Encapsulation of multiple emulsions in a single droplet

Introduction to encapsulation of multiple emulsions

Traditionally, multiple emulsions are produced using batch methods, with a 2-step mixing of immiscible phases: a vigorous mixing followed by a gentle one. This method is straightforward and enables the production of large batches, but is limited by a wide size distribution. This leads to numerous particle sizes and a low encapsulation rate of Active Pharmaceutical Ingredient (API) or double emulsion production (core-shell particles)¹.

Microfluidics emerged in the 2000’s as a powerful tool to overcome the limitations of batch methods as it allows a fine control over the size, monodispersity, and  structure of droplets². In particular, microfluidics is helpful when it comes to generating complex emulsions such as water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O) emulsions.

Based on the work of Li, E. Q. et al ³, a new tool called the Raydrop that had  the ability to make controlled encapsulations of multiple emulsions was demonstrated by the scientific experts at Secoya technologies.These devices, which enable multiple applications that create single and double emulsions, are of particular interest for the creation of solid microcapsules used for triggered release¹, or to co-encapsulate incompatible and cross-reactive solutions⁴.

We demonstrate in this application note that we can use two Raydrops placed in series to encapsulate multiple aqueous core droplets inside oil shell droplets with a precise control over droplet number and size.

Fluid reservoirs

RayDrop design

The RayDrop is a microfluidic droplet generator based on the alignment of two glass capillaries: the first one is terminated by a 3D printed nozzle and injects the droplet phase in the second one. At the junction of both capillaries, the continuous phase filling the cavity pinches the jet of the droplet phase, leading to the formation of droplets with a high monodispersity.

In this case, two RayDrop systems are placed in a series in order to perform an encapsulation of multiple emulsions. A first device R1 is mounted on the platform as done for simple emulsion generation, and a second Raydrop R2 is added immediately after (see figure 2). To efficiently control the formation of droplets inside the second RayDrop, an additional optical module (LED + camera) is placed on the platform.

The oil phase, containing the aqueous “core” droplets encapsulated in the R1 device, is transported as the “shell” phase (see Figure 4) towards the R2 device. Here, the oil phase is pinched by the continuous phase, which leads to the formation of monodisperse oil droplets, each containing multiple aqueous cores. The number of cores in each droplet can be controlled by tuning the flow-rates.

Nozzle information

The range of droplet sizes formed depends on the dimension of both the injection nozzle and the collection capillary. The dimensions used for the encapsulation of multiple emulsions in this application note are reported below.

Partial results of the encapsulation of multiple emulsions

The number of cores in a single droplet depends on the flow rate of the core phase.

During the formation of the multiple emulsions in RayDrop R2, the flow rates of the shell and of the continuous phases are kept fixed. Thus, only the flow rate of the core phase is changed. By increasing the core flow rate, the number of encapsulated core droplets increases. For a core flow rate of 0.5µl/min, most droplets contain one core, for a core flow rate of 2.3µl/min, most droplets contain four cores.