Optical Properties of Polymers

Engineering structures and dynamics of colloidal suspensions and granular matters has been explored to design specific structures and develop a new class of materials properties. Advances in molecular design has allowed achieving desirable properties via controlling the interparticle interactions which allow to tailor the particle configuration with external stimuli such as electric/magnetic fields, temperature, light, shear, pH, and combinations thereof. Leveraging such particle designs, stimulus-adaptive materials can be developed. Largely two classes of materials design are possible; 1) the formation of ordered colloidal crystals to target unique optical, mechanical, and acoustic properties based upon periodic structural conditions, and 2) disordered structures with adaptive rheological properties such as shear-thickening and jamming phenomena. Based on the understanding of above ordered and disordered systems, the goal is to control the phase transition across multiple phases from disorder to different crystallinities to obtain a single material system exhibiting tunable and adaptive properties.

First, state-of-the-art chemical and physical control of interparticle interactions can be employed to explore stimuli-responsive lattices. An example of molecular level manipulation includes click-chemistries, e.g., thia-Michael covalent bonds, that have shown multi-orders of magnitude change in the equilibrium constant and thus, the interaction potentials. This specific chemistry enables a more systematic control of interparticle interactions which overcomes limitations observed from other interaction pairs such as DNA-grafted particles with a narrow range of interaction potential. Further structural programming can be achieved with time-varying functions of external stimulus that allows access to out-of-equilibrium assembly pathways. This physical control presents ways to circumvent kinetic bottleneck and realize large-scale crystal formations. These understandings can be translated to bulk scale fabrication of a platform to print 2D/3D colloidal structures with tunable optical properties with anti-counterfeiting holographic structures.

Second, tuning the interparticle potential is crucial for the disordered particulate systems, such as nanocomposites and slurries. Non-Newtonian rheology of dense suspensions exhibit highly adaptive nature such as shear-thickening and shear-jamming phenomena, that serves as key principles for designing impact-mitigating smart fluids. I have recently developed a new pathway to the jamming transition using electrochemically redox-active particles. The redox reaction of disulfide crosslinkers exhibits irreversible swelling/shrinking as the reduced thiolates exchange with disulfides in the spherical geometry. This exchange kinetically arrests the thiolates far from the reductive-condition surface leading to the unbalanced reduction-oxidation rate. The complex redox mechanism embeds a structural memory that makes the jammed network mechanically robust and persistent against external stress. This opens enormous opportunities to explore different pathways to unique jamming structures, dynamics, and memory formation. Fundamental understanding of how jammed networks mitigate and propagate the external impact will eventually help enhance the maximum energy dissipation.

Overall, I aim to develop colloidal platforms that exhibit adaptivity based on the particle structures in any given environment and perform according to externally applied stimuli. The challenge is to unravel phase transition pathways across multi-phase boundaries from disorder to ordered, and to different crystallinities. Incorporating multiple responsiveness in a particle can lead a way to leverage sophisticated programming of stimuli in making of transformative and trainable materials.

Teaching interests

Throughout my academic career, I have guided undergraduate and graduate students and witnessed their growth as independent learner and researcher. These experiences taught me values of interactive teaching, learning process, and most importantly, the impact of teacher, advisor, and mentor on early-stage students. I believe that fostering young scientists involves not only disseminating the scientific knowledge but also helping students find their passion in problem-solving and active learning processes that are truly important for their continual engagement in sciences and engineering. I have a broad and extensive teaching experience that span holding math and science classes for K-12 students to TAing for graduate-level chemical engineering courses. To bolster my strength, I had recently participated in a Future Faculty Workshop sponsored by NSF to develop pedagogical skills in multiple formats from current, well-experienced faculty members in both research and undergraduate institutions. My goal in teaching is to create a healthy and engaging environment for students to grow. Specifically with my colloidal physics research background, my interests are in exploring particle/colloidal technology and developing courses that can help students to look beyond just the sciences but to find and address challenges where the technology is needed.