# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 3 Results
### LC Orientation in PDMS:PDMS and PDMS:Glass Microstructures
#### 3.2.4 Photolithography with the Use of SU-8 Negative Photoresist  
The final phase of the studies to evaluate the ways in which the mold manufacturing technology affects the orientation of the liquid crystal molecules within PDMS microstructures was to analyze the structures made from the SU-8 master. As was already mentioned, homeotropic alignment in PDMS microstructures, obtained thanks to the photolithographic processing of the mold, has been widely reported in the literature. A particular structure containing eight microchannels of different widths--20, 30, 40, 50, 100, 150, 200, and 250 --was created (see Figure 6a-d) to validate these reports. Due to the parameters of the technological processes, the height of the structures made of photoresist on the silicon wafer was about 30. All of the channels were merged with one entrance channel (see, e.g., Figure 6c) and terminated with radially distributed output channels (see, e.g., Figure 6h). The in/ outlets were drilled at the beginning/end of each 200-wide in/outflow channel, respectively. Such a design allowed us to get rid of the air bubbles during the infiltration with liquid crystalline material, and to better separate the channels from each other. Having each channel end with its independent outlet should minimize the flow and orientation issues occurring in the ladder-like-shaped structure from Figure 4. The PDMS:glass was filled with 5 CB NLC at 20. A slightly different time was required for each channel to be infiltrated entirely with the anisotropic liquid. This is mainly due to differences in the widths and lengths of the individual microchannels. Specifically, the microchannels' lengths vary from 28 to 30 mm in the central part of the structure where the channels are parallel to each other. The volumetric flow rate between 3 to 30 nl/min was estimated depending on the channel width. The photographs shown in Figure 6a-d were taken about 5 min after introducing the liquid crystalline material to the inlet. Even if flow-induced orientation is still visible within the entrance channel (Figure 6c,d), the molecular alignment is completely established in the tested channels. It is enough to provide the POM images of the two outer sections of the structure because the molecular ordering was unchanged along the whole length of the straight channels. The homeotropic alignment in the central part of the channels' widths, resulting from the pronounced VA on the top surfaces, is clearly recognizable for the microchannels wider than 40. When the aspect ratio (_h/w_) increases to more than one (for the narrower  
Figure 5: Microchannel in the PDMS:glass structure fabricated using the UV-cured photostencil film. The channel was infiltrated with E7 NLC and then observed between crossed polarizers. The photographs in the upper row show the entire length of the microchannel. The LC molecules in the central part of the channel are orthogonal to the azimuthal plane, leading to minimum light transmittance when the sample is placed between crossed polarizers (**left-hand side**). No change of transmitted intensity is observed when rotating the sample except in the vicinity of the channel walls (**right-hand side**). channels), the LC alignment on the sidewalls determines the molecular orientation in the channel cross-section. It is clearly visible when analyzing the results of the numerical simulations presented in Figure 6(k1-k3). The latter represents the molecular arrangement inside the waveguide channels, and was determined using a finite-difference relaxation scheme with an assumption of the stiff homeotropic boundary conditions on the PDMS surfaces. The numerical procedure was the same as described in [43]. The perfect VA alignment at all of the PDMS-LC intersections, including the top surfaces and sidewalls of the microchannels, was also proven in an analogical test sample consisting of channels with widths of 10, 20, 25, 30, 35, 40, 45, and 50 \(\upmu\)m (Figure 6e-h).