(309j) Vapor-Phase Polymerization to Modify the Surfaces of Pre-Assembled Microfluidic Devices

We have modified the interior surfaces of pre-assembled poly(dimethylsiloxane) (PDMS) microfluidic devices with a fluorinated coating via initiated chemical vapor deposition (iCVD).  The vapor phase polymerization is a solventless process that eliminates reagent solubility issues and PDMS-solvent compatibility issues associated with liquid-phase polymerization.  The channel surfaces of multiple devices can be modified simultaneously inside a single reaction chamber.  Our fluorinated coating consists of 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) cross-linked with ethylene glycol diacrylate (EGDA), resulting in poly(PFDA-co-EGDA) that is mechanically stable and retains the properties of a fluorinated polymer.  All materials used in this process are commercially available and do not require specialized synthesis.  Through iCVD we achieved complete coverage while retaining channel geometry, device material elasticity, and flow performance.  We tuned the functionality and surface energy of the films by varying the cross-linking density to find an optimal concentration of EGDA that stabilizes the film while maintaining a low surface energy.  The coating method was optimized for a confined geometry and can be extended to a variety of other commercially available vinyl monomers to achieve other surface functionalities.  Additionally, this approach can be used to functionalize other materials commonly used in microfluidic devices, including polycarbonate, silicon, glass, and PMMA.

We have demonstrated that the cross-linked fluorinated coating completely inhibits PDMS absorption of Rhodamine B and significantly suppresses swelling of the PDMS in the presence of hexane.  While PDMS has been widely deployed in microfluidic applications, the major drawback of this material is its inherent permeability.  Native PDMS is known to swell in the presence of organic solvents and to absorb low-molecular-weight molecules from flow streams.  Eliminating these weaknesses allows organic synthesis reactions and analytical techniques involving concentration-sensitive solutions to be implemented in PDMS devices.

Fluorinated coatings can also be used to modulate the behavior of multiphase liquid flows. We have utilized the fluorinated coating to facilitate continuous droplet formation in a two-phase system with a large viscosity mismatch.  The dispersed phase is an imidazolium-based ionic liquid (IL) that serves as the solvent in the synthesis of Au and Ag nanocrystals; the continuous phase is a fluorocarbon carrier that separates the reactant droplets.  We have shown that droplet flow is uniquely suited to fabricating monodisperse, spherical nanocrystals in the IL reaction system.  The modified channel surface allows for stable breakup of the IL input stream in the squeezing, transient, and dripping droplet formation regimes.  Droplet breakup was examined as a function of flow rate ratio, liquid-liquid interfacial tension, fluid viscosity and viscosity ratio, and liquid-solid interfacial tension at the channel walls.  Each region of the droplet breakup phase diagram allows for different rates of mixing in the isolated volumes.  We have developed a novel method of on-device quenching to control the size of the nanocrystals.  Quenching is dependent on the rate of mixing; therefore, there is a correlation between nanoparticle size and the operating flow regime.  This analysis of droplet formation is important for extending the use of microfluidic devices to other novel IL solvents with broad applications to environmentally sustainable synthetic chemistry.