Manipulating mechanism of particle’s motion has been extensively studied for the sample preparation in microfluidic applications including diagnostics, food industries, biological analyses and environmental monitoring. However, most of conventional methods need additional external forces such as electric field or pressure and complicated channel designs, which demand highly complex fabrication processes and operation strategies. In addition, these methods have inherent limitations of dilution or mixing during separation or preconcentration step, respectively, so that a number of studies have reported an efficient selective preconcentration process, i.e. conducting the separation and preconcentration simultaneously. In this work, a power-free spontaneous selective preconcentration method was suggested based on leveraging convective flow over diffusiophoresis near the water-absorbing nanoporous ion exchange medium, which was verified both by simulation and experiment. Especially, the velocity of the convective flow by an imbibition deviated from the original tendency of t−1/2 due to non-uniformly patterned nanoporous medium that has multiple cross-sectional areas. As a result, the direction of particle’s motion was controlled at one’s discretion, which led to the spontaneous selective preconcentration of particles having different diffusiophoretic constant. Also, design rule for maximizing the efficiency was recommended. Thus, this selective preconcentration method would play as a key mechanism for power-free lab on a chip applications.

Dielectric microstructures have been reported to have negative influences on perm-selective ion transportation because ions do not migrate in areas where the structures are located.However, the structure can promote the transportation if the membrane is confined to a microscopical scale. In such scale where the area to volume ratio is significantly large, the primary driving mechanisms of the ion transportation are transited from electro-convective instability (EOI) to surface conduction (SC) and electroosmotic flow (EOF). Here we provide rigorous evidence on how SC and EOF around the dielectric microstructures can accelerate the ion transportation by multi-physics simulations and experimental visualizations. The microstructures further polarize the ion distribution by SC and EOF so that ion carriers can travel to the membrane more efficiently. Furthermore, we verified, for the first time, that the arrangements of microstructures have a critical impact on the ion transportation. While convective flows are isolated in crystal pillar configuration, the flows show elongated pattern and create an additional path for ion current in the aligned pillar configuration. Therefore, the fundamental findings of the electrokinetic effects on the dielectric microstructures suggest an innovative application in an energy-efficient micro/nanofluidic devices.

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