# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 1 Introduction
For instance, LC:PDMS waveguiding structures may be used to obtain reconfigurable optical interconnectors and devices such as optical switches or wavelength demultiplexers [33; 36; 37]. Considering all of the possibilities for the use of LC:PDMS structures in practice, one must keep in mind that their functionality strongly depends on the molecular arrangement of the liquid crystalline material inside. Specifically, the anchoring conditions at the intersections between LC and PDMS surfaces should be identified and considered. Simultaneously, many studies define the physics of liquid crystals in microfluidic systems, including, e.g., the relationship between the flow and the molecular orientation [38] or the formation of topological defects in NLCs confined in complex networks [39]. Experimental works on the laminar flow of a nematic fluid through PDMS microchannels resulted in reproducible observations of different textures and topological defect structures, including formations of \(\pi\)-walls and surface-stabilized disclinations, depending on the channel depth and flow velocity [40]. It has been demonstrated that the nematic textures also retain high morphological stability after the flow termination [40], and the vertical anchoring of the rod-like molecules of NLC at every channel wall (i.e., PDMS surface) is obtained in stationary conditions [41; 42]. This stiff boundary condition has been applied in numerical simulations related to LC:PDMS waveguides and performed using the Monte Carlo [31; 32; 33; 34] and the finite difference [43] methods. It is evident that the macroscopic LC molecular arrangement in the microchannel strongly depends on its aspect ratio, resulting in homogeneous or homeotropic alignment for deep or shallow channels, respectively [44]. Such an orientation of LC molecules can be relatively easily changed and controlled by applying an appropriately formed electric field to LC:PDMS structures, thus giving an additional boost of flexibility to microfluidic waveguiding systems [34; 45; 43]. In this paper, the results of experimental investigations on the orientation of NLC molecules within various PDMS structures are presented. More precisely, the PDMS architectures fabricated using molds obtained by different techniques, including 3D printing, photostencil film processing, micromilling, and eventually photolithography, are considered. These efforts aim to better understand and determine LC orientation in PDMS as an essential part of creating novel optofluidic systems, introducing further research on LC:PDMS photonic structures. Indeed, it should be remembered that the necessary condition for receiving and controlling the anisotropic properties of LC material is to obtain a specific ordering of its molecules within the sample volume. Therefore, it is not surprising that structures made of PDMS attract considerable interest, as this material allows, in principle, for the intrinsic vertical orientation of liquid crystal molecules (with no orienting layers or additional processes required). This hypothesis agrees with the experimental results shown in the literature on the occasion of the intensive studies performed on the determination of the LC molecular arrangement in dynamic (flow) and static conditions in PDMS cells and in microchannels [38; 40; 44]. However, the main aim of our work is to check how the technology of PDMS-based structures' fabrication (and thus the quality of the PDMS surfaces) influences the alignment conditions. The latter significantly impacts the physical properties of liquid crystalline material and the electrooptical characteristics of related possible photonic devices. It means that the molecular arrangement must be strictly determined and controlled no matter what method is used to fabricate repeatable, high-quality LC-based devices of precisely specified characteristics. Unlike the studies shown in [40; 44], our LC:PDMS microchannels are targeted to be used eventually as waveguides.