# Difference between revisions of "High-Order Multiple Emulsions Formed in Poly(dimethylsiloxane) Microfluidics"

Birgit Hausmann

## Keywords

microfluidics, multiple emulsions, photoresponsive materials, sol–gel processes, wettability

## Overview

Multiple emulsions are nested sets of drops.[1,2] Drops of one kind of fluid are encapsulated inside drops of a second fluid, which themselves can be encapsulated inside drops of yet another fluid. Multiple emulsions are typically made in bulk using shear cells or porous membrane plates.[6] To form the multiple emulsions, the drops from one emulsification are fed back into the apparatus with additional fluids and are emulsified again. method poorly controlled, polydisperse emulsions

## Results and Discussion

Fig. 1
Fig. 2

With glass microcapillary devices, monodisperse multiple emulsions can be formed with controlled structure, albeit in much smaller quantities. but difficult to fabricate. A superior method would combine the control of microfluidic drop formation with increased scalability. method here combines the control of microfluidic drop formation with the scalability of lithographically fabricated devices.[10] We use linear arrays of poly(dimethylsiloxane) (PDMS) drop makers with alternating wettability. The precision of the fabrication allows us to carefully engineer the channels to optimize drop formation; this allows us to produce monodisperse drops of a controlled size. scale up to form quintuple emulsions. To form a water-in-oil single emulsion, we use a single drop maker with uniform hydrophobic wettability.[11] For the drop formation junction, we use pinned-jet flow focusing (PJFF). To form a double emulsion, we require two drop makers functionalized to have opposite wettability. the first hydrophilic and the second hydrophobic. to perfectly synchronize the devices, we hydrodynamically couple them using triggered drop formation. We design each nozzle 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 Weconfine our drops in a monolayer by sandwiching them between two plates that are 50mmapart. diameter distribution is narrow, with coefficient of variation (CV) of 2%, Similarly, the triple, quadruple, and quintuple emulsions all pack hexagonally because all are monodisperse

The emulsions pack hexagonally because they areconfinedinamonolayerandmonodisperse.

Experimental Section Preparation of devices: The devices are fabricated using softlithography in PDMS. [10] 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 [11] 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. Device operation: We form the multiple emulsions by injecting water and fluorocarbon oil with surfactants into the linear drop maker arrays. With the single emulsion device we form water-in-oil (W/O) single emulsions by injecting water into the first inlet and oil into the second inlet at 200 mL h�1 and 400 mL h�1, respectively. With the double-emulsion device we form O/W/O double emulsions by injecting the fluids into the first, second, and third inlets at 200, 400, and 600 mL h�1, respectively. With the tripleemulsion device we form W/O/W/O quadruple emulsions by With the quintuple-emulsion device we form W/O/W/O/W/O quintuple emulsions by injecting the fluids into the first, second, third, fourth, fifth and sixth inlets at 200, 400, 600, 800, 1400, and 2500 mL h�1, respectively.

The advantage of the presented process is that the infiltration of a preassembled porous structure is avoided, thus preventing the film from cracking during the drying step since cracking in "[...]conventional opal and inverse opal films tends to occur upon drying both at the colloidal assembly stage and at the infiltration stage due to a combination of dehydration and/or polymerization-induced contraction and associated local capillary forces". Indeed, the fabrication technique consists only of two steps (Fig. 1): 1) Polymer colloids (e.g. polystyrene (PS) or poly methyl methacrylate (PMMA)) assemble in a sol-gel precursor solution (e.g. $\mathrm {Si(OH_4), Ti(OH_4), Ge(OH_4)}$) during an evaporative deposition resulting in an opal film. 2) The template is removed. If the PMMA spheres are deposited from a hydrolyzed tetraethoxy silane (TEOS) solution, the interstitial spaces of the polymeric opal film are filled with silica gel matrix material which results in an inverse opal silica structure (I-$\mathrm {SiO_2}$). In that way defects as cracking, formation of domain boundaries and colloidal vacancies which are a main problem in conventionally assembled films can be omitted over a cm length scale.

Several observations were made: There's a critical TEOS-to-colloid ratio (approximately 0.15 mL TEOS solution per 20 mL PMMA suspension) beyond which crackfree films can be expected. Figure 2 shows a highly ordered crackfree film which was fabricated in optimized conditions: 1) suspending a vertically oriented glass slide in a mixture comprised of 0.15 mL of a 28.6 wt% TEOS solution with a 20 mL suspension of 280 nm diameter PMMA spheres (approximately 0.125 vol%), 2) allowing the solvent to slowly evaporate at 65 °C (deposition rate = 2 cm/day), and 3) treating the composite structure at 500 °C for 5 h in air.

The film thickness (number of layers) is linearly proportional to the colloidal volume fraction for a given TEOS-to-PMMA ratio. There were no crack formation observed beyond a 20 layers. For thicker films crack free domains were as large as 100$/mu m$ (Fig. 3A).

The growth direction was found to be along <110> direction as opposed to conventional film growth along <112> direction (Fig. 3C) and shows a fcc structure. Adding the silicate in the process seems to change the preferential growth direction along <110> and induces a "self-healing" of domains with a temporary different growth direction. The highest pore density in fcc films occurs along {111} planes where crack formation is most probable, which is confirmed for films with a thickness < 5$/mu m$ (when cleaved). But for films > 5$/mu m$ the formation occurs along <110> planes.

When explaining the effect of this new crackfree formation of films the authors refer to the phenomenon of the formation of large single crystals of calcite patterned at the micron scale: "[...]the transformation of the amorphous $\mathrm {CaCO_3}$ film into a defect-free, porous calcite crystal is facilitated by the micropatterned substrate that not only determines the pattern of porosity of the final single crystal, but also provides interfacial sites for stress and impurity release during the amorphous-to-crystalline transition. Similarly in the current system, the formation of a colloidal crystal and the associated interfaces between the polymerizing sol-gel solution and the assembling colloidal spheres may provide sites for the relaxation of tensile stresses encountered during the gelation process. Controlled solvent release during the polycondensation reaction can also occur at these interfaces and be channeled through the interconnected porous network to evaporate at the surface."

This evaporative deposition is also suitable for multifilm inverse opal structures of different pore sizes as well as deposition on curved surfaces (Fig. 4). The multifilm is created via multiple depostions of template/matrix layers.

The I-$\mathrm {SiO_2}$ films were also replicated in Si/MgO, Si and $\mathrm {TiO_2}$ while maintaining the porous structure and minor cracking in some cases.