Multiple emulsion droplet generation

Introduction to multiple emulsion

Traditionally, batch methods are used to produce emulsions in industry. The use of bulk mixing allows one to produce large quantities of emulsions but to the detriment of quality. The particle size distribution in a bulk mixer is large leading to numerous particles sizes and a low encapsulation rate  of Active Pharmaceutical Ingredient (API) or double emulsion production (core-shell particles)¹. Batch methods make the control of multiple emulsions more complex, i.e. the encapsulation of a precise number of droplets of liquid A in a droplet of liquid B.

The use of microfluidics provides monodispersed emulsions with  high control over both the size and structure can be obtained². Microfluidic tools are also used to create emulsions of varying compositions. It is possible to produce water–in-oil–in-water (W/O/W) emulsions or oil–in-water–in-oil (O/W/O) emulsions. A microfluidic device developed by Secoya Technologies – called the RayDrop – allows one to easily produce highly controlled emulsions. Examples of applications in single and double emulsion can be found on our website.

Following the work of Li, E. Q. et al ³, the experts of Secoya Technologies demonstrate the ability to produce multiple emulsions using the RayDrop. These multiple emulsions are precursors in the creation of solid microcapsules used for triggered release¹. These multicompartmental microspheres are also of interest to co-encapsulate incompatible solutions (which would react if they were in contact)⁴.

In this Application Note, aqueous droplets are obtained using the combination of two RayDrop devices placed in series. The influence of the fluidic parameters on the number of cores contained in the oil shell is underlined in this application note.

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 droplet phase, leading to the formation of droplets with a high monodispersity.

In this case, two RayDrop are placed in series. The first is placed on the platform as usual and the second is added after, as shown in Figure 2. To control the formation of droplets inside the second RayDrop, an additional camera and an additional LED light source are used. 

Droplets formed in RayDrop 1 (R1) are transported as the “shell” phase (see Figure 4) in RayDrop 2 (R2). Here, the “shell” phase is pinched by the continuous phase, leading to the formation of a shell droplet encapsulating multiple core droplets. 

Nozzle information

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

Results

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 while for a core flow rate of 2.3µl/min, most droplets contain four cores.