Article pubs.acs.org/Langmuir
Assembly of Colloidal Silica Crystals Inside Double Emulsion Drops Kathryn Shirk,† Colton Steiner,† Jin Woong Kim,‡ Manuel Marquez,§ and Carlos J. Martinez*,† †
School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States Department of Applied Chemistry, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 426-791, Korea § YNano LLC, Midlothian, Virginia 23113, United States ‡
S Supporting Information *
ABSTRACT: We investigated the assembly of colloidal silica crystals inside double emulsion drops generated in microcapillary microfluidic devices. The double emulsions are composed of an aqueous suspension of monodisperse silica particles in the inner drop surrounded by a PDMS oil drop that acts as a semipermeable membrane for the diffusion of water into or out of the inner drop in the presence of an osmotic gradient. Imposing a high osmotic pressure in the continuous phase induces water diffusion out of the inner drop, increasing the silica volume fraction (ϕsilica) and leading to the formation of a spherical colloidal silica crystal. Silica suspensions with no salt or low salt concentration ( 10−3 M, the electrostatic repulsion is reduced, and crystallization is suppressed. Crystals were preserved in a hydrogel matrix or inside a silicone rubber shell. This study demonstrates a robust path for controlled colloidal assembly inside double emulsion drops.
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INTRODUCTION Colloidal assembly in liquids is of great interest both from a fundamental soft matter physics point of view and for technological applications such as ceramics processing,1 pharmaceutical products,2 and coatings.3 Within this area, there is a growing interest in the assembly of colloidal particles from single and double emulsion drops to form highly uniform crystals and granules for applications in solar cells,4 immunoassays,5 and microlenses for light emitting diodes (LEDs).6 Early studies were performed by fabricating millimeter-size suspension drops on fluorocarbon oil7 or superhydrophobic surfaces.8 In these systems, the liquid in the drops is evaporated under ambient conditions, increasing the solids fraction until maximum packing (ϕmax) is reached. While the drop fabrication methodology used in these studies is simple, controlling both the drying process and drop generation is challenging and requires more advanced generation techniques such as those offered in microfluidic devices. Microfluidic devices allow the generation of single and double emulsion drops with sizes ranging from
