(6fv) Programmable 3D Transformation of Smart Soft Materials

My research interests lie in the intersection of soft matter, photonics, microfluidics and nanostructure engineering for development of smart soft materials and platforms. From this interdisciplinary approach, my independent research aims to exploit the fascinating interplay between material composition, geometry and prescribed deformation, and create novel structured materials and actuation devices with unprecedented properties and functions, including tunable optics, flexible electronics and bio-inspired transformable robotics. The early stage of my research program will span mainly two areas below.

1. Taming instabilities toward multi-functionality for reconfigurable active matter

In the course of evolution, nature has developed strategies to create materials with exquisite adaptability and optimized efficiency. Learning from nature, people have developed stimuli-responsive engineering polymers, known as a class of smart materials. Yet, there remains extensive untapped knowledge of the ways in which to harness functional, dynamic, and adaptable materials into the diverse integrated systems. To establish a rational strategy to control instabilities of soft polymeric materials, understanding of their phase behavior and active interactions with their surrounding is critical. My first research goal is to investigate dynamic behavior/mechanism of the responsive materials in terms of structure and physical property triggered by mechanical strain or different external field conditions by using various microscopy techniques. This knowledge should help resolve significant challenges of materials integration into complex and robust 3D structures. I believe this research regarding reconfigurable metamaterials can address broader technological and societal challenges, including those in the areas of renewable energy harvesting, water purification, drug delivery, etc.

2. Development of a new 4D printing based on photon upconversion and flow lithography

Two-photon polymerization (TPP), also known as direct laser writing, has been recognized as a promising technique for the fabrication of arbitrary 3D sub-micron features with high-order shapes and functions in photosensitive materials without photomasks. Typically, it requires an ultrashort pulsed laser to trigger non-linear two-photon absorption only in the focal spot volume, so called voxel. Unlike other 3D printing technologies, thus, it allows for the highest resolution or layer thickness with a previously unavailable freedom of design. Although TPP is a powerful and attractive technique, the major bottleneck in unleashing its potential lies in point-by-point writing with high power pulsed laser and long processing times unacceptable for practical applications. Here, I am interested in triplet fusion upconversion-assisted polymerization which allows for use of a non-coherent, continuous wave light at ultra-low excitation power as a pumping source to generate high-energy photons. Besides photochemistry, my interests include methods to develop high-throughput microfabrication by combining of flow lithography using digital micromirror device. I believe this approach provides a short processing time, offering possibility of mass production for high-fidelity freeform microstructures with programmed internal heterogeneity. If successful, I am utilizing them to synthesize novel functional building blocks like anisotropic, shape-deformable colloids for the assembled complex structures that dynamically respond to energy input from their surroundings.

Research Experiences:

For my postdoctoral research at the University of Massachusetts Amherst (Prof. Ryan C. Hayward, Polymer Science & Engineering), I am currently working on precise control of mechanical instabilities in responsive polymer patterns. I am particularly interested in programming robustly self-folding action in rigid origami without bifurcation of vertices which leads to undesired misfolding states. Using mathematical prediction, I have designed a new structural transformation of multilayer polymer films that can be dynamically reconfigured to change their shapes and properties on demand. I also have developed the active vertices to apply a driving force that avoids such bifurcation from a flat unfolded state using constrained deformation by external stimuli such as pH, temperature and light.

During my PhD at Georgia Institute of Technology (Prof. Elsa Reichmanis, Chemical Engineering), I focused on optofluidic encapsulation of triplet fusion photon upconversion. I investigated photon management in intermolecular energy transfer between metastable states and diffusion-mediated delayed fluorescence in polymer thin films and microcapsules. I had been leading the development of photo-induced interfacial polymerization and one-step emulsification of multi-phase fluids with microfluidic devices for high quantum efficiency and photochemical stability. Further, I demonstrated a photonic shell containing cholesteric liquid crystals with planar alignment to amplify spontaneous upconverted emission yield and tailor optical properties with tunable bandgaps. I believe the upconversion capsules have a wide application in photonics such as photonic ink, microlaser, volumetric display, solar energy harvesting, and photocatalytic water treatment.

