ELECTRORHEOLOGICAL CREEP RESPONSE OF TUMBLING NEMATICS

The shear creep response of homeotropic monodomains of 4,4'-n-octylcya nobiphenyl (8CB) and dilute solutions of a side-chain liquid-crystalli ne polysiloxane (LCP) in N-(4-methoxybenzylidene)-4-butylaniline (MBBA ) is studied in the absence and presence of electric fields applied al ong the director. In the absence of the field, oscillations in strain rate are observed for 8CB and the LCP/MBBA solutions, indicative of di rector-tumbling flow. In the presence of electric fields, the tumbling flow is suppressed and, with increasing field strength, a systematic evolution is observed toward flow-aligning, the apparent viscosity bei ng determined by the balance in hydrodynamic and electric torques. Due to a larger dielectric anisotropy, the tumbling flow of 8CB is suppre ssed at a critical field strength much lower than that of the LCP/MBBA solution. Moreover, a field-induced asymmetry of the doubler peaks is observed in the creep deformation for 8CB at 34 degrees C. Based on E ricksen's transversely isotropic fluid theory, simulation of the influ ence of electric fields on the flow-tumbling behavior is performed and qualitative agreement with the experimental results is obtained. (C) 1998 The Society of Rheology.