(7jb) Colloidal and Interfacial Phenomena Involving Anisotropic Fluid

Either bulk or interfaces of isotropic solvents have been widely used to direct assembly of molecules and colloidal particles, leading to a range of assemblies, elastomers and gels with complex mechanical, electronic and optical properties. Anisotropic fluid, such as liquid crystal (LC), possesses orientational ordering of constituent molecules which results in orientation-dependent properties (optical, mechanical and transport) that differentiate from isotropic liquids. My research aims to resolve the question how the orientational order in LC affects assembly of molecules and colloids. Here I present works on (i) LC topological defect-mediated molecular self-assembly, (ii) LC emulsion-templated colloidal assembly and particle synthesis, and (iii) non-monotonic responsive polymeric materials based on liquid crystal elastomer (LCE), which together provide the platform for realizing my goal to produce a remarkably diversity of colloidal and interfacial phenomena that are distinct from isotropic liquid.

Research Experience:

My first research topic is molecular self-assembly directed by topological defects in LCs. We found that nanoscopic environments defined by LC topological defects can selectively trigger processes of molecular self-assembly. By using fluorescence microscopy, cryogenic transmission electron microscopy and super- resolution optical microscopy, key signatures of self-assembly of amphiphilic molecules in topological defects are observed - including cooperativity, reversibility, and controlled growth of the molecular assemblies. By using amphiphiles that can be photocrosslinked, we also demonstrate preservation of molecular assemblies templated by defects, including nanoscopic “o-rings” synthesized from “Saturn-ring” disclinations. Our results reveal that topological defects in LCs are a versatile class of three-dimensional, dynamic and reconfigurable templates that can direct processes of molecular self-assembly in a manner that is strongly analogous to other classes of macromolecular templates (e.g., polymer—surfactant complexes).

The second part of my research discovers that LC droplets in LC-in-water emulsions can be utilized as templates for synthesis of anisometric microparticles with chemical surface patches. Remarkable control over the shape and surface morphology of polymeric particles templated from LC microdroplets can be achieved through chemical manipulation of configuration of the droplets. In addition, reversible adsorbate- driven ordering transitions in LC droplets are used to localize colloids at surfaces of water-dispersed LC droplets in a programmable manner. This approach can be used to synthesize either “Janus-like” microparticles or magnetically-responsive patchy microdroplets of LC with either dipolar or quadrupolar symmetry that exhibit distinct optical responses upon application of an external magnetic field.

My third research topic is development of a chemical platform for synthesis of a liquid crystalline material assuming a non-monotonic response with respect to the monotonic dynamics of a single stimulus. In this study, we synthesized a LCE polymer exhibiting two phase transitions – LC—isotropic and crystal—LC. Surprisingly, two deformations executed by such a LCE microplate proceeded in the opposite direction! By selecting the temperature range for thermal cycles, we programmed the LCE system to realize a rich palette of stimulus-dependent modes. Overall, these results demonstrate that LCEs consisting of multiple phase transitions can serve as a versatile class of adaptive materials for design of dynamic microstructures adopting non-monotonic stimulus-dependent responsiveness, which are unprecedented in traditional LCEs reported in previous studies.

Teaching Interests:

Chemical engineers applies fundamental physical sciences to design and produce chemicals and materials. This is exemplified in my PhD and postdoctoral research about colloidal and surface phenomena and polymer science involving LCs. My research experience has prepared me to teach a wide range of curriculum in Chemical Engineering (such as Thermodynamics, Polymer Science, Colloidal and Surface Science, and Transport Phenomena). In addition, I would like to work with undergraduate and graduate students in short-term projects in my course to deepen their understanding learned in the classroom, and promote student interest in scientific research and further graduate education.

Aside from my research, I also have been involved in a wide variety of educational and outreach activities that provided me with valuable experience to start an academic career. As a teaching assistant in my PhD study, I taught two intermediate level courses of Chemical Engineering at University of Wisconsin-Madison (UW). In my postdoctoral study at Harvard University, I was invited to give a lecture about “Intermolecular Interactions and Liquid Crystals” in a senior applied physics course of Chemistry in Materials Science and Engineering. Besides teaching, as a member of UW Materials Research Science and Education Center (MRSEC), I took roles and responsibilities for designing simple methods to gather public interest in science and engineering. As a consequence, I believe I have had valuable experience and gained confidence to advice students, and to influence public audience to gather interest to Chemical Engineering.

Selected Publications (23 total, 10 first author):

1. Wang, X.;* Zhou, Y.;* Kim, Y.-K.; Miller, D. S.; Zhang, R.; Martinez-Gonzalez, J. A.; Bukusoglu, E.; Zhang, B.; Brown, T. M.; de Pablo, J. J.; Abbott, N. L. “Patterned Surface Anchoring of Nematic Droplets at Miscible Liquid—Liquid Interfaces”. Soft Matter DOI: 10.1039/C7SM00975E 2017. (*contributed equally)

2. Wang, X.; Bukusoglu, E.; Abbott, N. L. “A Practical Guide to the Preparation of Liquid Crystal- Templated Microparticles”. Chemistry of Materials 29, 53-61, 2017.

3. Bukusoglu, E.;* Wang, X.;* Zhou, Y.; Martinez-Gonzalez, J. A.; Rahimi, M.; Wang, Q.; de Pablo, J.J.; Abbott, N. L. “Positioning Colloids at the Surfaces of Cholesteric Liquid Crystal Droplets”. Soft Matter 12, 8781-8789, 2016. (*contributed equally)

4. Wang, X.; Bukusoglu, E.; Miller, D. S.; Pantoja, M. A. B.; Xiang, J.; Lavrentovich, O. D.; Abbott, N.L. “Synthesis of Optically Complex, Porous and Anisometric Polymeric Microparticles by Templating from Liquid Crystalline Droplets”. Advanced Functional Materials 26, 7343-7351, 2016.

5. Wang, X.; Kim, Y.-K.; Bukusoglu, E.; Zhang, B.; Miller, D. S.; Abbott, N. L. “Experimental Insights into the Nanostructure of the Cores of Topological Defects in Liquid Crystals”. Physical Review Letters 116, 147801, 2016.

6. Wang, X.; Miller, D. S.; Bukusoglu, E.; de Pablo, J. J.; Abbott, N. L. “Topological Defects in Liquid Crystals as Templates for Molecular Self-Assembly”. Nature Materials 15, 106-112, 2016.