Electrokinetic concentration gradient generation using a converging-diverging microchannel

Creation of concentration gradients is important in the study of biological and chemical processes that are sensitive to concentration variations. This paper presents a simple method to generate a near-constant concentration gradient in electroosmotic flow in a converging-diverging microchannel. The mechanism is based on the enhanced diffusive mixing inside the converging-diverging microchannel section. By varying the converging-diverging geometry, the degree of diffusive mixing can be controlled. Different concentration gradients can be obtained by varying the applied electric field and microchannel's geometry. Concentration profiles with only 7% deviation over a channel length of 3 mm and 3% deviation over a channel length of 1 mm can be obtained for a 400 μm wide microchannel. In comparison with a simple T-shaped micromixer, the converging-diverging microchannel can produce desired concentration profiles with a much shorter mixing length (shorter by a factor of 2-3.5). A serially connected concentration gradient generator is also realized with the ability to generate two concentration gradient ranges in the same microchannel. Numerical simulations and experiments were carried out to investigate the factors that contribute to the generation of concentration gradients. The experiments were conducted in polydimethylsiloxane (PDMS)-glass chips produced by soft-lithography rapid prototyping.