# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 1 Introduction  
Microfluidic structures represent an essential part of modern photonic systems. Their varieties include, e.g., microfluidic chips, Lab-on-a-Chips (LOCs), Photonic Lab-on-a-Chips (PhLoCs), and micro-total-analysis-systems (\(\mu\)-TASs), thus combining applications and technologies known from chemistry, biology, and photonics [1, 2]. Owning to such solutions, space-consuming or expensive devices can be miniaturized, and the performed analyses or tests can be accelerated with significantly reduced costs [3, 4, 5]. This is particularly evident when inexpensive soft lithography is used for microfabrication, suiting many materials, including elastomeric ones. A considerable number of microfluidic structures are fabricated by a cast-molding technique using polydimethylsiloxane (PDMS). The latter may be successfully applied not only as a building material for the systems themselves but also as a mold for structures made of other materials [6, 7]. However, the first scenario is prevalent for PDMS-embedded photonic devices, relying essentially on the availability of high-quality models. The fabrication of microstructured master models with the required geometrical features is objectively a critical step in the manufacturing process. Various techniques may be applied for this purpose, including micromilling [8, 9, 10], 3D printing [9, 11, 12], polystyrene thermoplastic sheet shrinkage [13], capillary film processing [14], and photolithography [15]. In addition, a combination of different methods [16] or additional improvements of existing ones--e.g., the smoothing of the stamp's surfaces [17]--are also employed, resulting in high-quality micron-scale patterns to be accurately replicated in PDMS. In terms of optical applications, further benefits of such elastomer applications include its refractive index of about 1.41 @ $89 nm [18], isotropy, homogeneity, and high optical transparency in the spectral range covering UV, VIS, and NIR [19]. Examples of the microfluidic photonic structuresobtained in PDMS are optofluidic sensing devices [20], optical MEMS [21], microlenses [22], dye lasers [23], and reconfigurable optical waveguides [24]. A particular part of the PDMS-based microfluidic systems is constituted by structures filled with liquid crystals (LCs). Exploiting such unique birefringent and anisotropic materials may lead to many intriguing phenomena with various potential applications of the so-formed LC:PDMS architectures. One of them is the hierarchical filtration or sampling of liquid crystalline biomaterials and chemicals in electrically programmable microfluidic structures [25]. LOC-integrable systems can also be applied to enclose liquid crystal lasers utilizing cholesteric liquid crystal (CLC) confined between PDMS substrates [26]. Another usage of CLC is biosensing chips for albumin detection [27]. A 1-kHz optofluidic modulator was used to achieve novel analytical and imaging methods based on peristaltic nematogen microflows [28], and the PDMS-based tunable optofluidic birefringent lens utilized nematic liquid crystals (NLCs) to obtain different focal distances for p- and s-polarized light [29]. At the same time, the polymer-dispersed liquid crystal (PDLC) structures are another area of research that offers excellent opportunities in microfluidics. For example, a carbon nanotube-liquid crystal-PDMS composite structure may be indicated to serve as a highly resistive pressure sensor [30]. Studies on optical waveguides represent an important research area involving LC:PDMS structures [31; 32; 33; 34]. This is strongly encouraged by the low-cost and straightforward soft material technology allowing for arbitrary geometry. The maintenance of the polarization state for the light propagating along the microchannels embedded in PDMS and infiltrated with NLC indicates many potential arrangements for integrated optics and microfluidic circuits [35].