Trapping and manipulation of particles and droplets using micro-toroidal convection currents

This paper introduces a technique for generating toroid-shaped convective vortices in thin (80-400 /spl mu/m) layers of oil. Driven by micromachined heat sources suspended 5-250 /spl mu/m above the oil surface, the vortices are used to trap weed pollen with 25 /spl mu/m diameter and microdroplets of water with diameters ranging from 2-50 /spl mu/m. The speed of the convective flow pattern can be controlled by the input power to the heat source and the air gap between the heat source and the oil surface, while the radius of the collection region is determined primarily by the thickness of the oil layer. Flow velocities up to 1700 /spl mu/m/sec have been achieved in toroidal flows with radii ranging from 80-1120 /spl mu/m. When the heat source is connected to an XY scanning stage, the vortex can be swept across the liquid, effectively collecting particles in the areas over which it travels.