# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 3 Results  
### PDMS:PDMS Cells with Different Liquid Crystalline Materials  
The initial stage of the experimental studies was devoted to the confirmation of the NLC molecular alignment at the PDMS surfaces. For this purpose, the PDMS:PDMS cells were assembled in an analogical way, as is carried out in the typical glass-plate-based LC cells used for the testing of liquid crystalline components and materials. Due to the elasticity of the host material and the possibility of the entire structure collapsing, several types of cells with sub-pillars of different shapes were designed. Additionally, the pillars prevented any change in the cell gap over its total area. The molds for these PDMS:PDMS structures were made using a UV-cured photostencil film of the thickness of 200 \(\upmu\)m. Two exemplary empty cells with concentric semi-circular and rectangular sub-pillars are shown in Figure 2a,i, respectively. Many other geometries were also tested, giving no evidence of the sub-pillars' influence on long-distance ordering. The dimensions of the central part of the structure were approximately 9.3 \(\times\) 9.3 mm. The photographs do not show the two side channels with inlet/outlet holes to provide liquid crystalline material. The PDMS:PDMS cell from Figure 2a was filled with a 5 CB nematic liquid crystal at a temperature of about 20 \({}^{\circ}\)C. This anisotropic liquid enters the cell almost spontaneously, filling up its entire volume of about 17 \(\upmu\)L in 10 s (see Figure 2b). The chronological changes in the molecular orientation for 5 CB NLC, visualized by the polarizing microscope (POM) images, are presented sequentially in Figure 2b-h. The time elapsed since complete infiltration of the LC cell and the angular orientation of the polarizer and analyzer axes are indicated in each photo. It is worth noting that the initial flow-induced perturbed  
Figure 2: **(a,i)** Photographs of the empty PDMS:PDMS cells applied to determine the liquid crystalline alignment on the PDMS surface. Thin PDMS sub-pillars of semi-cylindrical (**a**) and rectangular (**i**) shapes are visible in the central part of the cell. (**b–h**) POM images of the sample infiltrated with 5 CB NLC taken after a particular time indicated on the bottom left corner of the photographs. (**j,k**) PDMS:PDMS cell with E7 NLC inserted in different angular positions between crossed polarizers and observed 60 min after infiltration. The black gridded pattern visible in some photographs is an artifact resulting from a photo-stitching procedure in the digital microscope. (**l**) Conoscopic image in the central part of the tested cells proving the homeotropic alignment. (**m**) Scheme of the polarization of the optical microscope. The passage from the typical orthoscopic mode to the conoscopic configuration is obtained by introducing a Bertrand lens between the ocular and the analyzer. planar molecular arrangement, with disclination points and lines (Figure 2b-d), was eventually transformed into the homogenous one (see Figure 2e,f) taken 40 min after infiltration). The light transmittance in the central cell area (and in the middle of the side channels) is at the same minimal level for the LC cell edge aligned at any angle to the crossed-polarizer axis. Moreover, for the specific angular positions of the cell, i.e., at 0 degrees (Figure 2e) and 45 degrees (Figure 2f), the minimal and maximal light transmittance in the close vicinity of the cell walls were obtained, respectively. Some defects in the structures (mainly related to the irregularities in the mold surfaces) did not influence the test results. The observations confirmed the homeotropic (vertical) alignment of 5 CB NLC molecules in the PDMS:PDMS cell. It was then checked every few days over a long time. Pictures of the same sample infiltrated with 5 CB NLC taken after 21 days (Figure 2f,g) proved the maintenance of the molecular orientation despite the leakage of LC material from the cell.