker (SYLGARD(tm) 184 Silicone Curing Agent), both from Dow Corning, in a weight ratio of 10:1 [18]. The prepared mixture was degassed in an extractor with a vacuum pump and then poured into a previously prepared form with a mold (Figure 1b). The PDMS was cross-linked at 75 \({}^{\circ}\)C for 90 min. After cross-linking, the PDMS was detached from the mold, resulting in a structure with a concave pattern (Figure 1c). The PDMS' elastomeric property allows for the peeling off of the mold without damaging the microstructures. At this stage of the fabrication process, the elastomer sample was immersed in liquid nitrogen to drill the vertical inlets and outlets in the outermost parts of the microchannels. The next step was to permanently bond the PDMS structure to the flat substrate, i.e., either a glass plate or an additional PDMS slab (Figure 1d). In both cases, the procedure of preparing the components for their joining was identical. Firstly, the PDMS layer with a concave pattern and the support (PDMS or glass) were washed with detergent, thoroughly rinsed with distilled water, and dried with compressed air. Next, the surfaces of both layers prepared for bonding were activated with plasma (using a low-pressure Atto Plasma Cleaner from Diener Electronic) for 20 s at a reduced pressure (stabilized at 0.3 mbar) and under oxygen conditions. After removing this from the chamber, the plasma-activated elements were merged to form a complete microstructure. Its two alternatives, differing by the substrate and hereafter referred to as a PDMS:PDMS or PDMS:glass structure, respectively, were ready to be filled with liquid crystalline material. Exemplary geometries of the samples are shown in Figure 1(e1-e3).  
A typical nematic liquid crystal, namely 4-cyano-4\({}^{\prime}\)-n-pentyl-biphenyl (5 CB) and the E7 nematic liquids mixture, both commercially available from Merck, were used for the tests. Such substances' physical and chemical parameters are well-known and widely described in the literature [46; 47; 48]. Liquid crystalline materials in the nematic phase (i.e., at room temperature) were injected through vertical inlets into the PDMS structure using syringes with a 0.4 mm needle.  
The microchannels were filled with NLC using a syringe with a 0.4 mm needle to put a droplet into the inlet. The liquid crystal was in an anisotropic phase at a temperature of about 20 \({}^{\circ}\)C. The process of the structure infiltration did not consider any additional forces other than spontaneous infiltration using the capillary forces.  
In order to study the electric field influence on LC molecular orientation, the LC:PDMS systems were featured with electrodes (see Figure 1(e3)) in the form of microchannels filled with the high-purity eutectic gallium-indium (EGaIn, from Sigma-Aldrich). This electrically conductive fluid metal (with a resistivity of 29.4 \(\times\) 10\({}^{-6}\)\(\Omega\cdot\)cm) comprises 75.5% gallium and 24.5% indium by weight. Its low viscosity and rheological properties at room temperature allow for the formation of metal electrodes within the microchannels with adequate cross-sectional dimensions [49].