"I know nothing except the fact of my ignorance."
- by Socrates

Current research projects
1. Assembly of Colloidal Dimers under Electric Fields (funded by NSF and NASA)
Although numerous theoretical work have shown that periodic structures of colloidal particles, arranged with well-controlled configuration and lattice symmetry, can exhibit unprecedented optical properties, the practical implementation is challenging. Our current knowledge of isotropic colloidal systems cannot be simply extended for building those structures of exquisite complexity because of our very limited control over colloidal interactions. To address this challenge, we will create, tune, and exploit a variety of anisotropic interactions by combining rational design in colloidal properties and appropriate application of external electric fields. Our primary objective is to elucidate the fundamental links between various types of microscopically anisotropic interactions and mesoscopically assembled phases, via a complementary experimental and theoretical approach. The colloidal dimer is an excellent model system that allows us to identify the specific impact of individual anisotropies (e.g., geometric, interfacial, and compositional anisotropy) and the synergistic effect of multiple anisotropies on the same colloidal model system.

2. Colloidal Molecules with Flexible Bonds (funded by NSF and NASA)
Flexibility is an important feature of molecular assembly. For example, non-covalent interactions between bio-molecules often allow them to transform between different morphologies (e.g., protein folding), triggered by changes in environmental conditions. Compared with rigid building blocks, colloidal molecules with flexible bonds are scarce in literature. Recently, we have successfully made monodisperse colloidal molecules by employing AC electric fields on spherical particles with isotropic properties. For example, polystyrene colloids (the "monomer") first aggregate into colloidal trimers in a unique way: one central sphere sits between two bottom ones. The bonds in trimers are so flexible that they can further assemble (with other monomers) into tetramers and pentamers, depending on frequencies. Those trimers can also assemble with other trimers or monomers into a variety of oligomers with uniform sizes and well-defined configurations, depending on both frequencies and particle concentrations. Those oligomers also possess flexible bonds and can further assemble into "macromolecules" including the honeycomb-like hierarchical structures. The striking similarity between chemical reactions of real molecules and the assembly of colloidal oligomers provides an efficient route to building "photonic molecules" with potential applications.

3. Self-propelling Motors and Smart Micro-robots (funded by CSM start-up)
The autonomous propulsion and active transport of nano-scale objects in a fluidic environment are essential for maintaining the bioactivities of all living species. Artificial motors have also become critically important for practical applications in targeted drug delivery, self-motile sensors for environment monitoring, and miniaturized surgeon. Because of the low Reynolds number, conventional swimming mechanisms that rely on inertial do not work at small scales. Although natural systems have evolved to possess extremely delicate biochemical motors, the development of synthetic motors lags way behind in terms of both complexity and efficiency. Current efforts in making self-propelling motors primarily rely on the conversion of chemical reaction energy (e.g., during the decomposition of hydrogen peroxide) into mechanical energy surrounding a properly designed particle. There are, however, serious drawbacks in the above systems. First, the chemical fuel of hydrogen peroxide is not biocompatible especially in high concentrations. Second, those motors are designed primarily for autonomous motion, while more important functions such as sensing, cargo transport, and targeted delivery have only been demonstrated in limited cases. Since cargoes are typically attached on the surface of those motors, they are unprotected from enzymatic degradation. Moreover, the surface attachment of cargo will unavoidably interfere with particle propulsion, which sensitively depends on its interfacial properties too. Third, the interfacial anisotropy on current motors are typically created via two-dimensional templating methods, which severely limits their potentials for scalable production. To overcome the above difficulties, we have pioneered the research using clusters of particles (instead of one particle) such as the dimer for building self-propelling motors. Since colloidal clusters naturally possess multiple compartments, they are excellent candidates for building multi-tasking motors where each modular compartment can perform a distinct function such as moving, sensing, imaging, or carrying cargoes. Moreover, the geometric, interfacial, and compositional anisotropies on colloidal clusters can all be conveniently tuned, which will allow us to exploit many other propulsion driving forces beyond the catalytic reaction.

