# High-Order Multiple Emulsions Formed in Poly(dimethylsiloxane) Microfluidics

Birgit Hausmann

## Keywords

microfluidics, multiple emulsions, photoresponsive materials, sol–gel processes, wettability

## Overview

Droplets encapsulated multiple times in droplets of alternating kinds of fluids (oil, water) were emulsified in a highly controlled way. PDMS microcapillary devices were used to guarantee monodispersity of higher order emulsions, at the expense of large quantity formation.

## Results and Discussion

Fig. 1 Ordered droplets (water and oil, alternating) formed by linear drop maker arrays. Photomicrographs of a) single, b) double, c) triple, d)quadruple, and e)quintuple emulsion drop maker arrays.The multiple emulsions produced by the arrays are shown to the right. (The scalebars are 100mm.)
Fig. 2 Hexagonally packed a) single, c) double, e) triple, g) quadruple, and i) quintuple emulsions in a monolayer. The diameter distributions diameter distribution (coefficient of variation (CV) of 2%) are shown for the b) single, d) double, f) triple, h) quadruple, and j) quintuple emulsions; the distributions for the outer drops and each of the nested inner drops are plotted individually.

Microfluidic drop formation of monodisperse emulsions in monolayers were combined with the scalability of lithographically fabricated devices. A single emulsion of water droplets in fluorocarbon oil (w/o) is formed by injecting water at 200mL/h in the first inlet of a microtube and oil in a second inlet at 400mL/h (Fig. 1a). The single drop maker has uniform hydrophobic wettability. To form a double emulsion of o/w/o droplets a third inlet is added to the linear drop maker where the fluid is injected at 600mL/h (Fig. 1b). By adding even more inlets and synchronizing the fluid speeds at each inlet even triple, quadruple and quintuple emulsion were formed (Fig. 1c-e). Droplets are confined in between two plates that are 50 $\mu m$ apart to guarantee a monolayer formation. Linear arrays of poly(dimethylsiloxane) (PDMS) drop makers with alternating wettability were fabricated such that drops form from each channel. The nozzle is desgined such that it is slightly narrower than the incoming emulsion from the previous drop maker: "This allows the incoming emulsion to obstruct the nozzle, perturbing flow, and triggering the formation of the outer drop."

In that way monodisperse higher order emulsion can be formed, which all pack hexagonally. Since the microcapillary devices fabrication is very difficult the scalability of the emulsification process is still restricted. Devices in PDMS were coated with a photoreactive sol-gel mixture which provides hydrophilic channels where exposed with UV light and hydrophobic channel parts where not exposed. In that way the devices were fabricated using softlithography. Hydrophilic channels are suited to form oil-in-water emulsions.