Pertaya-Braun, Natalya; Hardt, Steffen (Center of Smart Interfaces, TU Darmstadt, Germany)
In the present paper we demonstrate the use of flow focusing microfluidics technique for generating small droplets from which a solute is extracted to the continuous phase. The flow-focusing chip we present is used for the fast extraction of acetone from aqueous droplets. The chip was prepared by a polydimethylsiloxane (PDMS) molding technique using UV lithography of SU-8 resists. A mixture of water and acetone with a volumetric ratio of 2:1 was introduced into the flow focusing chip, and droplets of about 200 µm diameter were formed. Brightfield microscopy and a high-speed CMOS camera setup were used for studying the process. It was found that when the temperature of the chip rises, a substantial mass transfer of acetone from the droplet into the oil phase sets in. The higher the temperature, the faster is the extraction process. In order to estimate the volume loss during extraction, the volume of the droplet inside of the flow-focusing chip was measured as a function of time. It was found that after subsecond time periods the droplets have lost about 1/3 of their volume, which exactly corresponds to the dissolved amount of acetone, and indicates that extraction had been completed.