Introduction

Emulsions are a mixture of two or more liquids in which one is present as droplets, of microscopic or nanoscopic size, distributed throughout the other.

Emulsion drops are important in several every day and industrial contexts such as in foods (e.g. milk and butter), in the dyeing and tanning industries, in the manufacturing of synthetic rubber and plastics, in the preparation of cosmetics such as shampoos, in salves and pharmaceutical products, as well as in the petroleum industry for certain drilling muds, and for enhanced oil recovery.

Only recently has it become possible to produce monodisperse emulsions in research laboratories, using microfluidic devices for high throughput/control production. At the same time, research is ongoing on the single droplet level in order to control stabilizing agents structuring and dynamics for tailoring single droplets.

This can now enable production of designed emulsions, which not only in themselves could have new practical uses, but which can also be used as model systems in order to understand macroscopic emulsion properties. This in turn can enhance shelf-lives/quality and can also lead to new means of controlling flow/mechanical properties of emulsion products in general.

Most practical emulsions are formed from component liquids either spontaneously, or by means of mechanical agitation, provided that the liquids that are mixed have limited mutual solubility. Emulsions are stabilized by agents, which form films at the surface of the droplets (e.g., soap molecules, or colloidal particles ranging from the nano- to micro- sizes). The stability of emulsions (and thus the shelf-life and quality of emulsion products) depends on the type of stabilizing agent, and how this agent is processed onto the droplet interfaces

An emulsion in a microfluidic device is made by precisely fabricating one drop at a time. This process results in a highly monodisperse emulsion. One of the most attractive features of microfluidic techniques is that they enable the fabrication of double, triple, and even higher order emulsions, where the size and number of the encapsulated droplets can be manipulated with unprecedented accuracy.11,12

Drugs

Optical microphotograph of a water-in-oil-in-water emulsion prepared with a controlled-pore glass membrane. The median diameter of the microdroplets is 70 Ám.13