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The change of the rheological behavior of a bentonite suspension with the gap of the parallel-plate geometry in a rotational rheometer is investigated. An obvious gap-dependent behavior is found in both the gel and flowing states, based on which it is found that slip at the boundary plays an important role in the viscometric measurement at low shear rates, as well as in the small amplitude oscillatory shear experiment; shear banding is considered to take place at the vicinity of the yielding point, as well as in flows at low shear rates if the boundary slip is suppressed by using rough plates. Furthermore, relaxation behavior, similar to that of a polymeric system, is observed. The motion of the gel structure is slower with increasing gap, indicating a larger size of the networking structure. This is also the origin for the decrease of the yield stress observed with increasing gap. [ABSTRACT FROM AUTHOR]