# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 3 Results
### Change in the Molecular Alignment with the Use of an Electric Field  
Eventually, studies on the efficient reorientation of the liquid crystal molecules under the influence of an external electric field were performed. For this purpose, the micropattern with three channels (see Figure 7h)--the central one (to be filled with LC) and two side channels (to be injected with metal eutectic Ga-In)--was replicated in PDMS using two molds made of the Chromaline Super PHAT 200 photostencil film and SU-8 photoresist, respectively. The width of each channel was designed to be 200 um, as was the distance between them. In the case of the first technology, the manufacture of the mold is not so precise, and the main channel was again wider than anticipated. Depending on the mold used for fabrication, the height of the microchannels was about 200 um and 30 um, respectively. Both structures were made by bonding the PDMS layer to the glass substrate. The side channels (with the length of 10 mm for the section parallel to the main channel with the liquid crystalline material) were infiltrated with the eutectic gallium-indium using a syringe with a specially prepared needle. A sinusoidal alternating voltage (at the frequency of 1 kHz) was applied to the microstructured electrodes. At the peak-to-peak voltages of 300 V and 150 V, the complete reorientation of the E7 and 5 CB NLC molecules (with their long axes aligned with the direction of the electric field) was observed in the first (UV-cured photostencil film) and the second (SU-8) structure, respectively. The electrically-driven change in a molecular arrangement is shown in the POM images given in Figure 7a-f. This effect is particularly evident in the case shown in Figure 7f, where an initial part of the structure is presented with the electrode feeder channel just approaching the main channel. Two regions of entirely different molecular orientation (i.e., homeotropic and planar) were achieved. The disagreements in the voltages applied in both cases (i.e., for structures fabricated using different molds) may be related to the geometrical differences (the different dimensions and aspect ratios of the channels) and unidentified physical properties of the liquid crystal materials (e.g., different elastic constants and electric permittivities). However, only qualitative observations of the changes in the orientation of the liquid crystal molecules under the influence of an electric field were aimed for in this phase of the studies.  
Figure 7: (**a–f**) Change in the molecular orientation obtained in the central liquid crystalline channel due to the electric voltage application through fluid eutectic Ga-In electrodes (in the form of two side channels, the edges of which are marked by the dashed lines in the photographs from the POM). Tests were performed in the PDMS:glass samples fabricated by photostencil film processing, _h/w_\(\approx\) 1 (**a–d**) and SU-8 photolithography, _h/w_\(\approx\) 0.15 (**e**,**f**). More details about the liquid crystalline materials and voltages applied may be found in the text above. (**g**) Photograph of the part of the structure taken with reflected illumination, demonstrating the sample geometry (with the side channels infiltrated with EGaIn). (**h**) Photograph of the actual PDMS:glass system with two additional electric wires allowing for the electric biasing of the sample.