Tremendous advances in the fabrication of complex devices having characteristic sizes of nanometers to microns have opened up new frontiers for the application of chemical engineering fundamentals. Examples of such lilliputian machines include nanofluidic and microfluidic systems (NMFS), electronic circuits made of organic materials (organic or "plastic" electronics), and biochemical sensors. Transport and interfacial phenomena invariably play key roles in the production and operation of these technologies, but because of the small length scales involved, different forces and physical processes become important compared to larger-scale systems. Our research is centered around the investigation of fundamental problems in transport and interfacial phenomena, many of which are motivated by the above applications. Often, these problems involve deformable interfaces (fluid-fluid or fluid-solid) and rheologically complex materials (e.g., polymer solutions and polymer gels). Analysis, numerical simulation, and experiment are all applied in our endeavor, and tools from both continuum and molecular theory are employed. Research is currently focused in the areas described below:
Printing and coating technologies for the large-scale manufacture of nanoscale and microscale devices: There is a tremendous amount of interest in adapting traditional printing and coating technologies to mass-produce nanoscale and microscale devices, especially those involving organic electronic materials such as polymers. These printing and coating technologies are full of nanoscale and microscale flows that are still poorly understood, and there is a pressing need to better elucidate their fundamentals. We are currently performing research on a number of basic issues in this area including the dynamics of liquid transfer, wetting failure, and thin-liquid-film stability. Our work involves a combination of lubrication-theory-based analysis, numerical simulation, and flow-visualization experiments. In collaboration with Profs. M. S. Carvalho, L. F. Francis, and M. Tsapatsis, we have also recently launched research on the printing and coating of suspensions, processes that are central to the successful manufacture of emerging products in the energy and electronics industries.
Flow and deformation of viscoelastic materials: Viscoelastic materials---substances that can behave like both a liquid and a solid---are ubiquitous in polymer processing operations, printing and coating processes, and NMFS. We have examined a number of problems concerning the flow and deformation of these materials, including chaotic mixing of viscous polymeric liquids, instabilities in polymer-laden free shear flows and channel flows, shear banding in surfactant solutions, vibration-induced interfacial instabilities of viscoelastic fluids, and flow-induced interfacial instabilities near polymer gels. Currently, we are investigating self-assembly of topographical patterns on polymeric materials, which has application in the design of specialty coatings, NMFS, and biomaterials. Linear stability analysis, numerical simulation, and rheometry are among the tools we use in our work. We have also recently begun a collaboration with Profs. F. S. Bates and C. W. Macosko to study melt blowing of nanofibers, materials of interest in applications as diverse as filtration and tissue engineering.
Polymer dynamics near surfaces: The behavior of polymers near surfaces plays a key role in a variety of applications including NMFS, biosensors, and printing and coating processes. In many cases, the surface may be patterned chemically and/or topographically, and fluid flows and electric fields may be present. We are applying Brownian dynamics simulations to study how fluid flow, electric fields, and surface patterning can be designed to transport, separate, manipulate, and assemble macromolecules near surfaces. Various molecular theories are leveraged to guide the simulations and to understand the results. We have examined polymer dynamics in complex electroosmotic flows, polymer electrophoresis through narrow constrictions, and dendrimer adsorption. In collaboration with Prof. K. D. Dorfman, we are currently investigating the interaction of polyelectrolytes with surfaces and fluid flows.