Difference between revisions of "Functional patterning of PDMS microfluidic devices using integrated chemo-masks"

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A new method for patterning surface properties in PDMS (Polydimethylsiloxane) channels is presented here. Air reservoirs were created in the PDMS. As the polymer cured, the reservoirs diffused oxygen into nearby channel segments thus inhibiting functional polymer growth.  The placement of the reservoirs controlled the polymerization pattern.
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Polymersomes are a type of vesicles that can be created artificially. The ones explored in this article are formed from block-copolymers in a double emulsion. In this article, the authors explore the effects of different concentrations of diblock copolymers in the middle phase of a polymersome. It is known that if the concentration of diblock copolymers is too low, there will not be enough surfactant for a polymer monolayer to form and the structure will be unstable.  The authors look at the other extreme, where an excess concentration is used and they find the surprising result that a "dewetting" instability forms, leading to a polymer shell of varying thickness.   
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PDMS is an important and commonly used material in microfluidics.  It is used to quickly mold channels, valves and other microfluidic features.  Some microfluidic applications require spatially patterned surface properties which are difficult to create using existing techniques, often requiring extremely precise photomask alignments.  The method presented here uses simple oxygen reservoirs to robustly control the polymerization of PDMS.
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The air reservoirs can have irregular shapes and this method could enable patterning of 3-D devices if the chemo-masks are placed above or below, as well as beside flow channels.  Chemo masks modestly increase the footprint of PDMS devices.  
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The polymersomes are created with glass microcapillary devices.  The double emulsion (water in oil in water) was created one at a time with the final double emulsion being collected into deionized water and then the solvent from the double emulsions was evaporated to form the polymersomes. The interesting transition occurs when the oil is saturated with many block copolymers.  After evaporation, two phases coexist; an organic phase of monolayers and a solvated bilayer film.  The authors call this "dewetting" due to the transitions similarity to that from complete to partial wetting of liquid films at solid-vapor interfaces.  As best as I can understand, it's as if there are too many block copolymers in the oil that it no longer can tightly wrap around the water to form a zero contact angle and hence makes bulges.
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The authors go on to postulate an adhesion energy between the inner and outer phases due to this observation and they derive a Young-Dupre equation for the contact angle that relates the angle to this adhesion energy.  They then propose that the driving force behind the dewetting is a depletion interaction between the inner and outer interfaces that happens because of the extra block copolymers.  Their model shows agreement with their experimental results.  The authors further postulate that the rate of solvent evaporation during formation would lead to nonequilibrium effects.  They also test these ideas and find that the interfacial tension decreases as the surface are of the drop decreases.  In the extreme case, they find that sometimes a complete dewetting of the organic phase occurs leading the vesicles to break into smaller droplets. 
 
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In short, the paper explores the behavior of polymersomes during their formation and creates theories to explain the behavior for different concentrations of block copolymer and different speeds of evaporation.
 
  
 
==Soft Matter Connection==
 
==Soft Matter Connection==
  
 
This paper touches on several aspects of soft matter, including microfluidics, polymers (PDMS), and emulsions.  Microfluidics manipulate fluids at low Reynolds numbers and at a small scale where statistical mechnics become important.  PDMS is a important material from which microfluidics are made of and itself also exhibits soft matter traits.  Emulsions are forms of soft matter consisting of droplets of one fluid inside, surrounded by another.
 
This paper touches on several aspects of soft matter, including microfluidics, polymers (PDMS), and emulsions.  Microfluidics manipulate fluids at low Reynolds numbers and at a small scale where statistical mechnics become important.  PDMS is a important material from which microfluidics are made of and itself also exhibits soft matter traits.  Emulsions are forms of soft matter consisting of droplets of one fluid inside, surrounded by another.

Latest revision as of 22:08, 12 September 2010

Original entry by Caspar Floryan, APPHY 225 Fall 2010

Reference

"Functional patterning of PDMS microfluidic devices using integrated chemo-masks" Mark B. Romanowsky, Michael Heymann, Adam R. Abate, Amber T. Krummel, Seth Fraden and David A. Weitz. Lab on a Chip 10, 1521–1524 (2010).

Keywords

PDMS, Microfluidics, Emulsions, Patterning, Chemo-masks

Summary

Caspar wiki1 image1.jpg

A new method for patterning surface properties in PDMS (Polydimethylsiloxane) channels is presented here. Air reservoirs were created in the PDMS. As the polymer cured, the reservoirs diffused oxygen into nearby channel segments thus inhibiting functional polymer growth. The placement of the reservoirs controlled the polymerization pattern. PDMS is an important and commonly used material in microfluidics. It is used to quickly mold channels, valves and other microfluidic features. Some microfluidic applications require spatially patterned surface properties which are difficult to create using existing techniques, often requiring extremely precise photomask alignments. The method presented here uses simple oxygen reservoirs to robustly control the polymerization of PDMS. The air reservoirs can have irregular shapes and this method could enable patterning of 3-D devices if the chemo-masks are placed above or below, as well as beside flow channels. Chemo masks modestly increase the footprint of PDMS devices.

Caspar wiki1 image2.jpg

Soft Matter Connection

This paper touches on several aspects of soft matter, including microfluidics, polymers (PDMS), and emulsions. Microfluidics manipulate fluids at low Reynolds numbers and at a small scale where statistical mechnics become important. PDMS is a important material from which microfluidics are made of and itself also exhibits soft matter traits. Emulsions are forms of soft matter consisting of droplets of one fluid inside, surrounded by another.

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