A novel technique for response function determination of shear sensitive cholesteric liquid crystals for boundary layer investigations

A description of the design and setup of an experimental technique for measurement of the response function in shear sensitive liquid crystals has been reported. Utilizing the selective reflection characteristics of cholesteric liquid crystals, the method is capable of measuring the delay, rise, and relaxation times in response to a given dynamic shear stress as a function of the wavelength of the incident light. Application of a step input shear stress results in a liquid crystal time response that can be described as consisting of an initial delay, a shear induced helix deformation, and a relaxation to the initial state through diffusion processes. The method has been used for quantitative calibration of a shear sensitive liquid crystal by observing the peak in reflected light intensity, at a given wavelength, as a function of the shear stress. The selective reflection wavelength (λs) decreases linearly with an increase in shear stress (σ) and has a slope dλs/dσ≊10 nm/psi. The results suggest that in order to obtain quantitative information on the flow parameters over a test surface, three reflection spectra are required: one from a white surface and the other two from the liquid crystal surface one before and one in the presence of shear stress respectively. Free energy calculations for the elastic deformation of the liquid crystal suggest a time response of the liquid crystal for a step input in shear stress. Transport of the broken helix swarms has been suggested as a means for the helix relaxation to zero shear state even in presence of shear.