(7db) Theoretical and Computational Study of Soft Matter Systems: From Classical Challenges to Rational Design of New Materials

Modern chemical engineering becomes increasingly molecular-based. The development of theoretical and computational tools that capture the essential physics, address the molecular feature and quantify the systems across multiple time and length scales is vitally important. Soft materials are widely used from commodity to biomedical materials. Many phenomena in soft matter systems, ranging from classical experiments to emerging interest, cannot be satisfactorily explained, which necessitates theories that can provide fundamental insights to these seemingly complex problems. The large design space due to the rapid development of new synthetic and processing methods also requires predictive tools to accelerate the design process of new materials. My research focuses on using statistical mechanics, which bridges the molecular information and macroscopic phase/interfacial properties, to study a variety of soft matter systems, including electrolyte solutions, liquid crystals, membranes, colloidal dispersions, polymers, polyelectrolytes, polymer-protein conjugates and polymer networks/gels.

I did my PhD work at Caltech with Prof. Zhen-Gang Wang, where we developed renormalized Gaussian fluctuation theory to study the effect of self energy of ions on the double layer structure and interfacial properties. Our theory incorporates different components of self energy in a unified framework and involves a minimum set of intrinsic parameters. We show the consequence of the self energy by revealing three essential effects: the image charge effect, inhomogeneous screening effect and specific ion effect. We are able to quantitatively explain a couple of long-standing puzzles in physical chemistry and soft matter physics, such as the salt concentration dependence of surface tension, like-charge attraction and charge inversion, Hofmeister series in the interfacial affinity and strong interfacial adsorption of hydrophobic ions.

My current postdoctoral research in MIT with Professors Bradley Olsen, Alfredo Alexander-Katz and Jeremiah Johnson is to develop theoretical and computational tools to quantify the topology, gelation and elasticity of polymer networks. I developed a kinetic graph theory which demonstrates a universal dependence of loop defects on the preparation condition and reveals the intrinsic relation between different cyclic topologies. I developed a kinetic Monte Carlo simulation which quantifies the loop effects on the gel-point suppression and the change of critical exponents. I also developed a real network theory which bridges topological defects to gel elasticity. These theories and simulations show good agreement with experimental measurements, providing, for the first time, a quantitative understanding of gel elasticity based on molecular details. Using this platform, we are now seeking to optimize the mechanical properties through controlling the molecular connectivity of the network.

In the future, I would like to carry theoretical and computational research to solve fundamental challenges in soft matter systems and facilitates the design of new materials that meet the emerging requests in environment, energy and biology. I will develop simple models that capture the most essential features behind the complexity of the problems at hand. With the fundamental insights, I will develop more detailed theoretical and computational tools as low-cost, high-performance platform to aid the rational design of novel materials. Meanwhile, I will seek for extensive collaboration with experimentalists and data scientists to accelerate the material design process via coupling the predictive physics-based models, high-throughput experimentations and machine-learning techniques. I will initially focus on the following topics:

1) Complex interfacial phenomena of salt ions and polyelectrolytes driven by electrostatics, with fundamental challenges including charge inversion and like-charge attraction, and applications such as layer-by-layer deposition techniques and membrane fouling in water purification.

2) Phase behavior of polyelectrolyte hydrogels, with fundamental challenges including swelling-collapse transition and nanophase patterns, and applications such as stimuli-responsive materials and micro/nano soft particles used as actuators, sensors and microreactors.

3) Interfacial topology and properties of polymer networks, with fundamental challenges including erosion, friction, fracture and adhesion of gels, and applications such as self-healing materials and cell-matrix interactions.

Teaching Interests:

Teaching is extremely important for the professional development of my future students, and also benefits my own scientific research. Based on my teaching experience in Caltech and MIT, I feel that the traditional design of the chemical engineering courses cannot fully satisfy the emerging requests in both the scientific research and industrial employment. I suggest two modifications. 1. Besides the traditional courses that focus on the reactor scale, knowledge at molecular level such as synthetic chemistry, statistical (or even quantum) mechanics and molecular biology are needed. 2. Interdisciplinary courses referring to material science, environment, energy and biology need to be involved.

I would like the students in my class to be actively engaged in the class and get passion in learning and doing science, obtain clear basic concept, think critically and keep challenge themselves, and gain the ability of solving real problems. To achieve this goal, I will organize the course materials and develop the teaching methods by addressing the following aspects: 1) encourage the students to think about one problem from different perspectives; 2) guide the students to find the underlying physics that explains different phenomena; 3) design problem sets and term projects from recent research progresses.

As a graduate student at Caltech, I had the opportunity to work as a teaching assistant in graduate level thermodynamics, statistical mechanics and polymer physics. The evaluation from the students shows that I was one of the best teaching assistant in recent years. Besides these courses, my education and research experiences also make me well quantified to teach kinetics, transport process, interfacial phenomena, mathematics in chemical engineering and computer modeling and simulation. Currently as a postdoc in MIT, I had the opportunity to give guest lectures in the course of synthetic polymers. I am also training for mentoring undergraduate and graduate students, which is extremely helpful for my teaching skills. In the future, I plan to develop an in-depth course for soft matter physics that can meet the rapid development of this field. This interdisciplinary course will cover both the fundamental concepts and broad applications, which will benefit students from different background.