Norman J. Wagner

Norman J. Wagner

2014 Bingham Medalist

University of Delaware

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The 2014 Bingham Medal of The Society of Rheology is being presented to Norman J. Wagner of the University of Delaware, USA.

Wagner earned a Bachelor of Science in Chemical Engineering from Carnegie Mellon University in 1984. After an undergraduate thesis that yielded four publications on metal support interactions in supported catalysts, he turned to colloid science for his graduate research at Princeton. His dissertation, advised by William B. Russel, addressed “Nonequilibrium Statistical Mechanics of Concentrated Colloidal Dispersions” and included collaborations with Gerry Fuller (Stanford), Bruce Ackerson (Oklahoma), and Kees deKruif (Utrecht) that produced the first applications of optical techniques to the rheology of dispersions. After receiving his Ph.D. in 1988 he spent a year as a NATO Postdoctoral Fellow with Rudolph Klein and colleagues at the University of Konstanz, which introduced him to large-scale Brownian dynamics simulations of colloidal systems and further development of nonequilibrium statistical mechanics via mode coupling approximations. His exposure to simulations was reinforced by further postdoctoral work taking advantage of the computational facilities at Los Alamos National Laboratory.

Since joining the Department of Chemical Engineering at the University of Delaware Norm has deployed and further developed these tools, while characterizing the structure and rheology of a wide range of complex fluids, for example bimodal and polydisperse hard spheres, sticky spheres, liquid crystalline polymers (lyo- and thermotropic), polymeric and colloidal glasses, AOT microemulsions, dendrimers and hyperbranched polymers, wormlike micelles, and carbon nanotubes. His effort to understand as well as characterize this wide range of complex fluids stimulated a constant search for additional rheological tools, techniques to monitor the effect of shear on structure, and more powerful theory.

The outcomes of this quarter of a century of productivity can be characterized in a number of ways. The 180 publications in scholarly journals have generated more than 3,200 citations by others with a rate of citations that has doubled in the past four years. Much of the research focuses on colloidal dispersions, which is captured beautifully in the book Colloidal Suspension Rheology (Cambridge University Press, 2011) coauthored with Jan Mewis (KU Leuven). That book was molded by their short courses that the two have presented regularly for more than a decade at rheology meetings and a variety of other venues. Another more recent initiative that will benefit the community broadly is his demonstration of rheo-SANS (small-angle neutron scattering) instruments implemented at the D22 SANS beam line (Institut Laue-Langevin, Grenoble, France) for time-resolved large amplitude oscillatory experiments and the novel Rheo-SANS instrument developed at the National Center for Neutron Research (NCNR, NIST, Gaithersburg, MD). Results from these experiments provide clear connections between non-equilibrium structure and rheology.

One striking theme in Norm’s work is that of shear thickening in concentrated colloidal dispersions, a phenomenon initially highlighted by Rich Hoffman in 1974, who attributed it to a shear-induced order-disorder transition. Brady and Bossis (1985) with their early Stokesian dynamics simulations, D’Haene, Mewis, and Fuller (1993) through dichroism experiments, and Bender and Wagner (Journal of Rheology 40(5) 899-916 1996) with further flow dichroism measurements proved conclusively that hydrodynamic interactions generate very large, anisotropic aggregates of particles that produce the shear-thickening transition. The key distinction from an order-disorder transition is that hydrodynamic clustering is reversible, which an order-disorder transition is not. Norm’s group pursued that theme in several other well-cited papers (e.g. B.J. Maranzano & N.J. Wagner Journal of Rheology 45(5) 1205-1222 2001; Y.S. Lee & N.J. Wagner Rheologica Acta 42(3) 199-208 2003) and then joined forces with Brady for a popular summary in Physics Today (2009). In parallel with the science, Norm recognized a striking application described dramatically in “The ballistic impact characteristics of Kevlar® woven fabrics impregnated with a colloidal shear thickening fluid” (J. Materials Science 38(13) 2825-33 2003) and “Stab resistance of shear thickening fluid-treated fabrics” (Composites Science and Technology 67(3-4) 565-578 2007).

