475966 Microfluidic Design of Multi-Phase Emulsion Drops for Functional Materials Production

Sunday, November 13, 2016
Continental 4 & 5 (Hilton San Francisco Union Square)
Hyomin Lee, School of Engineering and Applied Sciences, Harvard University, Cambridge, MA

Research Interests:  

Emulsions are attractive for encapsulating, transporting, or controllably releasing a wide variety of chemical and biological active materials. Multi-phase emulsion drops are of particular interest due to the compartments of these emulsion drops that can be enlisted to impart functionality. By using the precise flow control of multi-phasic fluids in microfluidics, in conjunction with fluid-fluid interface design by proper selection of materials in each phase, I have produced novel functional materials that otherwise would have been inaccessible. For example, I developed a new strategy of combining bulk and microfluidic emulsification to achieve highly efficient encapsulation and enhanced retention of fragrance in polymer microcapsules. Additionally, I developed novel triple emulsion drops with an ultra-thin intermediate layer consisting of either hydrogel or fluorocarbon oil that separates the innermost cargo from the polymeric shell. These emulsion drops have interesting properties and great technological potential for encapsulation and controlled release of a wide variety of active materials. Moreover, I have worked on several collaborative projects through which we have designed ideal polymer network in emulsion templated microcapsules for encapsulation and triggered release of challenging actives, and semi-permeable polymeric shell in microcapsules for micro-sensor, and micro-reactor applications, just to name some.

Research Experience:

My academic career path has been a blend of many fields of science and engineering. My formal training at MIT (with Prof. Robert E. Cohen and Prof. Michael F. Rubner) was in Chemical Engineering, which I have studied the mechanisms that govern the structure and function of polymer thin films for developing a diverse range of functional films including antifogging coatings, switchable surfaces and stimuli-responsive hydrogels. Specifically, I have introduced a framework in which poly(vinyl alcohol) can be incorporated into multilayered thin films by investigating hydrogen-bonding interaction of polymers in aqueous media. By tailoring the interlayer diffusion of hydrogen-bonding polymers during the assembly, I also designed a complex hetero-structured architecture that sequentially dissolves with increase in local pH condition. Additionally, I have developed a new concept in controlling water condensation. By thorough characterization and analysis of a model system, a new physical theory, “zwitter-wettablility”, was proposed whereby the film readily absorbs water vapor while simultaneously exhibiting hydrophobic character to liquid water. Moreover, this zwitter-wettable concept was further extended to real-life applications where the optimally designed film resulted in a significantly enhanced antifog and even frost-resistant behavior.

Teaching Interests:

Aside from my research career, I also have extensive teaching experience. I TAed an undergraduate course in Chemical Engineering at MIT in which I lectured undergrads, held weekly office hours, and developed course materials; I mentored undergraduate students as a part of their undergraduate thesis at MIT; I also mentored local community college students as a part of an educational outreach program from MIT CMSE; and, lastly, I am currently mentoring and supervising the research of graduate students on a large number of projects.

Future Direction:

As a new faculty, I would like to continue studying soft materials at interfaces, and engineer new hierarchical structures for industrial applications. In particular I would like to leverage the knowledge that I obtained from the two topics that I worked on during my Ph.D. and postdoc: polymer science/engineering and microfluidics, respectively. Many intriguing phenomena occur at fluid-fluid/solid-fluid interfaces and I believe my expertise will provide new perspective in designing functional materials.

My first thrust as a faculty is to take advantage of molecular rearrangement of polymers at surfaces to design novel polymer film structures. It has been demonstrated that configuration of a liquid drop on a given surface depends on the force balance across liquid-solid-air interfaces, which is governed by the surface topology as well as the surface chemistry. However, by separately tuning the surface and bulk properties of a film, unique surfaces can be prepared in which water drops exhibit lower equilibrium contact angle than typical hydrocarbon oils that have much lower surface tension than water. While these engineered surfaces have great potential for applications such as self-cleaning surfaces, and antifogging coatings, how to modulate the rate of rearrangement by materials design is still unclear. I believe that studying this in depth in conjunction with phase change heat and mass transfer will give rise to new applications such as contaminant separation from water vapor for reducing pollutant in air and water.

My second thrust is to utilize the middle phase that separates the innermost drop from the continuous phase in multi-phase emulsion drops to study polymers at interfaces and engineer new functional materials. It has been demonstrated that the middle phase in double-emulsion drops can serve as a template to form polymer microcapsules for encapsulation of actives. Yet, there has been little progress toward the production of microcapsules with membranes that fulfills the practical demands. Moreover, I believe that many well-known scientific findings observed in bulk such as micro-phase separation of block copolymers, and polymer network formation using various chemistries can be translated into this compartment provided by the multi-phase emulsion drops. This will span a wide range of new interesting scientific and engineering questions which will open the door in the field by creating a new route to design and fabricate advanced particles with unique functionalities.

Besides material design my interests include methods to scale up the fabrication of these emulsion drops to produce practical quantities. While microfluidic devices have been massively parallelized to produce highly monodisperse single emulsion drops in high-throughput, extending this framework to produce double-emulsion drops consisting of fluids that have high viscosities have been difficult to achieve. This problem will require extensive fundamental studies of hydrodynamics and interactions of fluids with surfaces. I will use simulation-guided 3D-microfluidic device geometry design, and develop a simple and robust method to selectively modify the surface wetting characteristics of a microfluidic device to solve this challenge. The results of this study will bridge the gap between lab-bench scale to industry-level scalable generation which is both fundamentally and practically important.

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