cf 2 group unifying research theme is the interaction of flow and liquid micro- and nano-structure. Most engineered materials are formed and/or processed in the liquid state; they are complex fluids because they possess intrinsic length scales that are well-separated from the macroscopic length scales of the process (usually tens of micrometers to meters) and the nanoscopic length scales of the solvent (usually smaller than one nanometer). For example, in polymer solutions and melts the intrinsic length scale is the length of the polymer (usually hundreds of nanometers to few micrometers), which is well separated from the finer length scales (solvent diameter in solution, polymer diameter in melts). The large scale microstructural features relax on timescales that overlap the flow time scales; thus, the dynamic morphology can differ dramatically from the equilibrium one, and this changing morphology affects the flow and produced intriguing nonlinear dynamical phenomena that are not observed in flowing liquids of low-molecular weight.
We are studying how flexible (polystyrene, long DNA) and semiflexible (PBZT, actin) macromolecules interact with the flow at the molecular level by applying high-resolution fluorescence microscopy, shear and extensional rheometry, and non-equilibrium Brownian Dynamics.
We are applying similar molecular models to understanding the collapse
of semiflexible objects in bad solvent into toroidal
shapes—e.g., DNA in solution with multivalent ions (with Prof. MacKintosh, Vrije Universiteit
We are also studying the translocation of polymers and biopolymers in nanopores by Brownian Dynamics, Molecular Dynamics, and statistical
mechanics (with Profs. Kolomeisky and Clementi,
Rheometry, neutron and light
scattering, AFM, TEM, and molecular modeling, are applied to analyzing how the
degree of intramolecular crosslinking
affects the solution and flow behavior of polymer nanoparticles,
controlling the transition from particles to coils—particoils
(with Prof. Wong,
Detailed single-cell mechanics models are being developed based on nonlinear viscoelasticity and massively parallel finite element computations to understand how flow affects the stress on the cell membrane during growth, in an effort to controlling the in-vitro growth of biomaterials (with Prof. Zygourakis, Rice University).
Cell deformation models are coarsened for application to hemolysis in medical devices such as blood pumps (with
The solution behavior of Single-Walled NanoTubes in superacids is being studied by rheometry, optical microscopy, and scattering, to design optimal liquid crystalline solutions for successfully spinning macroscopic, continuous, neat fibers of SWNTs (with Prof. Smalley, Rice University).
We are simplifying the single-molecule models of nearly rigid polymers
by Galerkin-projecting them onto convenient basis
functions to yield highly efficient, yet accurate, stochastic models (with
The molecular models that are being developed and applied to understand single-molecule behavior of macromolecular solutions (from flexible to nearly rigid) are being coarsened through projection techniques based on an extension of local equilibrium thermodynamics in order to develop equations for expectation values of microstructural features of flowing macromolecular liquids; the coarse-grained models are used in massively-parallel finite elements codes for modeling, analyzing, and optimizing processes on larger length scales—from microfluidics (~mm), to coating (with Prof. Carvalho, Pontificia Universidade Catolica do Rio de Janeiro) and ink-jet printing (with Prof. Basaran, Purdue University) (~ 10 to 500 mm), to polymer processing (~ 100 mm to beyond few mm).
We also investigate the rheological behavior of emulsions, and relate it
their physical-chemical properties, which we can control in the formulation
(with A. Peña,
specific areas of research
Free surface flows
Computational modeling of process flows
Visualization of flowing single DNA molecules
Rheology and phase behavior of Single-Walled Carbon Nanotubes
Rheology and microstructure of Polymeric Nanoparticles (particoils)
Rheology and microstructure of Emulsions
Single-molecule behavior of semiflexible macromolecules