Contactless mass transfer for intra-droplet extraction

Abstract

This study demonstrates the possibility of "contactless" mass transfer between two aqueous slugs (droplets) separated by an oil slug in Taylor flow inside milli-channels. Separation of the alternating aqueous slugs at the outlet was performed by switching a couple of solenoid valves at branched outlets according to signals obtained by an optical sensor at the branch. Transfer of bromothymol blue (BTB) from acidic to basic aqueous slugs was performed for demonstration. In some cases, aqueous slugs separated by oil, merged catching on each other due to the velocity difference. Interfacial tension which was affected by the solute concentration was responsible for the velocity difference. Position-specific mass transfer activity at the rear end of the aqueous slugs was found on the course of the experiment. A meandering channel decreased the velocity difference and enhanced mass transfer. Almost complete (93%) transfer of BTB was achieved within a short residence time of several minutes under optimized conditions. The presented system opens a way for advanced separation using minimum amounts of the oil phase and allows concentrating the solute by altering relative lengths of the sender and receiver slugs.

Figure 1

A system used to form water-oil slugs of two compositions and their separation (sorting): (a,b) overview of the system, (c) slug formation section with 4 solenoid valves, (d) slug sorting section with 2 solenoid valves and a tee equipped with an optical sensor.

Figure 2

The output of the optical sensor attached to the sorting tee for the colorless aqueous slugs (pure water, “w”), and a colorless carrier oil (dodecane, “o”) at the total flow rate of 2.0 mL min⁻¹, and set aqueous slug length of 15 mm.

Figure 3

Schematics of the BTB transportation.

Figure 4

Photographs of BTB transfer experiment: (a) run 1, straight channel and 50 vol% aqueous flow (i) color and distance change and (ii) slug merging at the outlet sorting tee, (b) run 2, straight channel and 23 vol% aqueous flow, (c) run 3, meandering channel and 50 vol% aqueous flow. The total flow rate was 10 mL/min, and the channel volume – 9.0 mL. Aqueous slugs were 15 mm long for (a,c); 7.5 mm long for (b). End caps of the slugs are highlighted for clarity.

Figure 5

BTB transfer mechanism: (a) experimental evaluation with 3 receiver slugs between 2 sender slugs; (i) entire configuration and, (ii)-(iv) closed-up images of receiver slugs taken at the fixed observation point of 70 cm from the inlet tee (total flow rate 10 mL min⁻¹, aqueous slugs length 15 mm, 50% aqueous fraction). (b) proposed scheme of “rear-to-rear” mass transfer.