Electrically Actuated Concentration of Microparticles through Levitation and Convective Flows in Evaporating Droplets.

We present an electrically actuated approach for creating a well-defined centered microparticle cluster within a sessile droplet on an interdigitated microelectrodes. The method is demonstrated with different aggregation shapes and particle types including biological cells for 3D microtissue development. AC voltage application induces particle levitation and enhanced-convection through accelerated evaporation. Radial long-range fluid convection evolves along the substrate surface toward the droplet's center and suspended microparticles aggregate within the central stagnation zone, in an interesting occurrence that is opposite to the well-known coffee ring effect. This remarkable approach could open new opportunities in immunoassays, rare cell counting, and 3D cell cultures.