Early on in my career at KAIST (Prof. Seung-Man Yang, Chemical Engineering), I was involved in research on creation of nanostructures with free-defects over a large area by multi-coherent light without a photomask (i.e. holographic interference lithography, HIL). I developed a new type of hydrogel photoresist that undergoes viscoelastic deformation and produced 3D photonic crystals with tunable structural color. I also worked on hierarchical porous electrodes by colloidal templating and HIL for photovoltaic cells and batteries.

Teaching Interests:

During my teaching career development, I enjoyed serving as a teaching assistant for undergraduate courses in “Chemical Process Safety” and “Unit Operations Lab” at Georgia Tech. Chemical Process Safety requires students to tackle real safety problems with chemical engineering knowledge. As a TA, I prepared to guide their quizzes and projects with many aspects such as toxicology, release & dispersion models and case studies. For the course of UO Lab, I was assigned as a TA of the fluidized bed to give students a brief introduction and standard operation procedures to run a series of experiments. I evaluated their knowledge from their reports so I also learned how to clarify concepts to help them understand. These experiences have built my confidence and an interest in teaching. Besides, I am excited to teach a wide range of the stem courses in Chemical Engineering such as “Chemical Reaction Engineering”, “Mass, Momentum and Energy Transport”, and “Thermodynamics”. From my multi-disciplinary research experience, I am also confident in teaching specific courses of “Nano-chemical Technology”, “Photochemistry and Photophysics”, “Fluidic Mechanics”, “Capillarity and Wetting”, and “Polymer Chemistry”. I look forward to the opportunity to not only teach existing courses but also work to develop new ones that I can contribute to. Also, I have a lot of experience in mentoring undergraduate students from SURE and REU (NSF MRSEC), and graduate students to develop and trim their own creative ideas. Based on these teaching and mentoring experience, I have learned about the roles and responsibilities of the principal investigator regarding how to communicate better with my students and encourage them to proceed.

Selected Publications:

1. Kang,-H.; Kim, S.-H.; Fernandez-Nieves, A.; Reichmanis, E., Amplified Photon Upconversion by Photonic Shell of Cholesteric Liquid Crystals, Journal of American Chemical Society 2017, 139, 5708.
2. Kang, J.-H.; Lee, S. S.; Guerrero, J.; Fernandez-Neives, A.; Kim, S.-H.; Reichmanis, E., Ultrathin Double-Shell Capsules for High Performance Photon Upconversion, Advanced Materials 2017, 29, 1606830.
3. Kang, J.-H.; Reichmanis, E., Low-Threshold Photon Upconversion Capsules Obtained by Photoinduced Interfacial Polymerization, Angewandte Chemie International Edition 2012, 51, 11841.
4. Kim, Y.; Cho, C.-Y.; Kang,-H.; Cho, Y.-S.; Moon, J. H., Synthesis of Porous Carbon Balls from Spherical Colloidal Crystal Templates, Langmuir 2012, 28, 10543.
5. Kim, H.-N.*; Kang, -H.*; Jin, W.-M.; Moon, J. H., Surface Modification of 2D/3D SU-8 Patterns with a Swelling-Deswelling Method, Soft Matter 2011, 7, 2989. (*equally contributed)
6. Shin, J.-H.; Kang, -H.; Jin, W.-M.; Park, J. H.; Cho, Y.-S.; Moon, J. H., Facile Synthesis of TiO2 Inverse Opal Electrodes for Dye-Sensitized Solar Cells, Langmuir 2011, 27, 856.
7. Jin, W.-M.; Kang, -H.; Moon, J. H., Fabrication of 3D Copper Oxide Structure by Holographic Lithography for Photoelectrochemical Electrodes, ACS Applied Materials and Interfaces 2010, 2, 2982.
8. Kang, -H.; Moon, J. H.; Lee, S.-K.; Park, S.-G.; Jang, S. G., Yang, S.; Yang, S.-M., Thermoresponsive Hydrogel Photonic Crystals by Three-Dimensional Holographic Lithography, Advanced Materials 2008, 20, 3061.
9. Ha, S.-J.; Kang, J.-H.; Choi, D.H.; Nam, S.K.; Reichmanis, E.; Moon, J.H., Upconversion-assisted Dual-band Luminescence Solar Concentrator Coupled for High Power Conversion Efficiency Photovoltaic Systems, ACS Phtonics 2018, in revision.