Water in Oil Emulsions
Water in oil emulsions comes from the mixing of oil as a continuous phase and water as a dispersed phase to form colloidal systems. Mixing is done mostly using external forces, done mechanically using a magnetic stirrer or using ultrason. The first step in emulsion and formulation is the production of water droplets. In order to keep production stable, specific a.d dedicated agents called “surfactants” are added in the continuous phase to provide good emulsion stability. By using a dedicated system with efficients surfactant, it’s possible to create a small size of dispersed oil droplet. Encapsulation of active ingredients inside these small water droplets increases bioavailability and also the shelf life of food, beverages and drug formulation.
Oil does not mix with water under normal conditions, butut with proper mixing and using stabilizing agents, we can obtain oil in water emulsion. The effectiveness of this system is enhanced with a small size of dispersed oil droplets. It increases the bioavailability of pharmaceutical products, and it also increases the shelf life of food and beverages.
Water in oil emulsions (W/O) form the basis manufacturing techniques widely used in the industrial and R&D environments to manufacture droplets (e.g., for compartmentalization applications), hydrogel beads (e.g., alginate, agarose, or polyacrylamide), and polymer beads (e.g., acrylic, vinyl, and ethyl-based). Of particular importance is the ability to produce high-quality, monodisperse droplets and the ability to do so reproducibly and at a viable production rate.
The combination of Fluigent pressure pump units and Raydrop microfluidic devices,developed and manufactured by Secoya, enable smooth fluid delivery, precision flowrate control, automation, and the reproducibility necessary to generate high-quality water in oil emulsions.
Introduction to Water In Oil Emulsion
Emulsions are colloidal systems consisting of two liquid phases, oil and water, one of which is dispersed into the other. Water-in-oil (W/O) emulsions consist of an aqueous phase dispersed, in the form of small droplets, into a continuous oil phase. Water in oil emulsions have a high potential in cosmetic, biological, pharmaceutical, agricultural, and food industries. Their structure is suitable for the delivery of hydrophilic compounds, which, in turn, may bring different functions to the emulsified system, such as antimicrobial and antioxidant activities.
In cell biology, compartmentalization of biological agents in discrete aqueous droplets dispersed in an oil phase is a popular alternative to the microwell strategy. Water-in-oil emulsion (W/O) has an aqueous volume from femtoliter to nanoliter, each representing an isolated micro-reactor or micro-incubator. The maximized surface-to-volume ratio of the micro-droplets enables highly efficient mass and heat transfers between the internal and external phases, resulting in precise and rapid perturbation of the microenvironment in the droplet.
Water in oil emulsions produced with bulk emulsion techniques are not uniform in size. Another limitation of bulk emulsion droplets is that multistep processing of droplets is difficult, although some strategies for reagent delivery such as nanodroplet fusion, uncaging of substrates, and adding of hydrophobic substrates through the oil phase are possible. This is why microfluidic devices were created, where up to 10,000 highly monodisperse aqueous droplets per second generate in a continuous oil phase. Biocompatible surfactant oil formulations have been developed to prevent droplet coalescence, allow oxygen diffusion, and prevent molecules leaking out into the oil phase. Water in oil emulsions can be divided, fused, incubated, analysed, sorted and broken up. Integration of these steps with control over timing can potentially create a system for biological experimentation with a level of control akin to experiments on the macroscopic scale.
The choice of microfluidics for droplet ‘management’ also allows access to typical advantageous engineering features of this format, e.g. the potential for automatisation, the low cost of microfluidic devices and improved heat.
Water in oil emulsions: Materials and methods
Reagents
Droplet Phase: Water (Mili Q)
Continuous phase: Decane + 2% (wt) SPAN 80
| Reagent | Supplier | Catalogue number | CAS Number |
| Water | Ultrapure 18.2 MΩ – cm | – | 7732-18-5 |
| Decane | Sigma Aldrich | D901 | 124-18-5 |
| SPAN 80 | Sigma Aldrich | 8.40123 | 1338-43-8 |
Microfluidic Setup
The microfluidic setup was composed of:
How to make water in oil emulsions?
Protocols steps
With the use of Fluigent pressure-based flow controller units and the Raydrop microfluidic device (figure 1), we have generated water in oil emulsions with control of particle size by adjusting the flow of the continuous phase and the dispersed phase.
Results
| Continuous phase flowrate (μl/min) | Droplet phase flowrate (μl/min) | Droplet diameter (μm) | Production rate (Hz) |
| 100 | 5 | 71 | 445 |
| 100 | 10 | 75 | 754 |
| 100 | 15 | 87 | 725 |
| 50 | 5 | 85 | 259 |
| 50 | 10 | 91 | 422 |
| 50 | 15 | 92 | 613 |
| 25 | 5 | 89 | 225 |
| 25 | 10 | 93 | 396 |
| 25 | 15 | 94 | 575 |
| 15 | 15 | 94 | 575 |
Figure 4: Images of water droplets in decane generated using Fluigent equipment and Raydrop microfluidic device
Conclusion
Fluigent pressure based flow controller units and Raydrop microfluidic devices were successfully used to generate high-quality, monodisperse droplets of water in hydrocarbon oil. The droplet size was controlled in the range of 75 – 94 μm by adjusting the continuous and dispersed phase flowrates. The peak stable droplet production rate was recorded for 75 μm droplets at 754 Hz. The production techniques developed here can be extended to the generation of hydrogel, protein, or polymer beads by adding appropriate post-processing steps.