Effect of sample thickness gap

A study of the effect of gap thickness on band formation in HPC + water solutions was conducted [87]. The apparatus used had a parallelism of about 20 pm over a plate length of about 10 cm. Note that this was a linear and not a rotary parallel plate apparatus. The gap was varied between 150 and 1000 pm. A rest time of 5-6 h was allowed between sample loading and initiation of shear. Since the time scale of changes in the band structure was slow, of the order of minutes, there was plenty of time for detailed examination. 

A study of band formation in thermotropes by Zachariades et al. [96] commented that deformation appeared to be non-uniform and dependent on the thickness of the sample, and that non-uniform deformation occurs because the domains close to the polymer-wall interface deform more effectively by shearing versus the domains in the bulk of the sample which may slip or rotate... They found band structures after solidification of sheared thin samples, but not sheared thick samples. A personal communication from J.F. Fellers which reported a gap dependence was referenced in [56]. 

The consensus appears to favor the position that band formation is a bulk phenomenon and not due to instability or surface effects however wall effects may produce gap dependence. 

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