High-Order Multiple Emulsions Formed in Poly(dimethylsiloxane) Microfluidics
microfluidics, multiple emulsions, photoresponsive materials, sol–gel processes, wettability
Droplets encapsulated multiple times in droplets of alternating kinds of fluids (oil, water) were emulsified in a highly controlled way. PDMS microcapillary devices were used to guarantee monodispersity of higher order emulsions, at the expense of large quantity formation.
Results and Discussion
he control of microfluidic drop formation with the scalability of lithographically fabricated devices A single emulsion of water droplets in fluorocarbon oil (w/o) is formed by injecting water at 200mL/h in the first inlet of a microtube and oil in a second inlet at 400mL/h (Fig. 1a). The single drop maker has uniform hydrophobic wettability. To form a double emulsion of o/w/o a third inlet is added to the linear drop maker where the fluid is injected at 600mL/h (Fig. 1b). By adding even more inlets and synchronizing the fluid speeds at each inlet even triple, quadruple and quintuple emulsion were formed (Fig. 1c-e).
In that way monodisperse higher order emulsion can be formed, which all pack hexagonally. Since the microcapillary devices fabrication is very difficult the scalability of the emulsification process is restricted. Linear arrays of poly(dimethylsiloxane) (PDMS) drop makers with alternating wettability were fabricated such that drops form from each channel. The nozzle is desgined such that it is slightly narrower than the incoming emulsion from the previous drop maker: "This allows the incoming emulsion to obstruct the nozzle, perturbing flow, and triggering the formation of the outer drop."
A superior method would combine the control of microfluidic drop formation with increased scalability. method here . We use
Weconfine our drops in a monolayer by sandwiching them between two plates that are 50mmapart. Preparation of devices: The devices are fabricated using softlithography in PDMS.  All devices are fabricated at a fixed channel height of 50 mm. The PDMS devices are bonded to a glass plate using oxygen-plasma treatment To spatially control wettability, the devices are coated with a photoreactive sol–gel within 15 minutes after plasma bonding. The devices are filled with the photoreactive sol–gel mixture and heated with a hotplate set to 225 8C; this vaporizes the solvent in the mixture and deposits the coating. The coating makes the channels hydrophobic by default; to spatially pattern wettability, we graft patches of hydrophilic polyacrylic acid onto the interface using utraviolet (UV) light-initiated polymerization. To accomplish this we fill the coated channels with the hydrophilic monomer solution and expose them to spatially patterned UV light. When exposed to light, the photoinitiator silanes embedded in the sol–gel release radicals that initiate polymerization of the acrylic acid monomers in solution. The resulting acrylic acid polymers are grafted to the sol–gel interface, tethered by covalent linkages with the photoinitiator silanes. This results in a dense covering of polyacrylic acid of the interface, making it very hydrophilic, suitable for forming oil-in-water emulsions.