We describe a shear device designed to allow the visualization of susp
ensions or emulsions under an optical microscope. The optical axis of
the microscope is perpendicular to the plane of shear. The detailed ho
w profile is determined by numerical simulation with two different asp
ect ratios of the flaw cell. We use this shear device to investigate t
he structure of dilute suspensions of colloidal magnetic particles sub
mitted to a magnetic field and to a flaw. (C) 1998 The Society of Rheo
logy. [S0148-6055(98)00802-5].