An emulsion is a mixture of two immiscible liquids, in which one liquid is dispersed in the form of droplets in the other liquid that forms a continuous phase. Emulsion droplets have become an indispensable research tool in many scientific fields, such as macromolecule delivery, oil recovery, food processing, and hazardous material handling. Microfluidics technology has the ability to create three-dimensional flow patterns, which can achieve precise control of the mixing of immiscible and miscible fluids.
Figure.1 Overview of some of the applications for controlled emulsions made using microfluidics. (Shah R. K, et al. 2008)
In contrast to the bulk emulsification method, the emulsion in the microfluidic chip is made by precisely making one drop at a time, a process that produces a highly monodispersed emulsion. The use of microfluidic chip technology can produce double, triple, or even higher-order emulsions, and the size and number of droplets can be manipulated with unprecedented precision.
Single Emulsion in Microfluidic Devices
Single emulsion is the simplest form of emulsion, including oil-in-water (o/w) and water-in-oil (w/o). It consists of droplets dispersed in different fluids, usually stabilized with surfactants.
We use the microfluidic continuous flow method to generate monodisperse droplet emulsions of consistent size, fluid mixing, and concentration. Different formation techniques correspond to different desired results:
Segmented Flow-The size of the formed block is larger than the knot size.
Flow Focusing-The formation of droplets in a wider size range relative to the junction size.
Mixing of Multiple Reagents-Droplets can be used as a small reactor to achieve a high degree of chemical control or to form gel beads.
Double Emulsion in Microfluidic Devices
Double emulsions are also called emulsion liquid dispersion systems, including oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O). It is widely used in various applications such as drug delivery, cosmetics and food production.
The microfluidic generation of the double emulsion requires that the flow channels formed by the droplets have hydrophobicity and hydrophilicity on separate surface parts. In order to produce precise double emulsions, we choose two main microfluidic technologies:
Direct Interface - Using two chips placed in series. The first emulsion is produced in the first chip and then encapsulated in a stream of droplets in the second chip, and the second chip is attached and fluidly sealed to the first chip.
Indirect Interface - This is similar to the above method, but the two chips are separated by a tube, which is more suitable for generating multiple emulsions.
Multiple Emulsion in Microfluidic Devices
The encapsulation of drug molecules or active pharmaceutical ingredients (API) in PLGA particles is usually used to create specific particles for personalized medicine, API solubilization and controlled release. The droplet microfluidic technology is a very effective tool for producing PLGA particles with high monodispersity and size uniformity.
Our microfluidic droplet production method for polymer multiple emulsions is a two-step continuous process:
- Emulsify the aqueous solution in the organic continuous phase
- Emulsify the primary emulsion in the second water phase
The result is a water-in-oil-in-water emulsion, and once formed, the emulsion is dried to produce monodisperse PLGA microparticles containing a plurality of aqueous droplets. By adjusting the flow rate during any continuous process stage, the size and production throughput can be easily adjusted.
- Shah R. K, et al. (2020). "Designer Emulsions Using Microfluidics." Materials Today. 20: 11(4): 18-27.
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