# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 4 Discussion and Conclusions  
The fabrication of the master mold is undoubtedly the most expensive stage of the cast-molding technique applied for the PDMS-based microstructures' replication. In photonic systems for which high geometrical resolution is required, photolithography is still preferably used to obtain the best-quality master mold. Such financial investment is particularly uneconomic when new microstructures are designed and pre-tested. In fact, in the initial stage of the development, the better option is to test microdevices fabricated using rapid and cheap technologies with readily accessible materials. Therefore, time-consuming and expensive processes utilizing sophisticated cleanroom facilities and complex technological procedures may be avoided. However, this is only reasonable if all of the cases lead to qualitatively consistent results. In this work, LC:PDMS structures obtained using different technologies were tested for the possibility of obtaining a stable alignment of liquid crystal molecules. A summary of the applicability of the fabrication methods may be found by analyzing the data presented in Table 1. Please note that all of the parameters and descriptions refer exclusively to the structures of the type described in this paper. Some quantitative parameters, such as the deviation of the final PDMS channel width from the assumed one (in the design) and typical size deviations (estimated as the standard deviation from the mean value for many samples of the same type) for a 200-um-wide channel have been selected, among others, for the benchmarking. When comparing the possible most-diminutive individual mold dimensions (width/height/channels separation), the entire fabrication procedure (with all its subprocesses) must be considered. For example, the process of washing away the uncured material is essential when a capillary film is used for the mold. This may lead to the unwanted damage of the mold or its resizing. Moreover, particular attention has to be devoted to the implementation of the perfect dose of UV light when exposing the film to obtain the channels of the required geometry. In the case of SLA 3D printing, the mold size is greatly affected by the excess resin that solidifies throughout the process, generally widening the channels. This is especially noticeable when the designed separation between the channels is too small. Eventually, the target sizes of PMMA adds are highly dependent on the endmills used. As far as the quality of the LC molecular arrangement obtained in the studied structures is concerned, it should be remembered that each mold fabrication method is characterized by its technology and the material applied, which significantly affects the final surface roughness of the mold. This has been concluded as one of the critical parameters promoting the uniform and stable alignment of liquid crystal molecules. SLA 3D printing and micro-milling were found to be inefficient for this purpose. The molecule anchoring was disrupted, making it hard to determine the LC molecular orientation accurately. Moreover, the appearance of some defects in molecular arrangement when using the samples has been observed. It seems that the application of the photostencil film is straightforward and cheap, which makes it a rapid prototyping method. This allowed for the homeotropic alignment in the center of LC:PDMS channels to be obtained with the LC molecular arrangement being stable in time. However, only relatively simple patterns may be designed, with the lowest possible dimensions of the geometrical features of the sample being far from acceptable for use in optical system applications. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Model Fabrication Technique**} & \multirow{2}{*}{\ \begin{tabular}{c} **Exemplary** \\ **Micronechanics** \\ **Widths,** \(w\) \\ \end{tabular}  } & \multicolumn{2}{c}{**Max. **} & \multirow{2}{*}{ \begin{tabular}{c} **Lowest Possible** \\ **Dimensions 2** \\ **Widths,** \(w\) \\ \end{tabular}