| Abstract
| - We present a generic concept to create local concentrationgradients, based on the absorption of gases or vapors ina liquid. A multilayer microfluidic device with crossinggas and liquid channels is fabricated by micromilling andused to generate multiple gas−liquid contacting regions,separated by a hydrophobic membrane. Each crossing canacts as both a microdosing and microstripping region.Furthermore, the liquid and gas flow rate can be controlled independently of each other. The focus of thisconceptual article is on the generation of pH gradients,by locally supplying acidic or basic gases/vapors, suchas carbon dioxide, hydrochloric acid, and ammonia,visualized by pH-sensitive dyes. Stationary and movinggradients are presented in devices with 500-μm channelwidth, depths of 200−400 μm, and lengths of multiplecentimeters. It is shown that the method allows formultiple consecutive switching gradients in a single microchannel. Absorption measurements in a microcontactor with the model system CO2/water are presented toindicate the dependence of gas absorption rate on channeldepth and residence time. Achievable concentration rangesare ultimately limited by the solubility of used components. The reported devices are easy to fabricate, and theirapplication is not limited to pH gradients. Two proof ofprinciples are demonstrated to indicate new opportunities: (i) local crystallization of NaCl using HCl vapor and(ii) consecutive reactions of ammonia with copper(II) ionsin solution.
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