Aggregation and phase transition of colloids induced by optical tweezers and thermophoresis

Optical tweezers are an appropriate tool to manipulate colloidal systems. We present here a technique based on optical trapping and thermophoresis to create and manipulate large aggregates of colloidal particles. The system is a mixture in organic solvents of refractive index- and density-matched PMMA colloids that do not interact directly with the laser of the optical tweezers, and titania particles that can be trapped. Trapped titania colloids are heated by the laser beam of the optical tweezers and create a very localized temperature gradient in the fluid. Surrounding PMMA colloids experience this gradient and are subject to thermophoresis that causes their aggregation around the titania particle. With confocal imaging, we characterize the growth dynamics of such aggregates and show that they can induce the crystallization of colloids. The advantage of this method compared to direct optical trapping and direct heating of the fluid is that spherically symmetric aggregates can be created in the bulk of a sample. We suggest that this system could be used to study the evaporation of a nuclei of a dense phase in a colloidal gas by turning off the laser.