(Received 25 September 1996, Publication Date 23 May 1997)
The formation of a regular banded structure perpendicular to the direction of applied shear, in the lyotropic liquid crystal hydroxypropylcellulose (HPC) as observed by optical microscopy is presented. This is a phenomenon that has been the subject of much attention in the literature.
Use of a specialised optical shear cell allowed in-situ observation of the HPC whilst subjected to steady or oscillatory shear stress in a parallel plate geometry. Using the shear cell, textural observations of the formation of band structure during and after cessation of shear stress were obtained. Band formation was found to be dependent on the magnitude of reduction in steady shear rate. A reduction of the shear rate by more than 10 1/s was required for effective band formation. A larger reduction in shear rate led to more rapid and extensive band formation. When subjected to oscillatory shear, larger transverse bands were observed during oscillation at sufficiently high frequency and amplitude;formation of the smaller bands was also apparent on cessation. Higher oscillation frequencies yielded more extensive band formation.
Application of steady shear rate greater than 10 reciprocal seconds to HPC showed the gradual build up of wavy herring-bone texture. On relaxation of the shear stress the sample was seen to decelerate slowly with residual flow in the shear direction. The bands formed as a result of this crumpling motion inducing a sinusoidal ripple in the sample. The bands were also visible without crossed-polars indicating a periodic density distribution in addition to the orientational distribution. The texture observed was found to be dependent on shear rate; at applied shear rates of less than 10 1/s, the stable texture was vertically-striated whilst at shear rates greater than 10 1/s a herring-bone texture was observed. No transverse bands were seen on cessation of a shear rate of less than 10 1/s in the absence of the herring-bone texture.
Next Section: 1. Introduction
2. Experimental Apparatus and Technique
4. Summary and Conclusions