Reducing the shell thickness of double emulsions using microfluidics

Double emulsion drops are well-suited templates to produce capsules whose dimensions can be conveniently tuned by adjusting those of the drops. To closely control the release kinetics of encapsulants, the composition and thickness of the capsule shell must be precisely tuned; this is greatly facilitated if the shell is homogeneous in its composition and thickness. However, the densities of the two drops that form the double emulsion are often different, resulting in an offset of the two drop centers and therefore in an inhomogeneous shell thickness. This difficulty can be overcome if the shell is made very thin. Unfortunately, a controlled fabrication of double emulsions with thin shells is difficult. In this paper, we present a microfluidic squeezing device that removes up to 93 vol% of the oil from the shell of water–oil–water double emulsions. This is achieved by strongly deforming drops; this deformation increases their interfacial energy to sufficiently high values to make splitting of double emulsions into double emulsions with a much thinner shell and a single emulsion oil drop energetically favorable. Therefore, we can reduce the shell thickness of the double emulsion down to 330 nm. Because this method does not rely on solvent evaporation, any type of oil can be removed. Therefore, it constitutes a new way to produce double emulsions with very thin shells that can be converted into thin-shell capsules made of a broad range of materials.