Stability of microfluidic emulsions

Emulsion-like systems are used in every facet of modern life and much is known about their underlying stability, transport kinetics and interfacial structure making them excellent starting points for producing complex materials. Emulsions, dispersions of two or more immiscible liquids appear in foods, pharmaceutics, cosmetics, paints and pesticides. Unfortunately, emulsions are inherently unstable due to both internal and external forces acting on the droplets. Breakdown of emulsions can be caused by external stimuli including gravity, centrifugal forces, electrostatic or magnetic fields but the emulsion is fundamentally unstable without these influences due to internal forces at both inter-droplet and intermolecular levels.

Enhancing the stability of an emulsion can be achieved by the addition of "stabilisers" such as surfactants or surface-active polymers. These reduce the interfacial energy of the oil-water interface which helps slow (but does not prevent) breakdown of the emulsion. Ostwald ripening, the process whereby small droplets shrink and large droplets grow as material diffuses between droplets across the continuous phase can be prevented by having all droplets the same size. Microfluidic techniques (such as the flow-focussing chip shown above) are able to produce emulsions with very narrow droplet size distributions and the droplet sizes can be controlled by varying the feature sizes in the chips, the flow rates of the two fluids and the viscosities of the fluids.

With these techniques, it is possible to prepare droplets that are stable on the bench for years, as long as they are not heated or agitated. With a moderate amount of mild chemistry, it is possible to coat these droplets and further improve their stability.