Direct Measurement of Fluid Mechanical Properties in Micro Space: Liquid Crystal Orientation under Shear

The shear-induced molecular orientation of a nematic liquid crystal, 5CB (4-pentyl-4′-cyanobiphenyl) was studied in a newly-developed apparatus in which the liquid crystal was confined between optical windows at a film thickness that is continuously variable. The molecular orientation was measured in situ during shear in the presence and absence of DC electrical bias between the two surfaces. In this preliminary study we present findings for a spacing of 10μm, which is comparable to the spacing of actual liquid crystal display technology. Fourier transform infrared time resolved spectroscopy (FTIR-TRS) synchronized with the shear motion provided the molecular orientation as a function of time during the shear cycle. Only a slight orientation along the shear direction was found in the isotropic state but, in the homeotropic state, the molecular orientations in response to shear reflected a competition process between orientation in orthogonal directions. Shear promoted orientation parallel to the confining plates, and a DC bias voltage promoted orientation perpendicular to the confining plates.