4. Oil-Phase Delivery Through Bi-compartmental Colloids (funded by ACS-PRF)
Heavy oil is abundant in the world. The oil is, however, highly viscous and difficult to be extracted by conventional methods. As identified recently by the society of petroleum engineers, in-situ molecular manipulation to convert heavy oil catalytically into lighter grade oil could be a game-changing technology. Currently, efficient transport of catalysts through water flooding and targeted delivery towards the downhole oil phase is one of the grand challenges facing the oil industry. In this project, we will develop a unique type of colloidal surfactants that can encapsulate catalytic nanoparticles, transport within the reservoir media, detect the oil-water interfaces, and deliver catalysts to heavy oil. Specifically, we will synthesize bi-compartmental particles with both compositional and interfacial amphiphilicity. They serve as efficient surfactants for recognition and retention at water-oil interfaces. One compartment that encapsulates nano-catalysts is designed to swell and dissolve in heavy oil, facilitating the targeted delivery of catalysts in oil phase. By performing laboratory-scale core flood tests, we aim to study the efficiency of transport and delivery of nano-catalysts in porous media. Our scientific goal is to identify the important roles of geometric, compositional, and interfacial anisotropy on the emulsification and propagation of unconventional particles under pore scale two-phase flow. Our practical goal is to develop transformative technologies to recover abundant but currently inaccessible heavy oil.

5. Micro-/Nano-structures for Photon Management (funded by BAPVC-DOE)
Thin film solar cells can be fabricated with potentially lower cost and much less material. Efficient light trapping is critical in those physically "thin" photovoltaics. To make them optically "thick", simulations have suggested several strategies based on the dielectric, plasmonic, and photonic nanostructures. Industrial applications are, however, largely missing due to the challenge of fabricating low cost, large-area, and periodic arrays with reasonable control at the nanometer scale. Herein, we will develop a low-power AC electric-field assisted continuous coating process to create two- and three-dimensional nanostructures from both dielectric and metallic nanoparticles. Our technology offers the following advantages over standard top-down methods: (1) Precise control on the spacing and order of packing can be achieved through tunable dipole repulsions between particles, induced by the electric field. (2) Different shapes of particles can be readily deposited and their orientations on substrate (a key factor that affects light absorption) can be controlled by adjusting the frequency of AC field. (3) No template is required. (4) Solution-based processing and adaptability to existing flow coating process allow low cost, large-area, and continuous fabrication in 24 hours 7 days. (5) Both dielectric and metallic nanostructure can be fabricated based on the same set-up. We will investigate the interplay between electric field and convective assembly and study the effects of several key operating parameters. To demonstrate its efficiency and versatility, we will fabricate a variety of nanostructures on different types of thin solar cells for enhanced light absorption over a broad spectrum.

6. Transport of Radionuclide-carrying Colloids in Porous Media (funded by DOE-NEUP)
Independent of the methods of nuclear waste disposal, the degradation of packaging materials could lead to mobilization and transport of radionuclides into the geosphere. This process can be significantly accelerated due to the association of radionuclides with the backfill materials or mobile colloids in groundwater. The transport of these colloids is complicated by the inherent coupling of physical and chemical heterogeneities (e.g., pore space geometry, grain size, charge heterogeneity, and surface hydrophobicity) in natural porous media that can exist on the length scale of a few grains. In addition, natural colloids themselves are often heterogeneous in their surface properties (e.g., clay platelets possess opposite charges on the surface and along the rim). Both physical and chemical heterogeneities influence the transport and retention of radionuclides under various groundwater conditions. However, the precise mechanisms how these coupled heterogeneities influence colloidal transport are largely elusive. This knowledge gap is a major source of uncertainty in developing accurate models to represent the transport process and to predict distribution of radionuclides in the geosphere. The objective of this work is to identify the dominant transport mechanisms of radionuclide-carrying colloids in saturated porous media under the influence of pore-scale physical and chemical heterogeneities.