Optical techniques for characterizing rheology have a long history but the fundamental connections between the signal and the stresses have not always been clear. With J.W. Bender (J. Colloid and Interface Science 172 171 1995) Norm verified a fundamental stress-optical relationship that “enables the use of optical dichroism measurements to distinguish between thermodynamic and hydrodynamic contributions to the stress tensor.” This motivated them to “show that the shear thinning … is attributable to changes in the thermodynamic forces' contribution to the stress, consistent with both theory and simulation … [and] that shear thickening is attributable to increased hydrodynamic interactions.”

Another theme in Norm's work is the use of rheological techniques to deduce interparticle forces between colloidal particles. For example, Norm and Norbert Willenbacher of BASF employed torsional resonators at a range of frequencies (23-358 kHz) to determine the high frequency modulus of dispersions of polystyrene latices with grafted layers of poly(methacrylic acid). From those data they determined that the complex electrosteric forces were dominated by the excess osmotic pressure generated when the grafted layers overlap. Recent publications (e.g. Langmuir 28 1866-78 2012) demonstrate how far his group has come in this direction. This work with collaborators at NIST and Universidad de Guanajuato in Mexico employed TEM and densitometry to characterize the silica particles with grafted octodecyl chains; a stress-controlled cone and plate rheometer to characterize the linear viscoelasticity as function of temperature, frequency, and concentration; fiber-optic quasi-elastic light scattering to characterize the pair potential and detect the gel transition; and small-angle neutron scattering to characterize the structure as a function of temperature and volume fraction. This determines the gel line and the spinodal and verifies the liquid-crystal transition, i.e. the thermodynamics as well as the rheology.

Norm’s research on surfactant solutions with Eric Kaler might be exemplified by their well-cited paper (Langmuir 19(10) 4079-89 2003) on mixed cationic/anionic wormlike micelles. With rheology, flow birefringence, and small-angle neutron scattering over a broad range of solution compositions, they determined the microstructural length scales (persistence, entanglement, interaction, and contour lengths) that govern the rheology. They concluded that the characteristics of the added salt affect the persistence length and, hence, the microstructure and rheology. Varying surfactant and electrolyte concentration and composition systematically allowed them to span rheological behavior from nonionic wormlike micelles to polyelectrolytes.

The growing breadth of his capabilities is exemplified by two very recent papers, one characterizing the uptake of fluorescent dendimers into cancer cells in Biochimica et Biophysica Acta-Biomembranes and a second applying elastomers to control wrinkles in skin in ACS Macro Letters.

Finally, Norm’s long-standing and highly productive collaboration with Jan Mewis pulled him into classical issues that many modern rheologists avoid, that is, thixotropy. The basic elements of thixotropy are well reviewed in their paper (Advances in Colloid and Interface Science 147-48 214-227 2009).

Norm's work on shear thickening fluids has also captured the interest and imagination of many outside of the rheology community. He has shared this work in venues as diverse as National Public Radio (US), the Smithsonian Institution's "Going to Extremes" seminar series, and recently, a White House Office of Science and Technology Google Hangout, "We the Geeks: The Stuff Superheroes Are Made Of."

During his recent sabbatical year after completing his service as the Department Chair of Chemical and Biomolecular Engineering at the University of Delaware, Norm visited colleagues, friends, and family. In true Norm fashion, over a short four month period he packed in visits to Lund University, TU Berlin, KIT Karlsruhe, ETH Zurich, KU Leuven and IIT Madras, all the while keeping up with his research group of undergraduates, graduate students and postdocs back in Delaware and finding time to participate in neutron scattering experiments at the Institute Laue Langevin, Grenoble, France and the NIST Center for Neutron Research, Gaithersburg, MD. Nothing seems to charge up Norm more than the graveyard-shift at the neutron beamline.

Lastly, Norm is an indefatigable leader of outdoor adventures, whether it's an Earth Day hike in nearby White Clay Creek, an annual summer float with his research group and friends down the Brandywine river, or a conference-break jaunt to the summit of Saddle Peak in the Santa Monica Mountains. Norm also enjoys bow hunting, squash, and, in the spirit of Delaware's own Caesar Rodney, horseback riding.

Norm met his wife Sabine while he was a postdoc at the University of Konstanz and she was a student of International Politics, Linguistics, and English Literature. A native of Germany, Sabine teaches World Languages and Cultures at Archmere Academy in Wilmington. Together, Sabine and Norm travel frequently to visit family in Europe and to host study abroad experiences for American students. They live in a quiet neighborhood of rolling hills and cul-de-sacs just outside of Newark.