Recently, micro total analysis system (μ-TAS) utilizing micro-fabricated fluidic components has been extensively studied. In the field of μ-TAS, one of the important techniques is generation of gradients. The concentration gradient is utilized in various fields, such as analytical chemistry, optics, and biology. A number of techniques that generate concentration gradient in a micro-channel have been reported, which focus on generating concentration gradient in the "width" direction. However, in some applications, such as HPLC in analytical chemistry, generating concentration gradient in the "flow" direction is mandatory. In this study, we propose a micro-fluidic device that generates concentration gradient in the flow direction using diffusive mixing of droplets. The device generates and handles droplets using surface-modification of micro-channels by self-assembled-monolayer (SAM). We designed, fabricated, and experimentally evaluated the concentration gradient generating device. We successfully demonstrated generation of the concentration gradient of acetonitrile. The results indicated that the proposed micro-fluidic device is readily applicable to the fields of analytical chemistry and biology.
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