Dewetting Instability during the Formation of Polymersomes from Block-Copolymer-Stabilized Double Emulsions
Original entry by A.J. Kumar, APPHY 225 Fall 2009
Ryan C. Hayward, Andrew S. Utada, Nily Dan, and David A. Weitz, Langmuir 22, 4457-4461 (2006).
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.
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.
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.
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
Polymersomes are of great interest in soft matter. These vesicles could potentially be small model cells. Understanding their formation and their interface behaviors can help us to engineer such vesicles to carry a desired package to a target and then deliver it. This could have huge consequences in drug delivery or in doing droplet chemistry for lab-on-a-chip technology. This paper explores some of the interesting unanswered questions about what is really happening in a physical sense at the boundaries between layers.
 BM Discher, YY Won, DS Ege, J Lee, FS. "Polymersomes: tough vesicles made from diblock copolymers" 'Science', 1999 
 Wikipedia "Polymersome"