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

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Birgit Hausmann
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Birgit Hausmann [[Image:Abate2009 4.jpg|350px|thumb|right|]]
 
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Birgit Hausmann
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== Reference ==
 
== Reference ==
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== Keywords ==
 
== Keywords ==
Coassembly, colloidal assembly, crack-free films, inverse opals, nanoporous
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microfluidics, multiple emulsions, photoresponsive materials, sol–gel processes, wettability
  
 
== Overview ==
 
== Overview ==
A new synthesis of crack-free inverse opal films over cm length scales is presented. The two step process consists of a) a coassembly during a evaporative deposition of polymeric colloids in a hydrolyzed silicate sol-gel precursor solution (colloidal/matrix coassembly) and b) template  removal. The preferential grwoth direction is <110>. The synthesis of multilayered hierarchical films are also demonstrated. Furthermore, the inverse opal films were replicated in other materials as porous Si and <math>\mathrm{TiO_2}</math> while maintaining their morphology during the gas/solid displacement reaction.
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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 ==
 
== Results and Discussion ==
[[Image:Hatton2010 1.png|200px|thumb|left|'''Fig. 1''' Schematic for inverse opal synthesis: 1) Colloids assemble from a sol-gel solution 2) template removal]] [[Image:Hatton2010 2.png|500px|thumb|right|'''Fig. 2''' Highly ordered I-<math>\mathrm{SiO_2}</math> films formed from PMMA/sol-gel coassembly (Left scale bar is <math>10\mu m </math> and right scale bar is <math>1\mu m </math>)]]
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[[Image:Abate2009 1.png|600px|thumb|right|'''Fig. 1''' Ordered droplets (water and oil, alternating) formed by linear drop maker arrays. Photomicrographs of a) single, b) double, c) triple, d)quadruple, and e)quintuple emulsion drop maker arrays.The multiple emulsions produced by the arrays are shown to the right. (The scalebars are 100mm.)]]
[[Image:Hatton2010 3.png|500px|thumb|right|'''Fig. 3''' A) The film thickness is directly proportional to the colloidal concentration. The threshold thickness for cracking is indicated. B) A 1.5cm I-<math>\mathrm{SiO_2}</math> film. C) A cleaved film reveals the growth direction along <110>. Inset: fcc-lattice model]]
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[[Image:Abate2009 2.png|600px|thumb|right|'''Fig. 2''' Hexagonally packed a) single, c) double, e) triple, g) quadruple, and i) quintuple emulsions in a monolayer. The diameter distributions diameter distribution (coefficient of
[[Image:Hatton2010 4.png|250px|thumb|right|'''Fig. 4''' <math>SiO_2</math> inverse opal structures formed by colloidal coassembly. Schematics (left) vs. SEM images (right). (A) Synthesis of multilayered, hierarchical films with different pore sizes by successive layer
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variation (CV) of 2%) are shown for the b) single, d) double, f) triple, h) quadruple, and j) quintuple emulsions; the distributions for the outer drops and each of the nested inner drops are plotted individually.]]
deposition prior to template removal. (The top left and bottom left SEM images show the interface
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between layers before and after calcination, respectively.) (B) <math>\mathrm{SiO_2}</math> structures grown on topologically patterned substrates (Left), SEM fractured cross section of inverse opals grown in 4 μm wide, 5 μm deep channels on a Si substrate (Right). (C) Coassembly onto curved surfaces (Left), and SEM images (Right) of a <math>\mathrm{SiO_2}</math> inverse opal film layer (shown magnified, Inset) deposited
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onto a sintered, macroporous Ti scaffold structure.]]
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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. <math> \mathrm {Si(OH_4), Ti(OH_4), Ge(OH_4)}</math>) during an evaporative deposition resulting in an opal film. 2) The template is removed.
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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-<math> \mathrm {SiO_2}</math>). 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.
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Several observations were made:
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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.
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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<math>/mu m</math> (Fig. 3A).
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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.
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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<math>/mu m</math> (when cleaved). But for films > 5<math>/mu m</math> the formation occurs along <110> planes.
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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 <math> \mathrm {CaCO_3}</math> 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."
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Microfluidic drop formation of monodisperse emulsions in monolayers were combined 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 droplets 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). Droplets are confined in between two plates that are 50 <math>\mu m</math> apart to guarantee a monolayer formation.
  
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.  
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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."
  
The I-<math> \mathrm {SiO_2}</math> films were also replicated in Si/MgO, Si and <math> \mathrm {TiO_2}</math> while maintaining the porous structure and minor cracking in some cases.
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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 still restricted.
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Devices in PDMS were coated with a photoreactive sol-gel mixture which provides hydrophilic channels where exposed with UV light and hydrophobic channel parts where not exposed. In that way the devices were fabricated using softlithography. Hydrophilic channels are suited to form oil-in-water emulsions.

Latest revision as of 23:47, 20 September 2010

Birgit Hausmann
Abate2009 4.jpg

Reference

A. R. Abate and D. A. Weitz "High-Order Multiple Emulsions Formed in Poly(dimethylsiloxane) Microfluidics" Small 5(18), 2030-2032 (2009)

Keywords

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

Overview

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

Fig. 1 Ordered droplets (water and oil, alternating) formed by linear drop maker arrays. Photomicrographs of a) single, b) double, c) triple, d)quadruple, and e)quintuple emulsion drop maker arrays.The multiple emulsions produced by the arrays are shown to the right. (The scalebars are 100mm.)
Fig. 2 Hexagonally packed a) single, c) double, e) triple, g) quadruple, and i) quintuple emulsions in a monolayer. The diameter distributions diameter distribution (coefficient of variation (CV) of 2%) are shown for the b) single, d) double, f) triple, h) quadruple, and j) quintuple emulsions; the distributions for the outer drops and each of the nested inner drops are plotted individually.

Microfluidic drop formation of monodisperse emulsions in monolayers were combined 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 droplets 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). Droplets are confined in between two plates that are 50 <math>\mu m</math> apart to guarantee a monolayer formation.

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."

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 still restricted. Devices in PDMS were coated with a photoreactive sol-gel mixture which provides hydrophilic channels where exposed with UV light and hydrophobic channel parts where not exposed. In that way the devices were fabricated using softlithography. Hydrophilic channels are suited to form oil-in-water emulsions.