<p style="text-align: justify">&nbsp; An experiment of nematic liquid crystal 4′-Pentyl-4-biphenylcarbonitrile (5CB) flowing under horizontal temperature gradient in a rectangular microchannel is carried out. The microchannel has a length of 20 mm, a width of 100 μm, and a depth of 40 μm. The microchannel is made by following the well-known soft lithography procedures. A silicon wafer spin-coated with photoresist is exposed under ultraviolet through a chrome mask plate with prepared patterns on it. The silicon wafer is then rinsed successively by the photoresist developer, isopropanol, acetone and ethanol to completely remove the photoresist. The microchannel is fabricated by covering the treated silicon wafer with polydimethylsiloxane (PDMS) and baking subsequently, then bonding the microchannel base with a glass substrate by plasma surface treating. The inner walls and the glass substrate of the microchannel is functionalized by 0.1 wt.% N,N-Dimethyl-N-[3-(trimethoxysilyl)propyl]-1-octadecanaminium chloride (DMOAP), rinsed with deionized water and dried to generate homeotropic surface anchoring conditions. The microchannel is observed by a combined experimental setup, including an inverted polarized microscope, a syringe pump with a microliter syringe, and a high-speed CMOS camera connected to the microscope. An electric resistant wire connected to an DC power supply is embedded into one side of the microchannel to heat the side wall. Several K-type thermocouples are placed on both sides of the microchannel and connected to a data acquisition module to acquire the temperature near the side walls. Silica particles with the diameter of 2 μm are rinsed by deionized water, then centrifugated, dried, mixed with 5CB, and sonicated to acquire the velocity distribution in the microchannel by using the particle tracking velocimetry (PTV) method. The inlet volume flow rate of the liquid crystal is set at 1.0 μL/h. The liquid crystal is slightly heated on one side, with the temperature of the hot side wall lower than 306 K, the nematic-isotropic transition temperature of 5CB, so that the liquid crystal is maintained at the nematic phase. The molecules of 5CB can be regarded as ellipsoids, and the orientation of the liquid crystal molecules shows long-range order in nematic phase, showing birefringent optic characteristic like general crystals. The liquid crystal flowing inside the microchannel seems transparent under brightfield microscopy. However, under polarized optic microscopy (POM) with orthogonally crossed polarizers, the liquid crystal demonstrates a bright area because of its birefringence. Due to the homeotropic surface anchoring conditions of the side walls, the liquid crystal molecules near the side wall are aligned perpendicular to the wall, forming stripes along the wall in the view of POM.</p><p style="text-align: justify">&nbsp; The results show that when the horizontal temperature gradient across the microchannel is applied, an asymmetric flow emerges inside the microchannel. The horizontal flow velocity along the microchannel near the heated side becomes higher than the other side, resulting in a 20% velocity difference across the microchannel when the horizontal temperature gradient is 2.1 K. As the temperature gradient increases, the average velocity in the channel slightly increases, while the velocity difference increases significantly. The flow velocity of the heated region can be 50% higher than that of the other region under the horizontal temperature gradient of 3.7 K. The anisotropic characteristic of nematic liquid crystal that caused the asymmetric flow is discussed. This phenomenon can be applied in the Micro-Electro-Mechanical System (MEMS) region as a novel kind of microvalves in lab-on-a-chip devices, owing to various advantages over present microvalves, such as rapid reaction, continuous tuning, and high precision.</p>

Asymmetric Flow of Nematic Liquid Crystal Under Horizontal Temperature Gradient in Rectangular Microchannel

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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