Dynamics of Complex Fluids During Flow & Processing
Wesley R. Burghardt
Department of Chemical and Biological Engineering and Materials Science and Engineering, Northwestern University
The non-Newtonian flow characteristics of polymers and other complex fluids is intimately related to the ability of applied flow fields to significantly perturb the molecular or meso-scale structure. Consequently, modern research strongly emphasizes understanding of the molecular or microstructural origins of complex rheological behavior. In the case of polymers, process technology often relies on flow-induced structural changes (i.e. molecular orientation) to realize desired material properties in the resulting products. I will provide a brief survey of our efforts at the Advanced Photon Source to establish in situ x-ray scattering methods as a tool to directly probe the structure of complex polymer fluids during flow and processing. The combination of high flux with fast area detectors now enables real-time studies of transient structural dynamics, which can, in many cases, be directly linked to the macroscopic rheological behavior or to directly test predictions of theory (Figure).
Figure: Structure factor of a polymer bicontinuous microemulsion during shear flow. The top row show series of SAXS images collected on a poly(dimethyl siloxane)/poly(ethyl ethylene) microemulsion in the flow-velocity gradient plane (red feature in the center is parasitic scattering). The bottom row illustrates predictions of a Landau-Ginzburg microemulsion model under shear. From Caputo et al., Phys. Rev. E 66:041401 (2002).
In addition, high energy (short wavelength) x-rays allow new experimental concepts which extend the scope of possible studies. The presentation will survey available instrumentation ranging from shear cells for fundamental studies in simple flows, to in situ studies during processing via extrusion or injection molding.
I will then turn to a brief discussion of possible ways to exploit the potential for new advances in studying complex fluids under flow afforded by the unique characteristics of the planned ERL facility. Many classes of ordered complex fluids exhibit 'polydomain' structures in which local ordering only persists within micron-sized domains. There is considerable interest in the details of how flow affects the global orientation distribution of these domains. In typical in situ scattering experiments, the beam size is large, so that the measured response averages over the full distribution of domain orientations. A very narrow microfocus beam creates the potential for measurements at the 'single domain' level; however, the path length of the beam through the sample would have to be reduced to similar dimensions, a difficult proposition. Microfocus capabilities could also be exploited in studies of complex fluids in highly confined geometries (based on, for instance, microfluidics technology).
The ERL also promises advances in coherent scattering techniques. The dynamics of complex fluids can be elucidated both by dynamic studies at equilibrium (e.g. via photon correlation spectroscopy) or through interrogation of their response to applied deformation (the approach adopted in our research). Are there opportunities at this intersection? In general, flow and dynamic scattering are strange bedfellows, since velocity and/or velocity gradient fields impact the measured intensity correlation function. This might, however, present interesting opportunities. Specifically, homodyne dynamic light scattering has been shown to provide a means for direct, spatially-resolved measurements of velocity gradients [1,2] Certain classes of complex fluids are known to exhibit shear-banding instabilities in which portions of the fluid form thin regions of locally high velocity gradient [3]. I speculate on whether it might be possible to study such phenomena with narrow, coherent beams to obtain simultaneous measurements of the local velocity gradient and the fluid structure within the shear banded region.
References:
1. G Fuller, J. M. Rallison, R. L. Schmidt, and L. G. Leal, "The Measurement of Velocity Gradients in Laminar Flow by Homodyne Light Scattering Spectroscopy", J. Fluid. Mech. 100, 555 (1980)
2. J. Wang, D. Yavich, and L. G. Leal, "Time-resolved Velocity Gradient and Optical Anisotropy in Linear Flow by Photon Correlation Spectroscopy", Phys. Fluids, 6, 3519 (1994)
3. For instance, Y. T. Hu and A. Lips, "Kinetics and Mechanism of Shear Banding in an Entangled Micellar Solution", J. Rheol. 6, 1001 (2005)
