# Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems
## 2 Materials and Methods  
### Mold Manufacturing  
In this work, the LC molecular alignment within LC:PDMS structures was analyzed to check whether it is the same for molds made by three relatively simple methods (i.e., capillary film processing, 3D printing, and micromilling) and for the molds obtained in the photolithography process. Because a detailed description of low-cost fabrication techniques, including images of the molds' textures, was presented in a previous paper [9], only the essential information is shown here. Whatever approach is adopted, the microstructure's design with computer-aided design (CAD) software (e.g., AutoCAD) and the fabrication of suitable masks (except for micromilling and 3D printing techniques) are the first steps to initiate the mold manufacturing processes. A translucent film was used to create the PDMS cells of the required thickness (analogical to typical sandwich-type LC cells made of glass plates), and to obtain the microchannels in the PDMS slab. This commercially available material, also known as capillary or photostencil film, is offered in many different thicknesses down to 15 microns, depending on the supplier. Microfluidic structures with minimum widths of 50 \(\upmu\)m are accessible with this technology, while narrower patterns tend to peel off during development. The Chromaline Super PHAT 200 film from the IKONICS Corporation was used in this work (where the number in the product name corresponds to the film thicknesses). It consists of two parts, one of which is a photographsensitive emulsion layer and the other is a polyester support. The latter allows for the attachment of an appropriate piece of capillary film to the glass substrate. The photographsensitive layer of the capillary film was exposed through a negative mask (printed on a transparency film using a high-resolution laser plotter) to UV light (100-W Analytik Jena UV light-curing lamp with a wavelength of 365 nm and an intensity of about 20 mW/cm\({}^{2}\) for 2 min). Only the area where the mask is transparent was hardened by the UV radiation, and then the uncured capillary film was carefully washed out, leaving the convex structure of the mold. Drying the mold was the final stage of the fabrication process, taking less than 1 h in total. Our experiences showed that the master of this type is basically disposable, which is not the case for the SLA 3D printing which was also tested to obtain molds with a specific geometry. Specifically, the Form 2 desktop 3D printer with a standard clear resin (RS-F2-GCL-04) from the Formlabs Company was applied. Starting with a CAD model sent to the printer software, by optimizing the printing parameters (i.e., a single layer thickness of 25 \(\upmu\)m and a laser spot size of about 140 \(\upmu\)m in diameter), it was possible to produce molds with satisfactory-quality surface finish and pattern features. After a few hours of solidifying the resin layer by layer, the mold with a height of about 200 \(\upmu\)m was cleaned with isopropyl alcohol. Using the highest possible printing resolution did not avoid the inhomogeneities of the mold structure manifested by the subsequent layers of the resin distinguished on its walls and by its upper surface roughness caused by the laser beam shape. This implies that the better quality and repeatability of the mold may be achieved by mechanical micromilling in poly(methylmethacrylate) (PMMA) material, keeping the fabrication process within a similar timeframe of several hours. For this purpose, high-precision equipment (Mini-Mill/3 from Minitech Machinery Corporation with Kyocera endmills) was used. The relevant milling conditions--including the spindle rotational speed, feed rates, and axial depths of cut--were identified in order to enhance the surfaces' quality and obtain the micro-scale features in the PMMA mold.