Double Emulsion Generation

An emulsion consists of liquid droplets dispersed in another, immiscible liquid, e.g. water droplets in oil or oil droplets in water. Emulsions are studied and applied in widely varying fields such as the food, medical, pharma, cosmetics, chemistry and agricultural industries. Microfluidics can be used to generate highly monodisperse emulsions by bringing together both liquids on the micron scale in a controlled way, and numerous designs for microfluidic droplet or emulsion generators have been described.

A solution for diverse applications

Hydrophilic chips are preferred for making oil-in-water (O/W) emulsions, while water-in-oil (W/O) emulsions are best prepared in a hydrophobic chip.

So depending on the chip material, a hydrophobic or hydrophilic nanocoating may be required to obtain the desired surface wettability. Beside ‘simple’ O/W or W/O emulsions, more complex combinations are also possible, e.g. double emulsions consisting of three alternating oil and water layers (i.e. O/W/O or W/O/W). For some applications, double emulsions have considerable benefits. For example, it has been shown that water-soluble drugs are released at a controlled rate from double emulsions over a longer period of time compared to single emulsions.

Microfluidic double emulsions

Microfluidic double emulsions are generally prepared in two steps, first making a single emulsion (say O/W), and then adding the second oil layer to yield an O/W/O double emulsion (right image).

As mentioned above generation of the first O/W emulsion requires a hydrophilic chip, while a hydrophobic chip is needed for the second emulsification step. This problem can be solved by using two separate chips (a hydrophobic and a hydrophilic one).

However, a more practical and elegant solution is to use local surface modification to apply a hydrophobic and/or hydrophilic nanocoating and make the right combination of hydrophobic and hydrophilic channels in a single microfluidic chip (left image).