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

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== Overview ==
 
== Overview ==
Multiple emulsions are nested sets of drops.[1,2] Drops of one
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Droplets encapsulated in droplets of alternating kinds of fluids (oil, water) were emulsified in a highly controlled way.  
kind of fluid are encapsulated inside drops of a second fluid,
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Glass microcapillary devices were used to produce higher order monodispersed emulsions.  
which themselves can be encapsulated inside drops of yet
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another fluid.
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Multiple emulsions are typically made in bulk
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using shear cells or porous membrane plates.[6] To form the
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multiple emulsions, the drops from one emulsification are fed
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back into the apparatus with additional fluids and are
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emulsified again. method poorly controlled, polydisperse emulsions
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== Results and Discussion ==
 
== Results and Discussion ==
 
[[Image:Abate2009 1.png|600px|thumb|left|'''Fig. 1''']]
 
[[Image:Abate2009 1.png|600px|thumb|left|'''Fig. 1''']]
 
[[Image:Abate2009 2.png|500px|thumb|right|'''Fig. 2''']]
 
[[Image:Abate2009 2.png|500px|thumb|right|'''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. <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.
 
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.
 
 
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<math>/mu m</math> (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<math>/mu m</math> (when cleaved). But for films > 5<math>/mu m</math> 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 <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."
 
 
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-<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.
 

Revision as of 23:51, 18 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 in droplets of alternating kinds of fluids (oil, water) were emulsified in a highly controlled way. Glass microcapillary devices were used to produce higher order monodispersed emulsions.

Results and Discussion

Fig. 1
Fig. 2