Rapid Electrokinetic Patterning (REP) of Hydrosol Colloids At a Planar Electrode Surface

Rapid Electrokinetic Patterning (REP) is a relatively new technique for controlling hydrosol colloids at a planar electrode surface.  REP uses AC electric fields and a heat source to generate micro-scale vortices for manipulating trapped particles.  An infrared laser was used as a heat source to characterize aggregate groups of particles.  A device was constructed using an indium-tin oxide (ITO) coated glass slide and cover slip.  The glass slide and cover slip were separated by 50 µm double-sided tape with a small region removed.  One-micron polystyrene spheres were inserted into a solution of water and KCl.  The infrared laser was focused on and partially absorbed by the ITO on the surface of the cover slip.  This generated a thermal gradient in the fluid medium, which generated a gradient in the electrical properties of the medium.  The AC electric field acted on this gradient to produce a net force on the medium away from the surface of the electrode.  Particles trapped at the electrode surface by the AC field were then corralled into a group.

Particles collected on the surface of an electrode were manipulated to vary the particle-particle spacing.  Aggregations were stretched into lines as well.  The spacing of particles was characterized in order to study the crystallinity of the aggregation.  The spacing was characterized in dot-shaped aggregations by laser power, AC frequency and voltage, medium conductivity, and number of particles in the aggregation.  

More complicated geometries, such as ellipses, are shown.  Particles were aggregated in a dot-shaped aggregation, then stretched into a line-shaped aggregation, and then into an elliptical shape.  Intersecting and colliding geometries are also discussed, such as two line-shaped aggregations brought into contact with each other.