Polymer Science Faculty Research

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

Ali Dhinojwala


The shear-induced molecular orientation of a nematic liquidcrystal, 5CB (4-pentyl-4'-cyanobiphenyl) was studied in anewly-developed apparatus in which the liquid crystal was confinedbetween optical windows at a film thickness that is continuouslyvariable. The molecular orientation was measured in situduring shear in the presence and absence of DC electrical bias betweenthe two surfaces. In this preliminary study we present findings for aspacing of 10 µm, which is comparable to the spacing ofactual liquid crystal display technology. Fourier transform infraredtime resolved spectroscopy (FTIR-TRS) synchronized with the shearmotion provided the molecular orientation as a function of time duringthe shear cycle. Only a slight orientation along the shear directionwas found in the isotropic state but, in the homeotropic state, themolecular orientations in response to shear reflected a competitionprocess between orientation in orthogonal directions. Shear promotedorientation parallel to the confining plates, and a DC bias voltagepromoted orientation perpendicular to the confining plates.