Electro-rheological response of liquid crystals under oscillatory squeeze flow

We have investigated the electro-rheological (ER) properties of liquid crystals in an oscillatory squeezing flow. A Micro Fourier Rheometer was utilized to estimate the complex viscosity and phase shift. A clear dip in the phase angle was observed, centred around a frequency which increased with electric field strength. It was found that this critical frequency was proportional to the inverse of the response time required for the liquid crystal molecules to align under the electric field. Similar ER effects to those observed under steady shearing were obtained in the oscillatory flow, but the ER effect at high frequency is smaller than that under no electric field. In other words, a negative ER effect was obtained under low electric fields. A rheological model for the dynamic response was obtained through the control theory and the transfer function thus derived will be useful for estimating the output response in a flow or motion control system.