SHEAR MELTING OF COLLOIDAL CRYSTALS OF CHARGED SPHERES STUDIED WITH RHEOLOGY AND POLARIZING MICROSCOPY

The effect of a shear flow on a colloidal crystalline phase of charge stabilized silica spheres was examined both by theological measurement s and by observing the process with a polarizing microscope. The micro scopical observations show that a flow breaks up the crystal phase int o crystalline regions coexisting with ''shear melted'' regions. Increa sing the flow rate causes the shear melted regions to grow at the expe nse of the crystalline regions. In theological experiments this transi tion occurs when on increasing the shear rate a critical shear stress is reached. During the transition the stress does not increase with sh ear rate until the entire crystalline phase has melted. The critical s tress divided by the high-frequency limit of the storage modulus assum es a constant value independent of volume fraction and ionic strength.