# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 3 Results
### LC Orientation in PDMS:PDMS and PDMS:Glass Microstructures
#### 3.2.2 Micromilling in PMMA  
Our previous research showed that micromilling in PMMA might be an accurate method to obtain molds with suitable geometrical parameters [9]. Particular attention should be paid to the undistorted sidewalls, which are almost perfectly perpendicular to the base surface. Another advantage is the potential to create complex structures. One of the many tested approaches was the trial samples with five different channels fabricated on both PDMS and glass substrates. The channels' widths were 50, 100, 150, 200, and 250 \(\upmu\)m, respectively (see Figure 4a). Two perpendicular channels were used as interconnectors, and the full structure was terminated with four openings (not shown in the photo). This arrangement ensured the air-bubble-free infiltration of the entire length of the channels tested. The photographs in Figure 4 were taken for the whole structure filled with E7 NLC, and when the flow of the liquid crystalline material was terminated. Upon the closer examination of the POM images at 0 and 45 degree angles to the polarizers' axes, discontinuities in the LC orientation with the disclination points were visible. This was particularly emphasized in the vicinity of the channels' interconnectors. Moreover, the segments with different molecular arrangements along the channels could be distinguished. Identical orientation along the entire length of the channels was not achieved. This conclusion was reached for both the PDMS:PDMS (Figure 4a-f) and PDMS:glass (Figure 4g,h) structures of the leader type. Specifically, the POM image of the molecular orientation escape in a microchannel with the aspect ratio equal to one, resulting in a single singularity at the channel center, is shown in Figure 4e,f. It is worth underlining that the microchannel itself is of much better quality than that described in the Section 3.2.1 (please see the photographs in Figure 3a,b for comparison), indicating a much better quality of the micromilled mold. The higher integer singularities were also observed in this channel due to the same tendency of the molecules to orient orthogonally to the sidewalls and to the top wall. The design of the sample with a  
Figure 3: Photographs of the single microchannel (260-\(\upmu\)m-wide) embedded in the PDMS:PDMS structure made with the 3D-printed mold. The channel observed 5 min after infiltration with an E7 NLC when aligned at angles of 0 degrees (**a**) and 45 degrees (**b**) to the crossed-polarizers axes. (**c**) Image of the same channel taken 3 days later.  
single channel was also tested to resolve whether the above observations were not the result of the structure's geometry and complexity. In particular, a 200-\(\upmu\)m-wide microchannel made in the PDMS:glass architecture was studied. After the channel infiltration (with the volumetric flow rate in the range of about 1.5-4.0 \(\upmu\)L/min), a consistent orientation along the entire length of the channel was eventually achieved. Figure 4i,j shows polarizing optical micrographs of the equilibrium orientation of the LC molecules, which can be perturbated by the flow. Unfortunately, a complete lack of molecular alignment was observed a few days after the filling of the structure (see Figure 4k).