We investigate particle dynamics in nearly hard sphere colloidal crystals submitted to a steady shear flow. Both the fluctuations of single colloids and the collective motion of crystalline layers as a whole are studied by using a home-built counter rotating shear cell in combination with confocal microscopy. Firstly, our real space observations confirm the global structure and orientation as well as the collective zigzag motion as found by early scattering experiments. Secondly, dynamic processes accompanying the shear melting transition are followed on the particle level. Local rearrangements in the crystal are seen to occur more frequently with increasing shear rate. This shear-enhanced particle mobility is quantified by measuring the random particle displacements from time-tracked particle coordinates. We find that shear induced melting takes place when these random displacements reach 12% of the particle separation, reminiscent of the Lindemann criterion for melting in equilibrium systems. In addition, a dynamic criterion for melting, based on the relative importance of the long time self diffusion compared to the short time self diffusion, is discussed.
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