Microfluidic Device for Droplet Pairing by Combining Droplet Railing and Floating Trap Arrays

Abstract

Droplet microfluidics are characterized by the generation and manipulation of discrete volumes of solutions, generated with the use of immiscible phases. Those droplets can then be controlled, transported, analyzed or their content modified. In this wide droplet microfluidic toolbox, no means are available to generate, in a controlled manner, droplets co-encapsulating to aqueous phases. Indeed, current methods rely on random co-encapsulation of two aqueous phases during droplet generation or the merging of two random droplets containing different aqueous phases. In this study, we present a novel droplet microfluidic device to reliably and efficiently co-encapsulate two different aqueous phases in micro-droplets. In order to achieve this, we combined existing droplet microfluidic modules in a novel way. The different aqueous phases are individually encapsulated in droplets of different sizes. Those droplet populations are then filtered in order to position each droplet type towards its adequate trapping compartment in traps of a floating trap array. Single droplets, each containing a different aqueous phase, are thus paired and then merged. This pairing at high efficiency is achieved thanks to a unique combination of floating trap arrays, a droplet railing system and a droplet size-based filtering mechanism. The microfluidic chip design presented here provides a filtering threshold with droplets larger than 35 μm (big droplets) being deviated to the lower rail while droplets smaller than 20 μm (small droplets) remain on the upper rail. The effects of the rail height and the distance between the two (upper and lower) rails were investigated. The optimal trap dimensions provide a trapping efficiency of 100% for small and big droplets with a limited double trapping (both compartments of the traps filled with the same droplet type) of 5%. The use of electrocoalescence enables the generation of a droplet while co-encapsulating two aqueous phases. Using the presented microfluidic device libraries of 300 droplets, dual aqueous content can be generated in less than 30 min.

Keywords: droplet; microfluidics; pairing.

Figure 1

Droplet chip principle. The two aqueous phases are sequentially individually encapsulated in droplets from different sizes. The droplets are then filtered by size to position each droplet type towards its trapping location. The droplets are then trapped in pairs, each one containing a single droplet from each type. The droplet pairs are then merged.

Figure 2

Schematic of some steps performed in the device. (i) Droplet generation. Scheme of the 2D 90° droplet generation system with the aqueous inlet and the oil inlets. The width of the microchannels is indicated and the droplet length in the confined configuration is indicated as L. (ii) Droplet railing. Scheme of the droplet size filtering system. Top: time lapse of a big droplet shifting from the original rail to the wider rail. Bottom: time lapse of a small droplet remaining on the original narrow rail. (iii) Droplet trapping and pairing. The left scheme shows the droplet trapping and pairing area with 5 traps filled with 2 droplets of each type. The electrodes used for droplet merging are represented (in orange). The right scheme shows top and side views of a single droplet trap; the critical trap dimensions for an efficient droplet trapping are indicated in the scheme. The trap height and the rail height are identical.

Figure 3

Droplet rail filtering system. (A) (i) Time-lapse of a small droplet continuing on the narrow rail. (ii) Time-lapse of a big droplet being deviated to the wider rail (B) Graph of the ratio of deviated droplets as a function of the droplet radius for the experimental case presented in (A). The droplets either remain on the narrow rail (low-pass filtering) or are deviated to the wider rail (high pass filtering) according to their size. (C) Number of droplets sorted according to the droplet radius for different gap distances between rails (called d). The droplet threshold transfer is indicated in orange. (D) Number of droplets sorted according to the droplet radius for different rail heights. The droplet threshold transfer is indicated in orange. Videos are available in Videos S1 and S2.

Figure 4

Droplet trapping array. (A) Brightfield image of big droplets trapped. (B) Time-lapse of a big droplet being trapped in the second trap of the array. (C) Time-lapse of a small droplet being trapped at the 90th trap of the array. (D) Brightfield image of six traps containing a single droplet of each type (two different dyes). The white arrows indicate the trajectories of the droplets. (E) Histogram of the big droplet trapping efficiency in big traps and small traps (double trapping) as well as the small droplet trapping efficiency in small traps under the conditions of no rail, a single rail or two rails present, for the same trap dimensions (big trap diameter of 60 μm, small trap diameter of 40 μm and trap height of 40 μm). (F) Histogram of the small and big droplet (double trapping) trapping in small traps according to the small droplet trap diameter (big traps have a diameter of 60 μm and height of the traps 40 μm). Videos are available in the Video S3.