(7f) Developing Molecular Theories/Simulations to Understand and Optimize Soft Matter Systems: From Ions to Polymers to Gels

Modern chemical engineering becomes increasingly molecular-based. The development of theoretical frameworks that capture the essential physics, address the molecular feature and quantify the systems across multiple time and length scales is vitally important. 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, 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 only 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 use theory and simulation to quantify the topology and elasticity of polymer networks by addressing cyclic defects (loops). I develop a kinetic graph theory which demonstrates a universal dependence of loop fractions on the preparation condition and reveals the intrinsic relation between different cyclic topologies. I also develop a real network theory which quantifies the individual negative impacts of different loops on the elasticity. These theories shows excellent agreement with experimental data, providing, for the first time, a quantitative understanding of gel elasticity based on molecular details. By bridging the topology and elasticity, we are now seeking to optimize the mechanical property of the polymer networks through controlling the molecular connectivity.

In the future, I would like to carry theoretical and simulational research to solve fundamental challenges in physical chemistry and soft matter physics. I will build simple models that capture the essential molecular characteristics (constitution, interaction and connectivity), which are then incorporated into the framework of statistical mechanics and further solved by using or developing modern analytical, numerical and computational methods This methodology facilitates the study of inhomogeneous systems, fluctuation effects and nonequilibrium processes, where most of the challenges exist. On the other hand, I will seek for extensive collaboration with experimentalists to use this fundamental knowledge for rational design of new materials and processes. I will initially focus on the following topics:

1) Complex interfacial and kinetic phenomena driven by electrostatics;

2) Conformation, association and interfacial behavior of polyelectrolyte solutions;

3) Topology of polymer/polyelectrolyte networks vs. phase/interfacial behaviors.

Teaching Interests:

Teaching is extremely important for the professional development of my future students, and also benefits my own research work. In my future course, I would like my students to obtain clear basic concepts, the ability of critical thinking, the training of solving real problems and the knowledge about current challenges in this field. 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 assistants 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 am also training for mentoring undergraduate and graduate students, which is extremely helpful for my teaching skills. The course about soft materials right now is limited due to the rapid progress in this field and the lack of systematic textbooks. I plan to develop an in-depth course that convers a broad spectrum of soft matter systems in the future.