Dynamic assembly by electrokinetic microfluidics

Dynamic assembly induced by electrokinetic flows was investigated in a two-dimensional tapered microchannel. Vortices formed both at the outlet of the small end and inside the channel. These vortices led to electrolyte deposition on oppositely charged channel surfaces during the two-stage assembly process. The primary assembly occurred in the stagnation region of a double vortex, which was formed by a nonuniform surface ζ-potential distributed along the tapered channel. This was followed by a slower secondary assembly resulting from circulation of the vortex inside the channel. Since electrolyte deposition kept changing the surface charge distribution, the double vortex migrated in the channel. Consequently, the assembly started from the small end and gradually extended toward the large end. Continuous feeding of diluted electrolytes resulted in a uniform assembly on the channel surface, useful for grafting functional molecules and nanoparticles in long micro- and nanoscale channels. When multiple-dosage feeding was applied, an asymmetric assembly occurred, forming a gradient of surface property, which is useful for combinatorial techniques and the formation of multifunctional micro-/nanoconstructs. This new method provides a useful tool to overcome the limitation of the current self-assembly methods.