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 μm) layers of oil. Driven by micromachined heat sources suspended 5-250 μm above the oil surface, the vortices are used to trap weed pollen with 25 μm diameter and microdroplets of water with diameters ranging from 2-50 μm. 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 μm/sec have been achieved in toroidal flows with radii ranging from 80-1120 μm. 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.