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).