B.A., Rice University, 1969; M.Ch.E., Rice University, 1969
Ph.D., Stanford University, 1973
NATO Postdoctoral Fellow, Cambridge University, 1974
Honors and Awards
- Fellow, American Institute of Chemical Engineers, 2014
- Alpha Chi Sigma Award, American Institute of Chemical Engineers, 2010
- Award for Surface and Colloid Chemistry, American Chemical Society, 2007
- Debye (Visiting) Professor, University of Utrecht, 2001
- Bingham Medal, Society of Rheology, 1999
- American Academy of Arts and Sciences, 1995
- National Academy of Engineering, 1992
- William H. Walker Award, American Institute of Chemical Engineers, 1992
- Olaf A. Hougen Visiting Professor, University of Wisconsin, 1984
Formation of Thin Films from Colloidal Dispersions
The process of drying colloidal dispersions, i.e. evaporating the liquid, to create particulate solids or continuous polymer films is common to a range of important technologies, e.g. forming polymer films from latex dispersions, casting magnetic tapes, depositing highly porous coatings on ink jet papers, forming sol-gel glass, adding anti-reflection coatings to eyeglass lenses, encapsulating vitamins in beads, fabricating photonic crystals from silica sols, spray depositing thin film oxide fuel cells, and manufacturing photographic film. The objective is generally to create a layer of specified thickness and controlled porosity with permeability, strength, optical, and other physical properties appropriate for the application. Processing of such films raises a number of interesting and difficult issues because of the conflicting constraints for successful film formation and desired performance properties. Our recent work offers a model based on rigorous mechanics for consolidation and cracking of immobilized, i.e. close packed, colloidal spheres that deform viscoelastically due to contact or interfacial forces. Predictions from this model identify the conditions under which air-water, polymer-water, or polymer-air interfacial energies suffice to form void-free films and, if the particles do not deform viscously, why cracking often ensues. Several puzzles still remain. For example, cracks that follow a moving front across a drying film develop a characteristic spacing and often advance in a stick-slip fashion. Also, colloidal packings confined in a narrow channel open only at the end delaminate from the wall as well as cracking.
Electrohydrodynamic Patterning of Thin Polymer Films
Application of an electric field normal to a thin polymer film spin-coated on a silicon wafer can generate fascinating periodic patterns. The process requires positioning a second wafer parallel to and within 100 nm or so of the first and heating the film above the glass transition temperature of the polymer. This allows a uniform electric field to generate an instability in the film such that long wavelengths grow. If the upper surface is patterned discontinuously, the film is unconditionally unstable at edges and corners. Patterns emerge to form ordered arrays of pillars or concentric rings with periodicity reflecting the geometry of the template and the balance between surface tension and electrical forces. The evolution is controlled by a balance among interfacial tension, electric fields, and viscous stresses within the film. A linear stability analysis predicts wavelengths in quantitative agreement with the periodicity observed, whereas weakly nonlinear theory and fully nonlinear simulations elucidate the selection of hexagonal arrays of pillars under planar masks and the ability of patterns on the mask to produce long-range order. On longer time scales the patterns often coarsen, which is explained by the instability of pseudo-steady states to merging via collisions or Ostwald ripening. Our current objective is to scale the patterns down to wavelengths ≤ 100 nm by introducing ionic liquids in the gap.
Collapsing Colloidal Gels
An increasing fascination and better understanding of gelation in colloidal systems has brought into focus the puzzling abrupt collapse of some such gels. Recent research has demonstrated that “weak” gels collapse on a time scale that increases exponentially with the strength of attractions between the particles. Visualizations of the collapse reveal macroscopic fissures in some situations and density inhomogeneities followed by vertical streamers of fluid in others. Most dramatic are the gels that subside slightly then disintegrate catastrophically. Meanwhile, theorists have located the “gel line”, corresponding to the transition from a metastable fluid to an amorphous non-equilibrium solid, relative to the phase boundaries for the equilibrium fluid-crystal transition and the binodal for a metastable gas-liquid transition. Likewise, the mechanical properties of the metastable fluid and the gel are now understood with respect to the functional dependence on the strength of the interparticle attraction, e.g. an exponential dependence for the viscosity which correlates with the time to collapse. The element missing with respect is the driving force for the collapse. To understand that we seek to locate the thermodynamic state of collapsing gels within the equilibrium and non-equilibrium phase boundaries described above and incorporate the appropriate forcings into a predictive model.
- "Delayed Yield in Colloidal Gels: Creep, Flow, and Re-entrant Solid Regimes." Journal of Rheology, 60 (4), 783-807, 2016 [with B. Landrum and R. Zia ].
- "A Micro-mechanical Study of Coarsening and Rheology of Colloidal Gels: Cage Building, Cage Hopping, and Smoluchowski's Ratchet." Journal of Rheology, 58 (5), 1121-1157, 2014 [with R. Zia and B. Landrum].
- "Particle Ordering in Colloidal Thin Films Deposited by Flow-Coating." AIChE Journal, 60 (4) 1287-1302, 2014 [with C. Lau].
- "Divergence in the Low Shear Viscosity for Brownian Hard-sphere Dispersions: At Random Close Packing or the Glass Transition?" Journal of Rheology, 57 (6), 1555-1567, 2013 [with N. Wagner and J. Mewis].
- "Linear Viscoelasticity of ZrO2 Nanoparticle Dispersions with Associative Polymers." Rheologica Acta, 51 (9), 771-782, 2012 [with R. Ndong].
- "Rheology of Surface-modified Titania Nanoparticles Dispersed in PDMS Melts: The Significance of the Power Law." Journal of Rheology, 56 (1), 27-43, 2012 [with R. Ndong].