Complex fluids are a broad class of materials that are usually homogeneous at the macroscopic scale, but possess structure at an intermediate scale (e.g., colloids, blood, and polymers). The rheology and bulk flow behavior of such fluids are strong functions of their intermediate or structural scale. A prime example of this is the stretching and alignment of flexible polymer molecules in fluid flow, which has been connected to many poorly understood phenomena such as turbulence drag reduction, anomalous viscosity enhancement, and irregular flow. Recent studies have shown that the flow of viscoelastic fluids in parallel shear geometries (pipes/channels) is unstable to finite amplitude perturbations at low Reynolds numbers (Re). Results point to a subcritical transition that is akin to Newtonian turbulence in pipes except that elastic stresses rather than inertia is the driving force.
Intellectual Merit : The main goal of this proposal is to systematically investigate origins this newfound purely elastic subcritical transition in parallel shear flows and its broad consequences to complex fluid flows. Experiments with viscoelastic fluids will be performed in a long, straight microchannel using velocimetry and in-situ (local) pressure measurements, and single molecule tracers. In particular, single molecule experiments using fluorescent DNA molecules will be used to gain insight into the molecular origins of viscoelastic instabilities by measuring the conformation dynamics and statistics of polymer molecules in such flows. We will be able to address many important questions such as: i) What are the main molecular mechanisms leading to the onset of such nonlinear elastic instability? What are the molecular conformation dynamics before, during, and after such transition? ii) What kind of flow coherent structures develops during the transition? iii) Is the transition nonlinear/subcritical or is it a product of a linear growth amplification which is known to exist in Newtonian pipe flows? iv) Does this viscoelastic subcritical transition lead to "elastic turbulence"? v) How is the pressure drop related to the flow rate below and above the onset of irregular flow? Is there an increase in flow resistance or pressure drop?
From a scientific standpoint, the studies proposed here will provide much needed insight into the mechanisms by which the conformation dynamics of flexible molecules affects the stability of the bulk flow behavior using direct visualization of molecular tracers (fluorescent DNA molecules). Most previous investigations of this kind focus on the effect of fluid flow on polymer dynamics. The parallel pursuit of bulk flow behavior and direct molecular visualization will give rise to a comprehensive view of the molecular interactions with the applied fluid stresses. This, in turn, will lead to the development of more realistic and accurate theoretical and molecular models for the onset of flow instabilities in general. The use of microfluidics allows for an excellent test-bed for single molecule experiments since flows can be very well controlled.
Broader Impacts : This proposal outlines an integrated research and educational program that includes: i) training graduate students by offering graduate level courses in complex fluids, rheology, and nonlinear dynamics as well as research opportunities in these areas. A main goal is to increase the participation of historically under-represented minorities such as females, African-Americans, Native-Americans, and Hispanics in research; ii) recruiting undergraduate students for summer research internships from Historically Black Colleges and Universities that do not possess an engineering graduate program. The PI will also take advantage of the University of Pennsylvania's strong outreach infrastructure to involve K-12 teachers and high school students from West Philadelphia in the research program; iii) finally, the results of this research and educational program will be broadly disseminated and will have potentially important benefits to society. In particular, the results will offer new knowledge in complex fluid flow phenomena.
