STRUCTURAL TRANSFORMATIONS AND VISCOELASTIC RESPONSE OF SHEARED FINGERPRINT CHOLESTERIC TEXTURES

Analysis and computer simulations of structural transformations and vi scoelastic response functions are presented for the steady rectilinear simple shear flows of a flow-aligning cholesteric liquid crystal orie nted with the helix axis along the vorticity direction of the flow; th e helix orientation is known as the fingerprint texture. Computation a nd analysis of orientation and stress relaxation after cessation of sh ear are also given. The governing parameter that controls the orientat ion response to the imposed shear is the Ericksen number E (ratio of v iscous to elastic effects). For a sufficiently high value of E, a shea red cholesteric liquid crystal with a fingerprint texture originally a ligned along the vorticity axis is found to undergo the helix uncoilin g transition and adopt the nematic flow-aligned ordering. On the other hand if the Ericksen number is low, the helix rotates driven by vorti city and the orientation state is characterized by traveling twist wav es. The transition between the traveling twist wave mode and the stati onary nematic mode is found to be mediated by the expansion of the cho lesteric pitch, and occurs at a critical value of E, denoted by E,. Cl ose form solutions for the helix pitch as a function of the Ericksen n umber and for the critical Ericksen number E, for helix uncoiling are obtained and shown to be in good agreement with the numerical solution s. The viscoelastic modes during shear are identified and analyzed. Fo r steady shearing the classical three-region apparent viscosity curve is obtained. Stress relaxation after cessation of shear is computed an d explained in terms of the governing scales of the model and in terms of elastic storage modes that operate during shear. Typical oscillato ry underdamped stress relaxation oscillations generally present in liq uid crystalline materials are found.