Defect formation induced by a particle driven through a three-dimensional colloidal crystal

W e present experiments on a single particle driven through a threedimensional colloidal crystal. Using optical tweezers a tracer particle was trapped in the bulk of a colloidal crystal of host spheres. The crystal was moved past the trapped particle at constant velocities. Only the tracer particle was trapped because it had a high refractive index core, while no optical forces were exerted on the host spheres in the crystal as they had the same refractive index as the solvent. The tracer and host particles were imaged using quantitative three-dimensional confocal microscopy. Particle interactions were tuned to hard-sphere like by adding salt to the dispersion. At low driving speed of 0.27 particle diameters per second (a 0 /s), defects were only formed within a few diameters from the tracer. Occasionally, a defect survived and lived for some time before it was annealed out by the crystal. At higher driving speed of 2.1 a 0 /s, many more defects were introduced behind the trapped particle, which were not annealed out within the fi eld of view of the microscope. Host particles were sometimes observed to be pushed in front of the tracer particle for a while. Similar behavior was reported in recent computer simulations on a two-dimensional version of our experiment. 8 | Defect formation induced by a particle driven through a three-dimensional colloidal crystal