Orientation of Liquid Crystalline Molecules on PDMS Surfaces and within PDMS Microfluidic Systems

Abstract

The unique components of PDMS-based microfluidic systems are those combined with liquid crystalline materials. Their functionality, especially when it comes to optical applications, highly depends on the LC molecular arrangement. This work summarizes experimental investigations on the orientation of molecules within LC:PDMS structures according to the manufacturing technologies. The availability of high-quality molds to pattern PDMS is a significant barrier to the creation of advanced microfluidic systems. The possibility of using inexpensive molds in the rapid and reproducible fabrication process has been particularly examined as an alternative to photolithography. Different geometries, including an innovative approach for the electrical control of the molecular arrangement within PDMS microchannels, are presented. These studies are critical for novel optofluidic systems, introducing further research on LC:PDMS waveguiding structures.

Periodic liquid crystalline waveguiding microstructures

“…Designs of any shape, including complex patterns with a resolution down to single micrometers, may be easily achieved. Moreover, the spontaneous vertical alignment (VA) 33 of LC molecules on the PDMS surface simplifies the fabrication procedures, making additional orienting layers needless. The primary concern when fabricating electrically driven structures, especially with sophisticated geometry, is creating a suitable and efficient electrodes' system.…”

“…The resultant alignment of NLC molecules within PDMS-embedded microchannels strongly depends on the channels' aspect ratio 58 , the quality of the molds used in the fabrication process 33 , and the method of connecting the PDMS chip to a substrate 59 . Based on previous studies 32 , 33 , 60 , 61 , it has been concluded that the best results in terms of predictability, reproducibility, robustness, and stability of LC molecular alignment in LC:PDMS structures are obtained for the molds obtained in the photolithography process 33 and with the assistance of the plasma treatment, reducing the PDMS surface roughness 60 , 61 . In fact, such procedures have been applied when fabricating the samples shown in this paper.…”

“…We created LC:PDMS structures (see Fig. 2 ) using the typical cast-and-molding technique with the mold and waveguiding device fabrication processes described in our previous papers 31 – 33 , respectively. It must be underlined that all the samples presented in this work were obtained from high-quality molds fabricated using the photolithography process with UV illumination through the chrome mask 33 .…”

“…2 ) using the typical cast-and-molding technique with the mold and waveguiding device fabrication processes described in our previous papers 31 – 33 , respectively. It must be underlined that all the samples presented in this work were obtained from high-quality molds fabricated using the photolithography process with UV illumination through the chrome mask 33 . Specifically, SU-8 25 and SU-8 2010 negative photoresists (provided by Kayaku Advanced Materials Inc.) have been applied, providing a high-aspect ratio with nearly vertical sidewalls of the rib to be embossed in PDMS 34 , 35 .…”

“…Following the fabrication procedures described in Refs. 31 – 33 , it was possible to obtain the channels with a height of about 30 µm (for the structures shown in Fig. 9 a–c) and 12 µm (for the structure shown in Fig.…”